The Geography of the Region about Devil's Lake and the Dalles of the Wisconsin
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Title: The Geography of the Region about Devil's Lake and the Dalles of the Wisconsin
Author: Rollin D. Salisbury
Wallace Walter Atwood
Release date: November 27, 2011 [eBook #38148]
Language: English
Other information and formats: www.gutenberg.org/ebooks/38148
Credits: Produced by David Edwards, Joanna Johnston and the Online
Distributed Proofreading Team at http://www.pgdp.net (This
file was produced from images generously made available
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*** START OF THE PROJECT GUTENBERG EBOOK THE GEOGRAPHY OF THE REGION ABOUT DEVIL'S LAKE AND THE DALLES OF THE WISCONSIN ***
Produced by David Edwards, Joanna Johnston and the Online
Distributed Proofreading Team at http://www.pgdp.net (This
file was produced from images generously made available
by The Internet Archive)
Wisconsin Geological and Natural History Survey.
E. A. BIRGE, Director. C. R. VAN HISE, Consulting Geologist.
BULLETIN NO. V. EDUCATIONAL SERIES NO. 1.
THE GEOGRAPHY
OF THE
REGION ABOUT DEVIL'S LAKE
AND THE
DALLES OF THE WISCONSIN,
With Some Notes on Its Surface Geology.
BY
ROLLIN D. SALISBURY, A. M.,
_Professor of Geographic Geology, University of Chicago,_
AND
WALLACE W. ATWOOD, B. S.,
_Assistant in Geology, University of Chicago._
MADISON, WIS.
Published by the State.
1900.
Wisconsin Geological and Natural History Survey.
-------------------------------------------------------------------
BOARD OF COMMISSIONERS.
EDWARD SCOFIELD,
Governor of the State.
L. D. HARVEY,
State Superintendent of Public Instruction.
CHARLES K. ADAMS, President,
President of the University of Wisconsin.
EDWIN E. BRYANT, Vice-President,
President of the Commissioners of Fisheries.
CHARLES S. SLICHTER, Secretary,
President of the Wisconsin Academy of Sciences, Arts, and
Letters.
--------------------------------------------------------------
E. A. BIRGE, Director of the Survey.
C. R. VAN HISE, Consulting Geologist.
E. R. BUCKLEY, Assistant Geologist.
In charge of Economic Geology.
S. WEIDMAN, Assistant Geologist.
In charge of Geology of Wausau District.
L. S. SMITH, in charge of Hydrography.
S. V. PEPPEL, Chemist.
F. R. DENNISTON, Artist.
[Illustration: THE DALLES OF THE WISCONSIN.]
CONTENTS.
---------------------------------------------------------
PART I. THE TOPOGRAPHY WITH SOME NOTES ON THE SURFACE GEOLOGY.
CHAPTER I.
GENERAL GEOGRAPHIC FEATURES
I. The Plain Surrounding the Quartzite Ridges.
Topography
Structure
Origin of the Sandstone and Limestone
Origin of the Topography
II. The Quartzite Ridges
Topography
The Structure and Constitution of the Ridges
III. Relations of the Sandstone of the
Plain to the Quartzite of the Ridges
PART II. HISTORY OF THE TOPOGRAPHY.
CHAPTER II.
OUTLINE OF THE HISTORY OF THE ROCK FORMATIONS
WHICH SHOW THEMSELVES AT THE SURFACE.
I. The Pre-Cambrian History of the Quartzite
From loose Sand to Quartzite
Uplift and Deformation. Dynamic Metamorphism
Erosion of the Quartzite
Thickness of the Quartzite
II. The History of the Paleozoic Strata
The Subsidence
The Potsdam Sandstone (and Conglomerate)
The Lower Magnesian Limestone
The St. Peters Sandstone
Younger Beds
Climatic Conditions
Time involved
The Uplift
CHAPTER III.
GENERAL OUTLINE OF RAIN AND RIVER EROSION
Elements of Erosion
Weathering
Corrasion
Erosion without Valleys
The Beginning of a Valley
The Course of a Valley
Tributary Valleys
How a Valley gets a Stream
Limits of a Valley
A Cycle of Erosion
Effects of unequal Hardness
Falls and Rapids
Narrows
Erosion of folded Strata
Base-level Plains and Peneplains
Transportation and Deposition
Topographic Forms resulting from Stream Deposition
Rejuvenation of Streams
Underground Water
CHAPTER IV.
EROSION AND THE DEVELOPMENT OF STRIKING SCENIC FEATURES
Establishment of Drainage
Striking scenic Features
The Baraboo Bluffs
The Narrows in the Quartzite
Glens
Natural Bridge
The Dalles of the Wisconsin
The Mounds and Castle Rocks
CHAPTER V.
THE GLACIAL PERIOD.
The Drift
Snow Fields and ice Sheets
The North American ice Sheets
The Work of glacier Ice
Erosive Work of Ice. Effect on Topography
Deposition by the Ice. Effect on Topography
Direction of ice Movement
Effect of Topography on Movement
Glacial Deposits
The ground Moraine
Constitution
Topography
Terminal Moraines
Topography of terminal Moraines
The terminal Moraine about Devil's Lake
The Moraine on the main Quartzite Range
Constitution of the marginal Ridge
The Slope of the upper Surface of the Ice at the Margin
Stratified Drift
Its Origin
Glacial Drainage
Stages in the History of an Ice Sheet
Deposits made by extraglacial Waters during the maximum Extension
of the Ice
At the Edge of the Ice, on Land
Beyond the Edge of the Ice, on Land
Deposits at and beyond the Edge of the Ice in standing Water
Deposits made by extraglacial Waters during the Retreat of the Ice
Deposits made by extraglacial Waters during the Advance of the Ice
Deposits made by subglacial Streams
Relations of stratified to unstratified Drift
Complexity of Relations
Classification of stratified Drift on the Basis of Position
Extraglacial Deposits
Supermorainic deposits
The submorainic (basal) Deposits
Intermorainic stratified Drift
Changes in Drainage effected by the Ice
While the Ice was on
Wisconsin Lake
Baraboo Lake
Devil's Lake in glacial Times
After the Ice had disappeared
Lakes
Existing Lakes
Changes in Streams
Skillett Creek
The Wisconsin
The Driftless Area
Contrast between glaciated and unglaciated Areas
Topography
Drainage
Mantle Rock
LIST OF ILLUSTRATIONS.
------------------------------------------------------------
PLATES.
Plate
Frontispiece. The Dalles of the Wisconsin
I. General map of the Devil's Lake region
II. Local map of the Devil's Lake region
III. Fig. 1--Ripple marks on a slab of sandstone
Fig. 2--Piece of Potsdam conglomerate
IV. Lower Narrows of the Baraboo
V. Devil's Lake notch
VI. East bluff of Devil's Lake
VII. East bluff at the Upper Narrows of the Baraboo near Ableman's
VIII. Vertical shear zone face of east bluff at Devil's Lake
IX. Massive quartzite in situ in road through Upper Narrows near
Ableman's
X. Brecciated quartzite
XI. Northwest wall of the Upper Narrows
XII. Steamboat Rock
XIII. Fig. 1--A very young valley
Fig. 2--A valley at later stage of development
Fig. 3--Young valleys
XIV. Fig. 1--Same valleys as shown in Pl. XIII, Fig. 3,
but at a later stage of development
Fig. 2--Same valleys as shown in Fig. 1 in later stage of
development
XV. Diagram illustrating how a hard inclined layer of rock
becomes a ridge in the process of degradation
XVI. Skillett Falls
XVII. A group of mounds on the plain northwest from Camp Douglas
XVIII. Castle Rock near Camp Douglas
XIX. Fig. 1--Sketch of a young valley
Fig. 2--Same valleys as shown in Fig. 1 in later stage of
development
XX. Fig. 1--Sketch of a part of a valley at a stage of
development corresponding to the cross section
shown in Figure 21
Fig. 2--Sketch of a section of the Baraboo valley
XXI. Cleopatra's Needle
XXII. Turk's Head
XXIII. Devil's Doorway
XXIV. Talus slope on east bluff of Devil's Lake
XXV. Dorward's Glen
XXVI. Natural Bridge near Denzer
XXVII. The Navy Yard
XXVIII. Chimney Rock
XXIX. An island in the Lower Dalles
XXX. View in Lower Dalles
XXXI. Stand Rock
XXXII. Petenwell Peak
XXXIII. North American ice sheet
XXXIV. Owl's Head
XXXV. Cut in glacial drift
XXXVI. Glaciated stones
XXXVII. Topographic map of a small area about Devil's Lake
XXXVIII. Distorted laminae of silt and clay
FIGURES IN TEXT.
Figure
1. Profile across the Baraboo quartzite ranges through Baraboo
2. Profile across the Baraboo ranges through Merrimac
Transcriber's note: There is no figure 3.
4. Diagram showing the structure of the quartzite
5. Diagram showing the relation of the Potsdam sandstone to the Baraboo
quartzite
6. Diagram illustrating effect of faulting on outcrop
7. Diagram showing the disposition of sediments about an island
8. The same as 7 after subsidence
9. Diagram showing relation of Potsdam conglomerate to quartzite at
Devil's Lake
10. Cross section of a delta
11. The geological formations of southern Wisconsin
12. A typical river system
13. Diagram illustrating the relations of ground water to streams
14. Diagram illustrating the shifting of divides
15. Diagram showing topography at the various stages of an erosion cycle
16. Diagram illustrating the development of rapids and falls
17. Sketch looking northwest from Camp Douglas
18. Diagrammatic cross section of a young valley
19. Diagrammatic profile of a young valley
20. Diagrammatic cross section of a valley in a later stage of
development
21. The same at a still later stage
22. Diagram illustrating the topographic effect or rejuvenation of a
stream by uplift
23. Normal profile of a valley bottom
24. Profile of a stream rejuvenated by uplift
25. Diagram illustrating monoclinal shifting
26. Diagram showing the relation of the Potsdam sandstone to the
quartzite at the Upper Narrows
27. Diagrammatic cross section of a field of ice and snow
28. Shape of an erosion hill before glaciation
29. The same after glaciation
30. Diagram showing the effect of a valley on the movement of ice
31. The same under different conditions
32. Diagram showing the relation of drift to the underlying rock where
the drift is thick
33. The same where the drift is relatively thin
34. Diagrammatic representation of the effect of a hill on the edge of
the ice
35. The same at a later stage of the ice advance
36. Map showing the relation of the ice lobes during the Wisconsin epoch
of the glacial period
37. Sketch of the terminal moraine topography east of Devil's Lake
38. Cut through the terminal moraine east of Kirkland
39. Cross section of the marginal ridge of the moraine on the south
slope of the Devil's nose
40. Cross section of the marginal ridge of the moraine on the crest of
the quartzite range
41. Morainic outwash plain
42. The same in other relations
43. Skillett Creek and its peculiarities
44. The Wisconsin valley near Kilbourn city
45. Drainage in the driftless area
46. Drainage in the glaciated area
47. Section in the driftless region showing relation of the soil to the
solid rock beneath
PART I.
------------------------------------------------------------
THE TOPOGRAPHY.
WITH SOME NOTES ON THE SURFACE GEOLOGY.
GEOGRAPHY AND SURFACE GEOLOGY OF THE DEVIL'S LAKE REGION.
CHAPTER I.
GENERAL GEOGRAPHIC FEATURES.
This report has to do with the physical geography of the area in south
central Wisconsin, shown on the accompanying sketch map, Plate I. The
region is of especial interest, both because of its striking scenery,
and because it illustrates clearly many of the principles involved in
the evolution of the geography of land surfaces.
Generally speaking, the region is an undulating plain, above which rise
a few notable elevations, chief among which are the Baraboo quartzite
ranges, marked by diagonal lines on Plates I and II. These elevations
have often been described as two ranges. The South or main range lies
three miles south of Baraboo, while the North or lesser range, which is
far from continuous, lies just north of the city.
The main range has a general east-west trend, and rises with bold and
sometimes precipitous slopes 500 to 800 feet above its surroundings. A
deep gap three or four miles south of Baraboo (Plates II, V, and
XXXVII) divides the main range into an eastern and a western
portion, known respectively as the _East and West bluffs_ or _ranges_.
In the bottom of the gap lies Devil's lake (i, Plate II and Plate
XXXVII), perhaps the most striking body of water of its size in the
state, if not in the whole northern interior. A general notion of the
topography of a small area in the immediate vicinity of the lake may be
obtained from Plate XXXVII.
The highest point in the range is about four miles east of the lake, and
has an elevation of more than 1,600 feet above sea level, more than
1,000 feet above Lake Michigan, and about 800 feet above the Baraboo
valley at its northern base. The eastward extension of the west range
(Plate XXXVII) lying south of the lake, and popularly known as the
_Devil's nose_, reaches an elevation of a little more than 1,500 feet.
The lesser or North quartzite range (Plate II) rises 300 feet to 500
feet above its surroundings. It assumes considerable prominence at the
Upper and Lower narrows of the Baraboo (b and c, Plate II, c, Plate
XXXVII and Plate IV). The North range is not only lower than the
South range, but its slopes are generally less steep, and, as Plate II
shows, it is also less continuous. The lesser elevation and the gentler
slopes make it far less conspicuous. About three miles southwest of
Portage (Plate II) the North and South ranges join, and the elevation at
the point of union is about 450 feet above the Wisconsin river a few
miles to the east.
The lower country above which these conspicuous ridges rise, has an
average elevation of about 1,000 feet above the sea, and extends far
beyond the borders of the area with which this report is concerned. The
rock underlying it in the vicinity of Baraboo is chiefly sandstone, but
there is much limestone farther east and south, in the area with which
the Baraboo region is topographically continuous. Both the sandstone and
limestone are much less resistant than the quartzite, and this
difference has had much to do with the topography of the region.
The distinctness of the quartzite ridges as topographic features is
indicated in Plate XXXVII by the closeness of the contour lines on their
slopes. The same features are shown in Figs. 1 and 2, which represent
profiles along two north-south lines passing through Baraboo and
Merrimac respectively.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. I.
General map showing the location of the chief points mentioned in this
report. The location of the area shown in Plate XXXVII, centering about
Baraboo, is indicated.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. II.
Map of Area considered in this Report.]
[Illustration: Fig. 1.--Profile along a line extending due north and
south from Baraboo across the north and south ranges. The dotted
continuation northward, represents the extension of the profile beyond
the topographic map, Plate XXXVII.]
[Illustration: Fig. 2.--Profile north from Merrimac across the quartzite
ranges. The dotted continuation northward represents the extension of
the profile beyond the topographic map, Plate XXXVII.]
I. THE PLAIN SURROUNDING THE QUARTZITE RIDGES.
_Topography._--As seen from the top of the quartzite ridges, the
surrounding country appears to be an extensive plain, but at closer
range it is seen to have considerable relief although there are
extensive areas where the surface is nearly flat.
The relief of the surface is of two somewhat different types. In some
parts of the area, especially in the western part of the tract shown on
Plate II, the surface is made up of a succession of ridges and valleys.
The ridges may be broken by depressions at frequent intervals, but the
valleys are nowhere similarly interrupted. It would rarely be possible
to walk along a ridge or "divide" for many miles without descending into
valleys; but once in a valley in any part of the area, it may be
descended without interruption, until the Baraboo, the Wisconsin, the
Mississippi, and finally the gulf is reached. In other words, the
depressions are continuous, but the elevations are not. This is the
first type of topography.
Where this type of topography prevails its relation to drainage is
evident at a glance. All the larger depressions are occupied by streams
continuously, while the smaller ones contain running water during some
part of the year. The relations of streams to the depressions, and the
wear which the streams effect, whether they be permanent or temporary,
suggest that running water is at least one of the agencies concerned in
the making of valleys.
An idea of the general arrangement of the valleys, as well as many
suggestions concerning the evolution of the topography of the broken
plain in which they lie might be gained by entering a valley at its
head, and following it wherever it leads. At its head, the valley is
relatively narrow, and its slopes descend promptly from either side in
such a manner that a cross-section of the valley is V-shaped. In places,
as west of Camp Douglas, the deep, steep-sided valleys are found to lead
down and out from a tract of land so slightly rolling as to be well
adapted to cultivation. Following down the valley, its progressive
increase in width and depth is at once evident, and at the same time
small tributary valleys come in from right and left. At no great
distance from the heads of the valleys, streams are found in their
bottoms.
As the valleys increase in width and depth, and as the tributaries
become more numerous and wider, the topography of which the valleys are
a feature, becomes more and more broken. At first the tracts between the
streams are in the form of ridges, wide if parallel valleys are distant
from one another, and narrow if they are near. The ridges wind with the
valleys which separate them. Whatever the width of the inter-stream
ridges, it is clear that they must become narrower as the valleys
between them become wider, and in following down a valley a point is
reached, sooner or later, where the valleys, main and tributary, are of
such size and so numerous that their slopes constitute a large part of
the surface. Where this is true, and where the valleys are deep, the
land is of little industrial value except for timber and grazing. When,
in descending a valley system, this sort of topography is reached, the
roads often follow either the valleys or the ridges, however indirect
and crooked they may be. Where the ridges separating the valleys in such
a region have considerable length, they are sometimes spoken of as "hog
backs." Still farther down the valley system, tributary valleys of the
second and lower orders cross the "hog backs," cutting them into hills.
By the time this sort of topography is reached, a series of flats is
found bordering the streams. These flats may occur on both sides of the
stream, or on but one. The topography and the soil of these flats are
such as to encourage agriculture, and the river flats or alluvial plains
are among the choicest farming lands.
With increasing distance from the heads of the valleys, these river
plains are expanded, and may be widened so as to occupy the greater part
of the surface. The intervening elevations are there relatively few and
small. Their crests, however, often rise to the same level as that of
the broader inter-stream areas farther up the valleys. The relations of
the valleys and the high lands separating them, is such as to suggest
that there are, generally speaking, two sets of flat surfaces, the
higher one representing the upland in which the valleys lie, the lower
one representing the alluvial plains of the streams. The two sets of
flats are at once separated and connected by slopes. At the head of a
drainage system, the upland flats predominate; in the lower courses, the
river plains; in an intermediate stage, the slopes are more conspicuous
than either upper or lower flat.
Southwest from Devil's lake and northwest from Sauk City, in the valley
of Honey creek, and again in the region southwest from Camp Douglas, the
topography just described is well illustrated. In both these localities,
as in all others where this type of topography prevails, the intimate
relations of topography and drainage cannot fail to suggest that the
streams which are today widening and deepening the valleys through which
they flow, had much to do with their origin and development. This
hypothesis, as applied to the region under consideration, may be tested
by the study of the structure of the plain.
The second type of topography affecting the plain about the quartzite
ranges is found east of a line running from Kilbourn City to a point
just north of Prairie du Sac. Though in its larger features the area
east of this line resembles that to the west, its minor features are
essentially different. Here there are many depressions which have no
outlets, and marshes, ponds, and small lakes abound. Not only this, but
many of the lesser elevations stand in no definite relation to valleys.
The two types of topography make it clear that they were developed in
different ways.
_Structure._--Examination of the country surrounding the Baraboo ridges
shows that its surface is underlaid at no great depth by horizontal or
nearly horizontal beds of sandstone and limestone (see Plates XVI,
XXVIII, and Frontispiece). These beds are frequently exposed on opposite
sides of a valley, and in such positions the beds of one side are found
to match those on the other. This is well shown along the narrow valley
of Skillett creek just above the "Pewit's nest." Here the swift stream
is rapidly deepening its channel, and it is clear that a few years
hence, layers of sandstone which are now continuous beneath the bed of
the creek will have been cut through, and their edges will appear on
opposite sides of the valley just as higher layers do now. Here the most
skeptical might be convinced that the layers of rock on either side of
the narrow gorge were once continuous across it, and may see, at the
same time, the means by which the separation was effected. Between the
slight separation, here, where the valley is narrow, and the great
separation where the valleys are wide, there are all gradations. The
study of progressively wider valleys, commencing with such a gorge as
that referred to, leaves no room for doubt that even the wide valleys,
as well as the narrow ones, were cut out of the sandstone by running
water.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. III.
Illustration: FIG. 1.
Ripple marks on a slab of Potsdam sandstone.
Illustration: FIG. 2.
Piece of Potsdam conglomerate. The larger pebbles are about three inches
in diameter.]
The same conclusion as to the origin of the valleys may be reached in
another way. Either the beds of rock were formed with their present
topography, or the valleys have been excavated in them since they were
formed. Their mode of origin will therefore help to decide between these
alternatives.
_Origin of the sandstone and limestone._--The sandstone of the region,
known as the Potsdam sandstone, consists of medium sized grains of
sand, cemented together by siliceous, ferruginous, or calcareous cement.
If the cement were removed, the sandstone would be reduced to sand, in
all respect similar to that accumulating along the shores of seas and
lakes today.
The surfaces of the separate layers of sandstone are often distinctly
ripple-marked (Fig. 1, Pl. III), and the character of the markings is
identical in all essential respects with the ripples which affect the
surface of the sand along the shores of Devil's lake, or sandy beaches
elsewhere, at the present time. These ripple marks on the surfaces of
the sandstone layers must have originated while the sand was movable,
and therefore before it was cemented into sandstone.
In the beds of sandstone, fossils of marine animals are found. Shells,
or casts of shells of various sorts are common, as are also the tracks
and burrowings of animals which had no shells. Among these latter signs
of life may be mentioned the borings of worms. These borings are not now
always hollow, but their fillings are often so unlike the surrounding
rock, that they are still clearly marked. These worm borings, like the
ripple marks, show that the sand was once loose.
The basal beds of the sandstone are often conglomeratic. The
conglomeratic layers are made up of water-worn pieces of quartzite,
Plate III, Fig. 2, ranging in size from small pebbles to large bowlders.
The interstices of the coarse material are filled by sand, and the whole
cemented into solid rock. The conglomeratic phase of the sandstone may
be seen to advantage at Parfrey's glen (a, Plate XXXVII) and Dorward's
glen, (b, same plate) on the East bluff of Devil's lake above the Cliff
House, and at the Upper narrows of the Baraboo, near Ablemans. It is
also visible at numerous other less accessible and less easily
designated places.
From these several facts, viz.: the horizontal strata, the ripple-marks
on the surfaces of the layers, the fossils, the character of the sand,
and the water-worn pebbles and bowlders of the basal conglomerate,
positive conclusions concerning the origin of the formation may be
drawn.
The arrangement in definite layers proves that the formation is
sedimentary; that is, that its materials were accumulated in water
whither they had been washed from the land which then existed. The
ripple-marks show that the water in which the beds of sand were
deposited was shallow, for in such water only are ripple-marks made.[1]
Once developed on the surface of the sand they may be preserved by
burial under new deposits, just as ripple-marks on sandy shores are now
being buried and preserved.
[1] Ripple marks are often seen on the surface of wind-blown
sand, but the other features of this sandstone show that this
was not its mode of accumulation.
The conglomerate beds of the formation corroborate the conclusions to
which the composition and structure of the sandstone point. The
water-worn shapes of the pebbles and stones show that they were
accumulated in water, while their size shows that the water must have
been shallow, for stones of such sizes are handled only by water of such
slight depth that waves or strong currents are effective at the bottom.
Furthermore, the large bowlders show that the source of supply
(quartzite) must have been close at hand, and that therefore land
composed of this rock must have existed not far from the places where
the conglomerate is found.
The fossils likewise are the fossils of aquatic life. Not only this, but
they are the fossils of animals which lived in salt water. The presence
of salt water, that is, the sea, in this region when the sand of the
sandstone was accumulating, makes the wide extent of the formation
rational.
From the constitution and structure of the sandstone, it is therefore
inferred that it accumulated in shallow sea water, and that, in the
vicinity of Devil's lake, there were land masses (islands) of quartzite
which furnished the pebbles and bowlders found in the conglomerate beds
at the base of the formation.
This being the origin of the sandstone, it is clear that the layers
which now appear on opposite sides of valleys must once have been
continuous across the depressions; for the sand accumulated in shallow
water is never deposited so as to leave valleys between ridges. It is
deposited in beds which are continuous over considerable areas.
Within the area under consideration, limestone is much less widely
distributed than sandstone. Thin beds of it alternate with layers of
sandstone in the upper portion of the Potsdam formation, and more
massive beds lie above the sandstone on some of the higher elevations of
the plain about the quartzite ridge. This is especially true in the
southern and southwestern parts of the region shown on Plate II. The
limestone immediately overlying the sandstone is the _Lower Magnesian_
limestone.
The beds of limestone, like those of the sandstone beneath, are
horizontal or nearly so, and the upper formation lies conformably on the
lower. The limestone does not contain water-worn pebbles, and the
surfaces of its layers are rarely if ever ripple-marked; yet the
arrangement of the rock in distinct layers which carry fossils of marine
animals shows that the limestone, like the sandstone beneath, was laid
down in the sea. The bearing of this origin of the limestone on the
development of the present valleys is the same as that of the sandstone.
_Origin of the topography._--The topography of the plain surrounding the
quartzite ridges, especially that part lying west of Devil's lake, is
then an erosion topography, developed by running water. Its chief
characteristic is that every depression leads to a lower one, and that
the form of the elevations, hills or ridges, is determined by the
valleys. The valleys were made; the hills and ridges left. If the
material carried away by the streams could be returned, the valleys
would be filled to the level of the ridges which bound them. Were this
done, the restored surface would be essentially flat. It is the
sculpturing of such a plain, chiefly by running water, which has given
rise to the present topography.
In the development of this topography the more resistant limestone has
served as a capping, tending to preserve the hills and ridges. Thus many
of the hills, especially in the southwest portion of the area shown in
Plate II, are found to have caps of the Lower Magnesian formation. Such
hills usually have flat tops and steep or even precipitous slopes down
to the base of the capping limestone, while the sandstone below,
weathering more readily, gives the lower portions of the hills a gentler
slope.
The elevations of the hills and ridges above the axes of the valleys or,
in other words, the relief of the plain is, on the average, about 300
feet, only a few of the more prominent hills exceeding that figure.
The topography east of the line between Kilbourn City and Prairie du Sac
is not of the unmodified erosion type, as is made evident by marshes,
ponds and lakes. The departure from the erosion type is due to a mantle
of glacial drift which masks the topography of the bedded rock beneath.
Its nature, and the topographic modifications which it has produced, will
be more fully considered in a later part of this report.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. IV.
The Lower Narrows of the Baraboo from a point on the South range.]
II. THE QUARTZITE RIDGES.
_Topography._--The South or main quartzite range, about 23 miles in
length and one to four miles in width, rises 500 feet to 800 feet above
the surrounding sandstone plain. Its slopes are generally too steep for
cultivation, and are clothed for the most part with a heavy growth of
timber, the banks of forest being broken here and there by cultivated
fields, or by the purple grey of the rock escarpments too steep for
trees to gain a foothold. With the possible exception of the Blue mounds
southwest of Madison, this quartzite range is the most obtrusive
topographic feature of southern Wisconsin.
As approached from the south, one of the striking features of the range
is its nearly even crest. Extending for miles in an east-west direction,
its summit gives a sky-line of long and gentle curves, in which the
highest points are but little above the lowest. Viewed from the north,
the evenness of the crest is not less distinct, but from this side it is
seen to be interrupted by a notable break or notch at Devil's lake
(Plates V and XXXVII). The pass across the range makes a right-angled
turn in crossing the range, and for this reason is not seen from the
south.
The North or lesser quartzite range lying north of Baraboo is both
narrower and lower than the south range, and its crest is frequently
interrupted by notches or passes, some of which are wide. Near its
eastern end occurs the striking gap known as the _Lower narrows_ (Plate
IV) through which the Baraboo river escapes to the northward, flowing
thence to the Wisconsin. At this narrows the quartzite bluffs rise
abruptly 500 feet above the river. At a and b, Plate II, there are
similar though smaller breaks in the range, also occupied by streams.
The connection between the passes and streams is therefore close.
There are many small valleys in the sides of the quartzite ranges
(especially the South range) which do not extend back to their crests,
and therefore do not occasion passes across them. The narrow valleys at
a and b in Plate XXXVII, known as Parfrey's and Dorward's glens,
respectively, are singularly beautiful gorges, and merit mention as well
from the scenic as from the geologic point of view. Wider valleys, the
heads of which do not reach the crest, occur on the flanks of the main
range (as at d and e, Plate II) at many points. One such valley
occurs east of the north end of the lake (x, Plate XXXVII), another
west of the south end (y, Plate XXXVII), another on the north face of
the west bluff west of the north end of the lake and between the East
and West Sauk roads, and still others at greater distances from the lake
in both directions. It is manifest that if the valleys were extended
headward in the direction of their axes, they would interrupt the even
crest. Many of these valleys, unlike the glens mentioned above, are very
wide in proportion to their length. In some of these capacious valleys
there are beds of Potsdam sandstone, showing that the valleys existed
before the sand of the sandstone was deposited.
_The structure and constitution of the ridges._--The quartzite of the
ridges is nothing more nor less than altered sandstone. Its origin dates
from that part of geological time known to geologists as the Upper
Huronian period. The popular local belief that the quartzite
is of igneous origin is without the slightest warrant. It appears to
have had its basis in the notion that Devil's lake occupies an extinct
volcanic crater. Were this the fact, igneous rock should be found about
it.
Quartzite is sandstone in which the intergranular spaces have been
filled with silica (quartz) brought in and deposited by percolating
water subsequent to the accumulation of the sand. The conversion of
sandstone into quartzite is but a continuation of the process which
converts sand into sandstone. The Potsdam or any other sandstone
formation might be converted into quartzite by the same process, and it
would then be a _metamorphic_ rock.
Like the sandstone, the quartzite is in layers. This is perhaps nowhere
so distinctly shown on a large scale as in the bluffs at Devil's lake,
and at the east end of the Devil's nose. On the East bluff of the lake,
the stratification is most distinctly seen from the middle of the lake,
from which point the photograph reproduced in Plate VI was taken.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. V.
The Notch in the South quartzite range, at Devil's Lake.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. VI.
The east bluff of Devil's lake, showing the dip of quartzite (to the
left), and talus above and below the level where the beds are shown.]
Unlike the sandstone and limestone, the beds of quartzite are not
horizontal. The departure from horizontality, technically known as the
_dip_, varies from point to point (Fig. 4). In the East bluff of the
lake as shown in Plate VI, the dip is about 14 deg. to the north. At the
Upper and Lower narrows of the Baraboo (b and c, Plate II) the beds
are essentially vertical, that is, they have a dip of about 90 deg.. Between
these extremes, many intermediate angles have been noted. Plate VII
represents a view near Ablemans, in the Upper narrows, where the nearly
vertical beds of quartzite are well exposed.
The position of the beds in the quartzite is not always easy of
recognition. The difficulty is occasioned by the presence of numerous
cleavage planes developed in the rock after its conversion into
quartzite. Some of these secondary cleavage planes are so regular and so
nearly parallel to one another as to be easily confused with the bedding
planes. This is especially liable to make determinations of the dip
difficult, since the true bedding was often obscured when the cleavage
was developed.
In spite of the difficulties, the original stratification can usually be
determined where there are good exposures of the rock. At some points
the surfaces of the layers carry ripple marks, and where they are
present, they serve as a ready means of identifying the bedding planes,
even though the strata are now on edge. Layers of small pebbles are
sometimes found. They were horizontal when the sands of the quartzite
were accumulating, and where they are found they are sufficient to
indicate the original position of the beds.
Aside from the position of the beds, there is abundant evidence of
dynamic action[2] in the quartzite. Along the railway at Devil's lake,
half a mile south of the Cliff House, thin zones of schistose rock may
be seen parallel to the bedding planes. These zones of schistose rock a
few inches in thickness were developed from the quartzite by the
slipping of the rock on either side. This slipping presumably occurred
during the adjustment of the heavy beds of quartzite to their new
positions, at the time of tilting and folding, for no thick series of
rock can be folded without more or less slipping of the layers on one
another. The slipping (adjustment) takes place along the weaker zones.
Such zones of movement are sometimes known as _shear zones_, for the
rock on the one side has been sheared (slipped) over that on the other.
[2] Irving: "The Baraboo Quartzite Ranges." Vol. II, Geology
of Wisconsin, pp. 504-519. Van Hise: "Some Dynamic Phenomena
Shown by the Baraboo Quartzite Ranges of Central Wisconsin."
Jour. of Geol., Vol. I, pp. 347-355.
[Illustration: Fig. 4.--Diagram made by plotting the different dips now
at hand along a section from A to B, Plate II, and connecting
them so as to show the structure indicated by the known data. The full
lines, oblique or vertical, represent the beds of quartzite. The
continuous line above them represents the present surface of the
quartzite, while the dotted lines suggest the continuation of the beds
which completed the great folds of which the present exposures appear to
be remnants.]
[Illustration: Fig. 5.--A diagrammatic section showing the relation of
the sandstone to the quartzite.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. VII.
The East Bluff at the Upper Narrows of the Baraboo near Ablemans,
showing the vertical position of the beds of quartzite. In the lower
right-hand corner, above the bridge, appears some breccia.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. VIII.
Vertical shear zone in face of east bluff at Devil's lake.]
Near the shear zones parallel to the bedding planes, there is one
distinct vertical shear zone (Plate VIII) three to four feet in width.
It is exposed to a height of fully twenty-five feet. Along this zone the
quartzite has been broken into angular fragments, and at places the
crushing of the fragments has produced a "friction clay." Slipping along
vertical zones would be no necessary part of folding, though it might
accompany it. On the other hand, it might have preceded or followed the
folding.
Schistose structure probably does not always denote shearing, at least
not the shearing which results from folding. Extreme pressure is likely
to develop schistosity in rock, the cleavage planes being at right
angles to the direction of pressure. It is not always possible to say
how far the schistosity of rock at any given point is the result of
shear, and how far the result of pressure without shear.
Schistose structure which does not appear to have resulted from shear,
at least not from the shear involved in folding, is well seen in the
isolated quartzite mound about four miles southwest of Baraboo on the
West Sauk road (f, Plate II). These quartzite schists are to be looked
on as metamorphosed quartzite, just as quartzite is metamorphosed
sandstone.
At the Upper narrows of the Baraboo also (b, Plate II), evidence
of dynamic action is patent. Movement along bedding planes with
attendant development of quartz schist has occurred here as at the lake
(Plate IX). Besides the schistose belts, a wide zone of quartzite
exposed in the bluffs at this locality has been crushed into angular
fragments, and afterwards re-cemented by white quartz deposited from
solution by percolating waters (Plate X). This quartzite is said to be
brecciated. Within this zone there are spots where the fragments of
quartzite are so well rounded as to simulate water-worn pebbles. Their
forms appear to be the result of the wear of the fragments on one
another during the movements which followed the crushing. Conglomerate
originating in this way is _friction conglomerate_ or _Reibungsbreccia_.
The crushing of the rock in this zone probably took place while the beds
were being folded; but the brecciated quartzite formed by the
re-cementation of the fragments has itself been fractured and broken in
such a manner as to show that the formation has suffered at least one
dynamic movement since the development of the breccia. That these
movements were separated by a considerable interval of time is shown by
the fact that the re-cementation of the fragmental products of the first
movement preceded the second.
What has been said expresses the belief of geologists as to the origin
of quartzite and quartz schists; but because of popular misconception on
the point it may here be added that neither the changing of the
sandstone into quartzite, nor the subsequent transformation of the
quartzite to schist, was due primarily to heat. Heat was doubtless
generated in the mechanical action involved in these changes, but it was
subordinate in importance, as it was secondary in origin.
Igneous rock is associated with the quartzite at a few points. At g
and h, Plate II there are considerable masses of porphyry,
sustaining such relations to the quartzite as to indicate that they were
intruded into the sedimentary beds after the deposition of the latter.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. IX.
A mass of quartzite _in situ_, in the road through the Upper Narrows
near Ableman's. The bedding, which is nearly vertical, is indicated by
the shading, while the secondary cleavage approaches horizontality.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. X.
Brecciated quartzite near Ablemans in the Upper Narrows. The darker
parts are quartzite, the lighter parts the cementing quartz.]
III. RELATIONS OF THE SANDSTONE OF THE PLAIN TO THE QUARTZITE OF THE
RIDGES.
The horizontal beds of Potsdam sandstone may be traced up to the bases
of the quartzite ranges, where they may frequently be seen to abut
against the tilted beds of quartzite. Not only this, but isolated
patches of sandstone lie on the truncated edges of the dipping beds of
quartzite well up on the slopes, and even on the crest of the ridge
itself. In the former position they may be seen on the East bluff at
Devil's lake, where horizontal beds of conglomerate and sandstone rest
on the layers of quartzite which dip 14 deg. to the north.
The stratigraphic relations of the two formations are shown in Fig. 5
which represents a diagrammatic section from A to B, Plate II. Plate XI
is reproduced from a photograph taken in the Upper narrows of the
Baraboo near Ablemans, and shows the relations as they appear in the
field. The quartzite layers are here on edge, and on them rest the
horizontal beds of sandstone and conglomerate. Similar stratigraphic
relations are shown at many other places. This is the relationship of
_unconformity_.
Such an unconformity as that between the sandstone and the quartzite of
this region shows the following sequence of events: (1) the quartzite
beds were folded and lifted above the sea in which the sand composing
them was originally deposited; (2) a long period of erosion followed,
during which the crests of the folds were worn off; (3) the land then
sank, allowing the sea to again advance over the region; (4) while the
sea was here, sand and gravel derived from the adjacent lands which
remained unsubmerged, were deposited on its bottom. These sands became
the Potsdam sandstone.
This sequence of events means that between the deposition of the
quartzite and the sandstone, the older formation was disturbed and
eroded. Either of these events would have produced an unconformity; the
two make it more pronounced. That the disturbance of the older formation
took place before the later sandstone was deposited is evident from the
fact that the latter formation was not involved in the movements which
disturbed the former.
Although the sandstone appears in patches on the quartzite ranges, it is
primarily the formation of the surrounding plains, occupying the broad
valley between the ranges, and the territory surrounding them. The
quartzite, on the other hand, is the formation of the ridges, though it
outcrops at a few points in the plain. (Compare Plates II and XXXVII.)
The striking topographic contrasts between the plains and the ridges is
thus seen to be closely related to the rock formations involved. It is
the hard and resistant quartzite which forms the ridges, and the less
resistant sandstone which forms the lowlands about them.
That quartzite underlies the sandstone of the plain is indicated by the
occasional outcrops of the former rock on the plain, and from the fact
that borings for deep wells have sometimes reached it where it is not
exposed.
The sandstone of the plain and the quartzite of the ridges are not
everywhere exposed. A deep but variable covering of loose material or
_mantle rock (drift)_ is found throughout the eastern part of the area,
but it does not extend far west of Baraboo. This mantle rock is so thick
and so irregularly disposed that it has given origin to small hills and
ridges. These elevations are superimposed on the erosion topography of
the underlying rock, showing that the drift came into the region after
the sandstone, limestone, and quartzite had their present relations, and
essentially their present topography. Further consideration will be
given to the drift in a later part of this report.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XI.
The northeast wall of the Upper Narrows, north of Ableman's, showing the
horizontal Potsdam sandstone and conglomerate lying unconformably on the
quartzite, the beds of which are vertical.]
PART II.
HISTORY OF THE TOPOGRAPHY.
CHAPTER II.
OUTLINE OF THE HISTORY OF THE ROCK FORMATIONS WHICH SHOW THEMSELVES AT
THE SURFACE.
I. THE PRE-CAMBRIAN HISTORY OF THE QUARTZITE.
_From loose sand to quartzite._--To understand the geography of a region
it is necessary to understand the nature of the materials, the sculpture
of which has made the geography.
It has already been indicated that the Huronian quartzite of
which the most prominent elevations of this region are composed, was
once loose sand. Even at the risk of repetition, the steps in its
history are here recounted. The source of the sand was probably the
still older rocks of the land in the northern part of Wisconsin. Brought
down to the sea by rivers, or washed from the shores of the land by
waves, the sand was deposited in horizontal or nearly horizontal beds at
the bottom of the shallow water which then covered central and southern
Wisconsin. Later, perhaps while it was still beneath the sea, the sand
was converted into sandstone, the change being effected partly by
compression which made the mass of sand more compact, but chiefly by the
cementation of its constituent grains into a coherent mass. The water
contained in the sand while consolidation was in progress, held in
solution some slight amount of silica, the same material of which the
grains of sand themselves are composed. Little by little this silica in
solution was deposited on the surfaces of the sand grains, enlarging
them, and at the same time binding them together. Thus the sand became
sandstone. Continued deposition of silica between and around the grains
finally filled the interstitial spaces, and when this process was
completed, the sandstone had been converted into quartzite. While
quartzite is a metamorphic sandstone, it is not to be understood that
sandstone cannot be metamorphosed in other ways.
_Uplift and deformation. Dynamic metamorphism._--After the deposition of
the sands which later became the quartzite, the beds were uplifted and
deformed, as their present positions and relations show. It is
not possible to say how far the process of transformation of sand into
quartzite was carried while the formation was still beneath the shallow
sea in which it was deposited. The sand may have been changed to
sandstone, and the sandstone to quartzite, before the sea bottom was
converted into land, while on the other hand, the formation may have
been in any stage of change from sand to quartzite, when that event
occurred. If the process of change was then incomplete, it may have been
continued after the sea retired, by the percolating waters derived from
the rainfall of the region.
Either when first converted into land, or at some later time, the beds
of rock were folded, and suffered such other changes as attend profound
dynamic movements. The conversion of the sandstone into quartzite
probably preceded the deformation, since many phenomena indicate that
the rock was quartzite and not sandstone when the folding took place.
For example, the crushing of the quartzite (now re-cemented into
brecciated quartzite) at Ablemans probably dates from the orogenic
movements which folded the quartzite, and the fractured bits of rock
often have corners and edges so sharp as to show that the rock was
thoroughly quartzitic when the crushing took place.
The uplift and deformation of the beds was probably accomplished slowly,
but the vertical and highly tilted strata show that the changes were
profound (see Fig. 4).
The dynamic metamorphism which accompanied this profound deformation has
already been referred to. The folding of the beds involved the
slipping of some on others, and this resulted in the development of
quartz schist along the lines of severest movement. Changes effected in
the texture and structure of the rock under such conditions constitute
_dynamic metamorphism_. In general, the metamorphic changes effected
by dynamic action are much more profound than those brought about in
other ways, and most rocks which have been profoundly metamorphosed,
were changed in this way. Dynamic action generates heat, but contrary to
the popular notion, the heat involved in profound metamorphism is
usually secondary, and the dynamic action fundamental.
At the same time that quartz schist was locally developed from the
quartzite, crushing probably occurred in other places. This is
_demorphism_, rather than metamorphism.
_Erosion of the quartzite._--When the Huronian beds were raised to the
estate of land, the processes of erosion immediately began to work on
them. The heat and the cold, the plants and the animals, the winds, and
especially the rain and the water which came from the melting of the
snow, produced their appropriate effects. Under the influence of these
agencies the surface of the rock was loosened by weathering, valleys
were cut in it by running water, and wear and degradation went on at all
points.
The antagonistic processes of uplift and degradation went on for
unnumbered centuries, long enough for even the slow processes involved
to effect stupendous results. Degradation was continuous after the
region became land, though uplift may not have been. On the whole,
elevation exceeded degradation, for some parts of the quartzite finally
came to stand high above the level of the sea,--the level to which all
degradation tends.
Fig. 4 conveys some notion of the amount of rock which was
removed from the quartzite folds about Baraboo during this long period
of erosion. The south range would seem to represent the stub of one side
of a great anticlinal fold, a large part of which (represented by the
dotted lines) was carried away, while the north range may be the core of
another fold, now exposed by erosion.
Some idea of the geography of the quartzite at the close of this period
of erosion may be gained by imagining the work of later times undone.
The younger beds covering the quartzite of the plains have a thickness
varying from zero to several hundred feet, and effectually mask the
irregularities of the surface of the subjacent quartzite. Could they be
removed, the topography of the quartzite would be disclosed, and found
to have much greater relief than the present surface; that is, the
vertical distance between the crest of the quartzite ridge, and the
surface of the quartzite under the surrounding lowlands, would be
greater than that between the same crest and the surface of the
sandstone. But even this does not give the full measure of the relief of
the quartzite at the close of the long period of erosion which followed
its uplift, for allowance must be made for the amount of erosion which
the crests of the quartzite ranges have suffered since that time. The
present surface therefore does not give an adequate conception of the
irregularity of the surface at the close of the period of erosion which
followed the uplift and deformation of the quartzite. So high were the
crests of the quartzite ranges above their surroundings at that time,
that they may well be thought of as mountainous. From this point of
view, the quartzite ranges of today are the partially buried mountains
of the pre-Potsdam land of south central Wisconsin.
When the extreme hardness of the quartzite is remembered and also the
extent of the erosion which affected it (Fig. 4) before the next
succeeding formation was deposited, it is safe to conclude that the
period of erosion was very long.
_Thickness of the quartzite._--The thickness of the quartzite is not
known, even approximately. The great thickness in the south range
suggested by the diagram (Fig. 4) may perhaps be an exaggeration.
Faulting which has not been discovered may have occurred, causing
repetition of beds at the surface (Fig. 6), and so an exaggerated
appearance of thickness. After all allowances have been made, it is
still evident that the thickness of the quartzite is very great.
II. THE HISTORY OF THE PALEOZOIC STRATA.
_The subsidence._--Following the long period of erosion, the irregular
and almost mountainous area of central Wisconsin was depressed
sufficiently to submerge large areas which had been land. The subsidence
was probably slow, and as the sea advanced from the south, it covered
first the valleys and lowlands, and later the lower hills and ridges,
while the higher hills and ridges of the quartzite stood as islands in
the rising sea. Still later, the highest ridges of the region were
themselves probably submerged.
[Illustration: Fig. 6.--A diagrammatic cross-section, showing how, by
faulting, the apparent thickness of the quartzite would be increased.]
_The Potsdam sandstone (and conglomerate)._--So soon as the sea began to
overspread the region, its bottom became the site of deposition, and the
deposition continued as long as the submergence lasted. It is to the
sediments deposited during the earlier part of this submergence that the
name _Potsdam_ is given.
The sources of the sediments are not far to seek. As the former land was
depressed beneath the sea, its surface was doubtless covered with the
products of rock decay, consisting of earths, sands, small bits and
larger masses of quartzite. These materials, or at least the finer
parts, were handled by the waves of the shallow waters, for they were at
first shallow, and assorted and re-distributed. Thus the residuary
products on the submerged surface, were one source of sediments.
From the shores also, so long as land areas remained, the waves derived
sediments. These were composed in part of the weathered products of the
rock, and in part of the undecomposed rock against which the waves
beat, after the loose materials had been worn away. These sediments
derived from the shore were shifted, and finally mingled with those
derived from the submerged surface.
So long as any part of the older land remained above the water, its
streams brought sediments to the sea. These also were shifted by the
waves and shore currents, and finally deposited with the others on the
eroded surface of the quartzite. Thus sediments derived in various ways,
but inherently essentially similar, entered into the new formation.
[Illustration: Fig. 7.--Diagram to illustrate the theoretical
disposition of sediments about an island.]
[Illustration: Fig. 8.--Same as Fig. 7, except that the land has been
depressed.]
The first material to be deposited on the surface of the quartzite as it
was submerged, was the coarsest part of the sediment. Of the sediment
derived by the waves from the coasts, and brought down to the sea by
rivers, the coarsest would at each stage be left nearest the shore,
while the finer was carried progressively farther and farther from it.
Thus at each stage the sand was deposited farther from the shore than
the gravel, and the mud farther than the sand, where the water was so
deep that the bottom was subject to little agitation by waves. The
theoretical distribution of sediments about an island as it was
depressed, is illustrated by the following diagrams, Figs. 7 and 8. It
will be seen that the surface of the quartzite is immediately overlain
by conglomerate, but that the conglomerate near its top is younger than
that near its base.
In conformity with this natural distribution of sediments, the basal
beds of the Potsdam formation are often conglomeratic (Fig. 9, Plate
III, Fig. 2, and Plate XXV). This may oftenest be seen near
the quartzite ridges, for here only is the base of the formation
commonly exposed. The pebbles and larger masses of the conglomerate are
quartzite, like that of the subjacent beds, and demonstrate the source
of at least some of the material of the younger formation. That the
pebbles and bowlders are of quartzite is significant, for it shows that
the older formation had been changed from sandstone to quartzite, before
the deposition of the Potsdam sediments. The sand associated with the
pebbles may well have come from the breaking up of the quartzite, though
some of it may have been washed in from other sources by the waters in
which the deposition took place.
[Illustration: Fig. 9.--Sketch showing relation of basal Potsdam
conglomerate and sandstone to the quartzite, on the East bluff at
Devil's lake, behind the Cliff house.]
The basal conglomerate may be seen at many places, but nowhere about
Devil's lake is it so well exposed as at Parfrey's glen (a, Plate
XXXVII), where the rounded stones of which it is composed vary
from pebbles, the size of a pea, to bowlders more than three feet in
diameter. Other localities where the conglomerates may be seen to
advantage are Dorward's glen (b, Plate XXXVII), the East bluff at
Devil's lake just above the Cliff house, and at the Upper narrows of the
Baraboo, above Ablemans.
While the base of the Potsdam is conglomeratic in many places, the main
body of it is so generally sandstone that the formation as a whole is
commonly known as the Potsdam sandstone.
The first effect of the sedimentation which followed submergence was to
even up the irregular surface of the quartzite, for the depressions in
the surface were the first to be submerged, and the first to be filled.
As the body of sediment thickened, it buried the lower hills and the
lower parts of the higher ones. The extent to which the Potsdam
formation buried the main ridge may never be known. It may have buried
it completely, for as already stated patches of sandstone are
found upon the main range. These patches make it clear that some
formation younger than the quartzite once covered essentially all of the
higher ridge. Other evidence to be adduced later, confirms this
conclusion. It has, however, not been demonstrated that the high-level
patches of sandstone are Potsdam.
There is abundant evidence that the subsidence which let the Potsdam
seas in over the eroded surface of the Huronian quartzite was gradual.
One line of evidence is found in the cross-bedding of the sandstone
(Plate XII) especially well exhibited in the Dalles of the Wisconsin.
The beds of sandstone are essentially horizontal, but within the
horizontal beds there are often secondary layers which depart many
degrees from horizontality, the maximum being about 24 deg.. Plates XXVII
and XII give a better idea of the structure here referred to than
verbal description can.
The explanation of cross-bedding is to be found in the varying
conditions under which sand was deposited. Cross-bedding denotes shallow
water, where waves and shore currents were effective at the bottom where
deposition is in progress. For a time, beds were deposited off shore at
a certain angle, much as in the building of a delta (Fig. 10). Then by
subsidence of the bottom, other layers with like structure were
deposited over the first. By this sequence of events, the dip of the
secondary layers should be toward the open water, and in this region
their dip is generally to the south. At any stage of deposition the waves
engendered by storms were liable to erode the surface of the deposits
already made, and new layers, discordant with those below, were likely
to be laid down upon them. The subordinate layers of each deposit might
dip in any direction. If this process were repeated many times during
the submergence, the existing complexity would be explained.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XII.
Steamboat rock,--an island in the Dalles of the Wisconsin.]
[Illustration: Fig. 10.--A diagrammatic cross-section of a delta.]
The maximum known thickness of the Potsdam sandstone in Wisconsin is
about 1,000 feet, but its thickness in this region is much less. Where
not capped by some younger formation, its upper surface has suffered
extensive erosion, and the present thickness therefore falls short of
the original. The figures given above may not be too great for the
latter.
_The Lower Magnesian limestone._--The conditions of sedimentation
finally changed in the area under consideration. When the sand of the
sandstone was being deposited, adjacent lands were the source whence the
sediments were chiefly derived. The evidence that the region was sinking
while the sand was being deposited shows that the land masses which were
supplying the sand, were becoming progressively smaller. Ultimately the
sand ceased to be washed out to the region here described, either
because the water became too deep[3] or because the source of supply
was too distant. When these relations were brought about, the conditions
were favorable for the deposition of sediments which were to become
limestone. These sediments consisted chiefly of the shells of marine
life, together with an unknown amount of lime carbonate precipitated
from the waters of the sea. The limestone contains no coarse, and but
little fine material derived from the land, and the surfaces of its
layers are rarely if ever ripple-marked. The materials of which it is
made must therefore have been laid down in quiet waters which were
essentially free from land-derived sediments. The depth of the water in
which it was deposited was not, however, great, for the fossils are not
the remains of animals which lived in abysmal depths.
[3] A few hundred feet would suffice.
The deposition of limestone sediments following the deposition of the
Potsdam sands, does not necessarily mean that there was more or
different marine life while the younger formation was making, but only
that the shells, etc., which before had been mingled with the sand,
making fossiliferous sandstone, were now accumulated essentially free
from land-derived sediment, and therefore made limestone.
Like the sandstone beneath, the limestone formation has a wide
distribution outside the area here under discussion, showing that
conditions similar to those of central Wisconsin were widely distributed
at this time.
The beds of limestone are conformable on those of the sandstone, and the
conformable relations of the two formations indicate that the deposition
of the upper followed that of the lower, without interruption.
The thickness of the Lower Magnesian limestone varies from less than 100
to more than 200 feet, but in this region its thickness is nearer the
lesser figure than the larger. The limestone is now present only in the
eastern and southern parts of the area, though it originally covered the
whole area.
_The St. Peters sandstone._--Overlying the Lower Magnesian limestone at
a few points, are seen remnants of St. Peters sandstone. The
constitution of this formation shows that conditions of sedimentation
had again changed, so that sand was again deposited where the conditions
had been favorable to the deposition of limestone but a short time
before. This formation has been recognized at but two places (d and
e) within the area shown on Plate XXXVII, but the relations at these
two points are such as to lead to the conclusion that the formation may
once have covered the entire region. This sandstone formation is very
like the sandstone below. Its materials doubtless came from the lands
which then existed. The formation is relatively thin, ranging from
somewhat below to somewhat above 100 feet.
The change from the deposition of limestone sediments to sand may well
have resulted from the shoaling of the waters, which allowed the sand to
be carried farther from shore. Rise of the land may have accompanied the
shoaling of the waters, and the higher lands would have furnished more
and coarser sediments to the sea.
_Younger beds._--That formations younger than the St. Peters sandstone
once overlaid this part of Wisconsin is almost certain, though no
remnants of them now exist. Evidence which cannot be here detailed[4]
indicates that sedimentation about the quartzite ridges went on not only
until the irregularities of surface were evened up, but until even the
highest peaks of the quartzite were buried, and that formations as high
in the series as the Niagara limestone once overlay their crests. Before
this condition was reached, the quartzite ridges had of course ceased to
be islands, and at the same time had ceased to be a source of supply of
sediments. The aggregate thickness of the Paleozoic beds in the region,
as first deposited, was probably not less than 1,500 feet, and it may
have been much more. This thickness would have buried the crests of the
quartzite ridges under several hundred feet of sediment (see Fig. 11).
[4] Jour. of Geol., Vol. III (pp. 655-67).
[Illustration: Fig. 11.--The geological formations of southern Wisconsin
in the order of their occurrence. Not all of these are found about
Devil's lake.]
It is by no means certain that south central Wisconsin was continuously
submerged while this thick series of beds was being deposited. Indeed,
there is good reason to believe that there was at least one period of
emergence, followed, after a considerable lapse of time, by
re-submergence and renewed deposition, before the Paleozoic series of
the region was complete. These movements, however, had little effect on
the geography of the region.
Finally the long period of submergence, during which several changes in
sedimentation had taken place, came to an end, and the area under
discussion was again converted into land.
_Time involved._--Though it cannot be reduced to numerical terms, the
time involved in the deposition of these several formations of the
Paleozoic must have been very long. It is probably to be reckoned in
millions of years, rather than in denominations of a lower order.
_Climatic conditions._--Little is known concerning the climate of this
long period of sedimentation. Theoretical considerations have usually
been thought to lead to the conclusion that the climate during this part
of the earth's history was uniform, moist, and warm; but the conclusion
seems not to be so well founded as to command great confidence.
_The uplift._--After sedimentation had proceeded to some such extent as
indicated, the sea again retired from central Wisconsin. This may have
been because the sea bottom of this region rose, or because the sea
bottom in other places was depressed, thus drawing off the water. The
topography of this new land, like the topography of those portions of
the sea bottom which are similarly situated, must have been for the most
part level. Low swells and broad undulations may have existed, but no
considerable prominences, and no sudden change of slope. The surface was
probably so flat that it would have been regarded as a level surface had
it been seen.
The height to which the uplift carried the new land surface at the
outset must ever remain a matter of conjecture. Some estimate may be
made of the amount of uplift which the region has suffered since the
beginning of this uplift, but it is unknown how much took place at this
time, and how much in later periods of geological history.
The new land surface at once became the site of new activities. All
processes of land erosion at once attacked the new surface, in the
effort to carry its materials back to the sea. The sculpturing of this
plain, which, with some interruption, has continued to the present day,
has given the region the chief elements of its present topography. But
before considering the special history of erosion in this region, it may
be well to consider briefly the general principles and processes of land
degradation.
CHAPTER III.
GENERAL OUTLINE OF RAIN AND RIVER EROSION.
_Elements of erosion._--The general process of subaerial erosion is
divisible into the several sub-processes of weathering, transportation,
and corrasion.[5]
[5] There is an admirable exposition of this subject in
Gilbert's "Henry Mountains."
_Weathering_ is the term applied to all those processes which
disintegrate and disrupt exposed surfaces of rock. It is accomplished
chiefly by solution, changes in temperature, the wedge-work of ice and
roots, the borings of animals, and such chemical changes as surface
water and air effect. The products of weathering are transported by the
direct action of gravity, by glaciers, by winds, and by running water.
Of these the last is the most important.
_Corrasion_ is accomplished chiefly by the mechanical wear of streams,
aided by the hard fragments such as sand, gravel and bowlders, which
they carry. The solution effected by the waters of a stream may also be
regarded as a part of corrasion. Under ordinary circumstances solution
by streams is relatively unimportant, but where the rock is relatively
soluble, and where conditions are not favorable for abrasion, solution
may be more important than mechanical wear.
So soon as sea bottom is raised to the estate of land, it is attacked by
the several processes of degradation. The processes of weathering at
once begin to loosen the material of the surface if it be solid; winds
shift the finer particles about, and with the first shower
transportation by running water begins. Weathering prepares the material
for transportation and transportation leads to corrasion. Since the goal
of all material transported by running water is the sea, subaerial
erosion means degradation of the surface.
_Erosion without valleys._--In the work of degradation the valley
becomes the site of greatest activity, and in the following pages
especial attention is given to the development of valleys and to the
phases of topography to which their development leads.
If a new land surface were to come into existence, composed of materials
which were perfectly homogeneous, with slopes of absolute uniformity in
all directions, and if the rain, the winds and all other surface
agencies acted uniformly over the entire area, valleys would not be
developed. That portion of the rainfall which was not evaporated and did
not sink beneath the surface, would flow off the land in a sheet. The
wear which it would effect would be equal in all directions from the
center. If the angle of the slope were constant from center to shore, or
if it increased shoreward, the wear effected by this sheet of water
would be greatest at the shore, because here the sheet of flowing water
would be deepest and swiftest, and therefore most effective in
corrasion.
_The beginning of a valley._--But land masses as we know them do not
have equal and uniform slopes to the sea in all directions, nor is the
material over any considerable area perfectly homogeneous. Departure
from these conditions, even in the smallest degree, would lead to very
different results.
That the surface of newly emerged land masses would, as a rule, not be
rough, is evident from the fact that the bottom of the sea is usually
rather smooth. Much of it indeed is so nearly plane that if the water
were withdrawn, the eye would scarcely detect any departure from
planeness. The topography of a land mass newly exposed either by its own
elevation or by the withdrawal of the sea, would ordinarily be similar
to that which would exist in the vicinity of Necedah and east of Camp
Douglas, if the few lone hills were removed, and the very shallow
valleys filled. Though such a surface would seem to be moderately
uniform as to its slopes, and homogeneous as to its material, neither
the uniformity nor the homogeneity are perfect, and the rain water would
not run off in sheets, and the wear would not be equal at all points.
Let it be supposed that an area of shallow sea bottom is raised above
the sea, and that the elevation proceeds until the land has an altitude
of several hundred feet. So soon as it appears above the sea, the rain
falling upon it begins to modify its surface. Some of the water
evaporates at once, and has little effect on the surface; some of it
sinks beneath the surface and finds its way underground to the sea; and
some of it runs off over the surface and performs the work
characteristic of streams. So far as concerns modifications of the
surface, the run-off is the most important part.
The run-off of the surface would tend to gather in the depressions of
the surface, however slight they may be. This tendency is shown on
almost every hillside during and after a considerable shower. The water
concentrated in the depressions is in excess of that flowing over other
parts of the surface, and therefore flows faster. Flowing faster, it
erodes the surface over which it flows more rapidly, and as a result the
initial depressions are deepened, and _washes_ or _gullies_ are started.
Should the run-off not find irregularities of slope, it would, at the
outset, fail of concentration; but should it find the material more
easily eroded along certain lines than along others, the lines of easier
wear would become the sites of greater erosion. This would lead to the
development of gullies, that is, to irregularities of slope. Either
inequality of slope or material may therefore determine the location of
a gully, and one of these conditions is indispensable.
Once started, each wash or gully becomes the cause of its own growth,
for the gully developed by the water of one shower, determines greater
concentration of water during the next. Greater concentration means
faster flow, faster flow means more rapid wear, and this means
corresponding enlargement of the depression through which the flow takes
place. The enlargement effected by successive showers affects a gully in
all dimensions. The water coming in at its head carries the head back
into the land (head erosion), thus lengthening the gully; the water
coming in at its sides wears back the lateral slopes, thus widening it;
and the water flowing along its bottom deepens it. Thus gullies grow to
be ravines, and farther enlargement by the same processes converts
ravines into valleys. A river valley therefore is often but a gully
grown big.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XIII.
FIG. 1.
A very young valley.
Illustration: FIG. 2.
A valley in a later stage of development.
Illustration: FIG. 3.
Young valleys.]
_The course of a valley._--In the lengthening of a gully or valley
headward, the growth will be in the direction of greatest wear. Thus in
Plate XIII, Fig. 1, if the water coming in at the head of the gully
effects most wear in the direction a, the head of the gully will
advance in that direction; if there be most wear in the direction b or
c, the head will advance toward one of these points. The direction of
greatest wear will be determined either by the slope of the surface, or
by the nature of the surface material. The slope may lead to the
concentration of the entering waters along one line, and the surface
material may be less resistant in one direction than in another. If
these factors favor the same direction of head-growth, the lengthening
will be more rapid than if but one is favorable. If there be more rapid
growth along two lines, as b and c, Plate XIII, Fig. 1, than between
them, two gullies may develop (Plate XIII, Fig. 2). The frequent and
tortuous windings common to ravines and valleys are therefore to be
explained by the inequalities of slope or material which affected the
surface while the valley was developing.
_Tributary valleys._--Following out this simple conception of valley
growth, we have to inquire how a valley system (a main valley and its
tributaries) is developed. The conditions which determine the location
and development of gullies in a new land surface, determine the location
and development of tributary gullies. In flowing over the lateral slopes
of a gully or ravine, the water finds either slope or surface material
failing of uniformity. Both conditions lead to the concentration of the
water along certain lines, and concentration of flow on the slope of an
erosion depression, be it valley or gully, leads to the development of
a tributary depression. In its growth, the tributary repeats, in all
essential respects, the history of its main. It is lengthened headward
by water coming in at its upper end, is widened by side wash, and
deepened by the downward cutting of the water which flows along its
axis. The factors controlling its development are the same as those
which controlled the valley to which it is tributary.
There is one peculiarity of the courses of tributaries which deserves
mention. Tributaries, as a rule, join their mains with an acute angle up
stream. In general, new land surfaces, such as are now under
consideration, slope toward the sea. If a tributary gully were to start
back from its main at right angles, more water would come in on the side
away from the shore, on account of the seaward slope of the land. This
would be true of the head of the gully as well as of other portions, and
the effect would be to turn the head more and more toward parallelism
with the main valley. Local irregularities of surface may, and
frequently do, interfere with these normal relations, so that the
general course of a tributary is occasionally at right angles to its
main. Still more rarely does the general course of a tributary make an
acute angle with its main on the down stream side. Local irregularities
of surface determine the windings of a tributary, so that their courses
for longer or shorter distances may be in violation of the general rule
(c, Fig. 43); but on the whole, the valleys of a system whose
history has not been interrupted in a region where the surface material
is not notably heterogeneous, follow the course indicated above. This is
shown by nearly every drainage system on the Atlantic Coastal plain
which represents more nearly than any other portion of our continent,
the conditions here under consideration. Fig. 12 represents the drainage
system of the Mullica river in southern New Jersey and is a type of the
Coastal plain river system.
_How a valley gets a stream._--Valleys may become somewhat deep and long
and wide without possessing permanent streams, though from their
inception they have _temporary_ streams, the water for which is
furnished by showers or melting snow. Yet sooner or later, valleys come
to have permanent streams. How are they acquired? Does the valley find
the stream or the stream the valley? For the answer to these questions,
a brief digression will be helpful.
[Illustration: Fig. 12.--A typical river system of the Coastal plain
type.]
In cultivated regions, wells are of frequent occurrence. In a flat
region of uniform structure, the depth at which well water may be
obtained is essentially constant at all points. If holes (wells 1 and 2,
Fig. 13) be excavated below this level, water seeps into them, and in a
series of wells the water stands at a nearly common level. This means
that the sub-structure is full of water up to that level. These
relations are illustrated by Fig. 13. The diagram represents a vertical
section through a flat region from the surface (s s) down below the
bottom of wells. The water stands at the same level in the two cells (1
and 2), and the plane through them, at the surface of the water, is the
_ground water level_. If in such a surface a valley were to be cut until
its bottom was below the ground water level, the water would seep into
it, as it does into the wells; and if the amount were sufficient, a
permanent stream would be established. This is illustrated in Fig. 13.
The line A A represents the ground water level, and the level at which
the water stands in the wells, under ordinary circumstances. The bottom
of the valley is below the level of the ground water, and the water
seeps into it from either side. Its tendency is to fill the valley to
the level A A. But instead of accumulating in the open valley as it does
in the enclosed wells, it flows away, and the ground water level on
either hand is drawn down.
[Illustration: Fig. 13.--Diagram illustrating the relations of ground
water to streams.]
The level of the ground water fluctuates. It is depressed when the
season is dry (A' A'), and raised when precipitation is abundant (A''
A''). When it is raised, the water in the wells rises, and the stream in
the valley is swollen. When it falls, the ground water surface is
depressed, and the water in the wells becomes lower. If the water
surface sinks below the bottom of the wells, the wells "go dry;" if
below the bottom of the valley, the valley becomes for the time being, a
"dry run." When a well is below the lowest ground-water level its supply
of water never fails, and when the valley is sufficiently below the same
level, its stream does not cease to flow, even in periods of drought. On
account of the free evaporation in the open valley, the valley
depression must be somewhat below the level necessary for a well, in
order that the flow may be constant.
It will be seen that _intermittent_ streams, that is, streams which flow
in wet seasons and fail in dry, are intermediate between streams which
flow after showers only, and those which flow without interruption. In
the figure the stream would become dry if the ground water level sank to
A' A'.
It is to be noted that a permanent stream does not normally precede its
valley, but that the valley, developed through gully-hood and
ravine-hood to valley-hood by means of the temporary streams supplied by
the run-off of occasional showers, _finds a stream_, just as diggers of
wells find water. The case is not altered if the stream be fed by
springs, for the valley finds the spring, as truly as the well-digger
finds a "vein" of water.
_Limits of a valley._--So soon as a valley acquires a permanent stream,
its development goes on without the interruption to which it was subject
while the stream was intermittent. The permanent stream, like the
temporary one which preceded it, tends to deepen and widen its valley,
and, under certain conditions, to lengthen it as well. The means by
which these enlargements are affected are the same as before. There are
limits, however, in length, depth, and width, beyond which a valley may
not go. No stream can cut below the level of the water into which it
flows, and it can cut to that level only at its outlet. Up stream from
that point, a gentle gradient will be established over which the water
will flow without cutting. In this condition the stream is _at grade_.
Its channel has reached _baselevel_, that is, the level to which the
stream can wear its bed. This grade is, however, not necessarily
permanent, for what was baselevel for a small stream in an early stage
of its development, is not necessarily baselevel for the larger stream
which succeeds it at a later time.
Weathering, wash, and lateral corrasion of the stream continue to widen
the valley after it has reached baselevel. The bluffs of valleys are
thus forced to recede, and the valley is widened at the expense of the
upland. Two valleys widening on opposite sides of a divide, narrow the
divide between them, and may ultimately wear it out. When this is
accomplished, the two valleys become one. The limit to which a valley
may widen on either side is therefore its neighboring valley, and since,
after two valleys have become one by the elimination of the ridge
between them, there are still valleys on either hand, the final result
of the widening of all valleys must be to reduce all the area which
they drain to baselevel. As this process goes forward, the upper flat
into which the valleys were cut is being restricted in area, while the
lower flats developed by the streams in the valley bottoms are being
enlarged. Thus the lower flats grow at the expense of the higher.
There are also limits in length which a valley may not exceed. The head
of any valley may recede until some other valley is reached. The
recession may not stop even there, for if, on opposite sides of a
divide, erosion is unequal, as between 1A and 1B, Fig. 14, the divide
will be moved toward the side of less rapid erosion, and it will cease
to recede only when erosion on the two sides becomes equal (4A and
4B). In homogeneous material this will be when the slopes on the two
sides are equal.
[Illustration: Fig. 14.--Diagram showing the shifting of a divide. The
slopes 1A and 1B are unequal. The steeper slope is worn more rapidly and
the divide is shifted from 1 to 4, where the two slopes become equal and
the migration of the divide ceases.]
It should be noted that the lengthening of a valley headward is not
normally the work of the permanent stream, for the permanent stream
begins some distance below the head of the valley. At the head,
therefore, erosion goes on as at the beginning, even after a permanent
stream is acquired.
Under certain circumstances, the valley may be lengthened at its
debouchure. If the detritus carried by it is deposited at its mouth, or
if the sea bottom beyond that point rise, the land may be extended
seaward, and over this extension the stream will find its way. Thus at
their lower, as well as at their upper ends, both the stream and its
valley may be lengthened.
_A cycle of erosion._--If, along the borders of a new-born land mass, a
series of valleys were developed, essentially parallel to one another,
they would constitute depressions separated by elevations, representing
the original surface not yet notably affected by erosion (see Plate XIV,
Fig. 1). These inter-valley areas might at first be wide or narrow, but
in process of time they would necessarily become narrow, for, once, a
valley is started, all the water which enters it from either side helps
to wear back its slopes, and the wearing back of the slopes means the
widening of the valleys on the one hand and the narrowing of the
inter-valley ridges on the other. Not only would the water running over
the slopes of a valley wear back its walls, but many other processes
conspire to the same end. The wetting and drying, the freezing and the
thawing, the roots of plants and the borings of animals, all tend to
loosen the material on the slopes or walls of the valleys, and gravity
helps the loosened material to descend. Once in the valley bottom, the
running water is likely to carry it off, landing it finally in the sea.
Thus the growth of the valley is not the result of running water alone,
though this is the most important single factor in the process.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XIV.
FIG. 1.
The same valleys as shown in Plate XIII, Fig. 3, in a later stage of
development.
Illustration: FIG. 2.
Same valleys as shown in Fig. 1, in a still later stage of
development.]
Even if valleys developed no tributaries, they would, in the course of
time, widen to such an extent as to nearly obliterate the intervening
ridges. The surface, however, would not easily be reduced to perfect
flatness. For a long time at least there would remain something of slope
from the central axis of the former inter-stream ridge, toward the
streams on either hand; but if the process of erosion went on for a
sufficiently long period of time, the inter-stream ridge would be
brought very low, and the result would be an essentially flat surface
between the streams, much below the level of the old one.
The first valleys which started on the land surface (see Plate XIII,
Fig. 3) would be almost sure to develop numerous tributaries. Into
tributary valleys water would flow from their sides and from their
heads, and as a result they would widen and deepen and lengthen just as
their mains had done before them. By lengthening headward they would
work back from their mains some part, or even all of the way across the
divides separating the main valleys. By this process, the tributaries
cut the divides between the main streams into shorter cross-ridges. With
the development of tributary valleys there would be many lines of
drainage instead of two, working at the area between two main streams.
The result would be that the surface would be brought low much more
rapidly, for it is clear that many valleys within the area between the
main streams, widening at the same time, would diminish the aggregate
area of the upland much more rapidly than two alone could do.
The same thing is made clear in another way. It will be seen (Plate XIV,
Figs. 1 and 2) that the tributaries would presently dissect an area of
uniform surface, tending to cut it into a series of short ridges or
hills. In this way the amount of sloping surface is greatly increased,
and as a result, every shower would have much more effect in washing
loose materials down to lower levels, whence the streams could carry
them to the sea.
[Illustration: Fig. 15.--Cross-sections showing various stages of
erosion in one cycle.]
The successive stages in the process of lowering a surface are suggested
by Fig. 15, which represents a series of cross-sections of a land mass
in process of degradation. The uppermost section represents a level
surface crossed by young valleys. The next lower represents the same
surface at a later stage, when the valleys have grown larger, while the
third and succeeding sections represent still later stages in the
process of degradation. Plate XIII, Fig. 3, and Plate XIV, Figs. 1 and
2, represent in another way the successive stages of stream work in the
general process of degradation.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XV.
Diagram illustrating how a hard inclined layer of rock becomes a ridge
in the process of degradation.]
In this manner a series of rivers, operating for a sufficiently long
period of time, might reduce even a high land mass to a low level,
scarcely above the sea. The new level would be developed soonest near
the sea, and the areas farthest from it would be the last--other things
being equal--to be brought low. The time necessary for the development
of such a surface is known as a _cycle of erosion_, and the resulting
surface is a _base-level plain_, that is, a plain as near sea level as
river erosion can bring it. At a stage shortly preceding the base-level
stage the surface would be a _peneplain_. A peneplain, therefore, is a
surface which has been brought toward, but not to base-level. Land
surfaces are often spoken of as young or old in their erosion history
according to the stage of advancement which has been made toward
base-leveling. Thus the Colorado canyon, deep and impressive as it is,
is, in terms of erosion, a young valley, for the river has done but a
small part of the work which must be done in order to bring its basin to
baselevel.
_Effects of unequal hardness._--The process of erosion thus sketched
would ultimately bring the surface of the land down to base-level, and
in case the material of the land were homogeneous, the last points to be
reduced would be those most remote from the axes of the streams doing
the work of leveling. But if the material of the land were of unequal
hardness, those parts which were hardest would resist the action of
erosion most effectively. The areas of softer rock would be brought low,
and the outcrops of hard rock (Plate XV) would constitute ridges during
the later stages of an erosion cycle. If there were bodies of hard rock,
such as the Baraboo quartzite, surrounded by sandstone, such as the
Potsdam, the sandstone on either hand would be worn down much more
readily than the quartzite, and in the course of degradation the latter
would come to stand out prominently. The region in the vicinity of Devil's
lake is in that stage of erosion in which the quartzite ridges are
conspicuous (Plate XXXVII). The less resistant sandstone has been
removed from about them, and erosion has not advanced so far since the
isolation of the quartzite ridges as to greatly lower their crests. The
harder strata are at a level where surface water can still work
effectively, even though slowly, upon them, and in spite of their great
resistance they will ultimately be brought down to the common level. It
will be seen that, from the point of view of subaerial erosion, a
base-level plain is the only land surface which is in a condition of
approximate stability.
_Falls and rapids._--If in lowering its channel a stream crosses one
layer of rock much harder than the next underlying, the deepening will
go on more rapidly on the less resistant bed. Where the stream crosses
from the harder to the less hard, the gradient is likely to become
steep, and a rapids is formed. These conditions are suggested in Fig. 16
which represents the successive profiles (a b, a c, d e, f e, g e,
and h e) of a stream crossing from a harder to a softer formation. Below
the point a the stream is flowing over rock which is easily eroded, while
above that point its course is over a harder formation. Just below a
(profile a b) the gradient has become so steep that there are rapids.
Under these conditions, erosion is rapid just beyond the crossing of the
hard layer, and the gradient becomes higher and higher. When the steep
slope of the rapids approaches verticality, the rapids become a _fall_
(profile a c).
[Illustration: Fig. 16.--Diagram to illustrate the development of a
rapid and fall. The upper layer is harder than the strata below. The
successive profiles of the stream below the hard layer are represented
by the lines a b, a c, d e, f e, g e, and h e.]
As the water falls over the precipitous face and strikes upon the softer
rock below, part of it rebounds against the base of the vertical face
(Fig. 16). The result of wear at this point is the undermining of the
hard layer above, and sooner or later, portions of it will fall. This
will occasion the recession of the fall (profile d e and f e). As the
fall recedes, it grows less and less high. When the recession has
reached the point i, or, in other words, when the gradient of the stream
below the fall crosses the junction of the beds of unequal hardness, as
it ultimately must, effective undermining ceases, and the end of the
fall is at hand.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XVI.
Skillett Falls, in the Potsdam formation, three miles southwest of
Baraboo. The several small falls are occasioned by slight inequalities
in the hardness of the layers.]
When the effective undercutting ceases because the softer bed is no
longer accessible, the point of maximum wear is transferred to the top
of the hard bed just where the water begins to fall (g, Fig. 16). The
wear here is no greater than before, though it is greater relatively.
The relatively greater wear at this point destroys the verticality of
the face, converting it into a steep slope. When this happens, the fall
is a thing of the past, and rapids succeed. With continued flow the bed
of the rapids becomes less and less steep, until it is finally reduced
to the normal gradient of the stream (h e), when the rapids disappear.
When thin layers of rock in a stream's course vary in hardness, softer
beds alternating with harder ones, a series of falls such as shown in
Plate XVI, may result. As they work up stream, these falls will be
obliterated one by one. Thus it is seen that falls and rapids are not
permanent features of the landscape. They belong to the younger period
of a valley's history, rather than to the older. They are marks of
topographic youth.
_Narrows._--Where a stream crosses a hard layer or ridge of rock lying
between softer ones, the valley will not widen so rapidly in the hard
rock as above and below. If the hard beds be vertical, so that their
outcrop is not shifted as the degradation of the surface proceeds, a
notable constriction of the valley results. Such a constriction is a
_narrows_. The Upper and Lower narrows of the Baraboo (Plate IV)
are good examples of the effect of hard rock on the widening of a
valley.
_Erosion of folded strata._--The processes of river erosion would not be
essentially different in case the land mass upon which erosion operated
were made of tilted and folded strata. The folds would, at the outset,
determine the position of the drainage lines, for the main streams would
flow in the troughs (synclines) between the folds (anticlines). Once
developed, the streams would lower their beds, widen their valleys, and
lengthen their courses, and in the long process of time they would bring
the area drained nearly to sea-level, just as in the preceding case. It
was under such conditions that the general processes of subaerial
erosion operated in south central Wisconsin, after the uplift of the
quartzite and before the deposition of the Potsdam sandstone. It was
then that the principal features of the topography of the quartzite were
developed.
In regions of folded strata, certain beds are likely to be more
resistant than others. Where harder beds alternate with softer, the
former finally come to stand out as ridges, while the outcrops of the
latter mark the sites of the valleys. Such alternations of beds of
unequal resistance give rise to various peculiarities of drainage,
particularly in the courses of tributaries. These peculiarities find no
illustration in this region and are not here discussed.
_Base-level plains and peneplains._--It is important to notice that a
plane surface (base-level) developed by streams could only be developed
at elevations but slightly above the sea, that is, at levels at which
running water ceases to be an effective agent of erosion; for so long as
a stream is actively deepening its valley, its tendency is to roughen
the area which it drains, not to make it smooth. The Colorado river,
flowing through high land, makes a deep gorge. All the streams of the
western plateaus have deep valleys, and the manifest result of their
action is to roughen the surface; but given time enough, and the streams
will have cut their beds to low gradients. Then, though deepening of the
valleys will cease, widening will not, and inch by inch and shower by
shower the elevated lands between the valleys will be reduced in area,
and ultimately the whole will be brought down nearly to the level of the
stream beds. This is illustrated by Fig. 15.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XVII.
A group of mounds on the plain southwest from Camp Douglas. The
base-level surface is well shown, and above it rise the remnants of the
higher plain from which the lower was reduced.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XVIII.
Castle Rock near Camp Douglas. In this view the relation of the erosion
remnant to the extensive base-leveled surface is well shown.]
It is important to notice further that if the original surface on which
erosion began is level, there is no stage intermediate between the
beginning and the end of an erosion cycle, when the surface is again
level, or nearly so, though in the stage of a cycle next preceding the
last--the peneplain stage (fourth profile, Fig. 15)--the surface
approaches flatness. It is also important to notice that when streams
have cut a land surface down to the level at which they cease to erode,
that surface will still possess some slight slope, and that to seaward.
No definite degree of slope can be fixed upon as marking a base-level.
The angle of slope which would practically stop erosion in a region of
slight rainfall would be great enough to allow of erosion if the
precipitation were greater. All that can be said, therefore, is that the
angle of slope must be low. The Mississippi has a fall of less than a
foot per mile for some hundreds of miles above the gulf. A small stream
in a similar situation would have ceased to lower its channel before so
low a gradient was reached.
The nearest approach to a base-leveled region within the area here under
consideration is in the vicinity of Camp Douglas and Necedah (see Plate
I). This is indeed one of the best examples of a base-leveled plain
known. Here the broad plain, extending in some directions as far as the
eye can reach, is as low as it could be reduced by the streams which
developed it. The erosion cycle which produced the plain was, however,
not completed, for above the plain rise a few conspicuous hills (Plates
XVII and XVIII, and Fig. 17), and to the west of it lie the highlands
marking the level from which the low plain was reduced.
Where a region has been clearly base-leveled, isolated masses or ridges
of resistant rock may still stand out conspicuously above it. The
quartzite hill at Necedah is an example. Such hills are known as
_monadnocks_. This name was taken from Mount Monadnock which owes its
origin to the removal of the surrounding less resistant beds. The name
has now become generic. Many of the isolated hills on the peneplain east
of Camp Douglas are perhaps due to superior resistance, though the rock
of which they are composed belongs to the same formation as that which
has been removed.
[Illustration: Fig. 17.--Sketch, looking northwest from Camp Douglas.]
CHARACTERISTICS OF VALLEYS AT VARIOUS STAGES OF DEVELOPMENT.
In the early stages of its development a depression made by erosion has
steep lateral slopes, the exact character of which is determined by many
considerations. Its normal cross-section is usually described as
V-shaped (Fig. 18). In the early stages of its development, especially
if in unconsolidated material, the slopes are normally convex inward. If
cut in solid rock, the cross section may be the same, though many
variations are likely to appear, due especially to the structure of the
rock and to inequalities of hardness. If a stream be swift enough to
carry off not only all the detritus descending from its slopes, but to
abrade its bed effectively besides, a steep-sided gorge develops. If it
becomes deep, it is a canyon. For the development of a canyon, the
material of the walls must be such as is capable of standing at a high
angle. A canyon always indicates that the down-cutting of a stream keeps
well ahead of the widening.
[Illustration: Fig. 18.--Diagrammatic cross-section of a young valley.]
Of young valleys in loose material (drift) there are many examples in
the eastern portion of the area here described. Shallow canyons or
gorges in rock are also found. The gorge of Skillett creek at and above
the Pewit's nest about three miles southwest from Baraboo, the gorge of
Dell creek two miles south of Kilbourn City, and the Dalles of the
Wisconsin at Kilbourn City may serve as illustrations of this type of
valley.
[Illustration: Fig. 19.--Diagrammatic profile of a young valley.]
The profile of a valley at the stage of its development corresponding to
the above section is represented diagrammatically by the curve A B in
Fig. 19. The sketch (Pl. XIX, Fig. 1) represents a bird's-eye view of a
valley in the same stage of development.
[Illustration: Fig. 20.--Diagrammatic cross-section of a valley at a
stage corresponding with that shown in Plate XIX, Fig. 2.]
At a stage of development later than that represented by the V-shaped
cross-section, the corresponding section is U-shaped, as shown in Fig.
20. The same form is sketched in Plate XIX, Fig. 2. This represents a
stage of development where detritus descending the slopes is not all
carried away by the stream, and where the valley is being widened faster
than it is deepened. Its slopes are therefore becoming gentler. The
profile of the valley at this stage would be much the same as that in
the preceding, except that the gradient in the lower portion would be
lower.
Still later the cross section of the valley assumes the shape shown in
Fig. 21, and in perspective the form sketched in Plate XX, Fig. 1. This
transformation is effected partly by erosion, and partly by deposition
in the valley. When a stream has cut its valley as low as conditions
allow, it becomes sluggish. A sluggish stream is easily turned from side
to side, and, directed against its banks, it may undercut them, causing
them to recede at the point of undercutting. In its meanderings, it
undercuts at various points at various times, and the aggregate result
is the widening of the valley. By this process alone the stream would
develop a flat grade. At the same time all the drainage which comes in
at the sides tends to carry the walls of the valley farther from its
axis.
[Illustration: Fig. 21.--Diagrammatic cross-section of a valley at a
stage later than that shown in Fig. 20.]
A sluggish stream is also generally a depositing stream. Its deposits
tend to aggrade (build up) the flat which its meanderings develop. When
a valley bottom is built up, it becomes wider at the same time, for the
valley is, as a rule, wider at any given level than at any lower one.
Thus the U-shaped valley is finally converted into a valley with a flat
bottom, the flat being due in large part to erosion, and in smaller part
to deposition. Under exceptional circumstances the relative importance
of these two factors may be reversed.
It will be seen that the cross-section of a valley affords a clue to its
age. A valley without a flat is young, and increasing age is indicated
by increasing width. Valleys illustrating all stages of development are
to be found in the Devil's lake region. The valley of Honey creek
southwest of Devil's lake may be taken as an illustration of a valley at
an intermediate stage of development, while examples of old valleys are
found in the flat country about Camp Douglas and Necedah.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XIX.
FIG. 1.
Sketch of a valley at the stage of development corresponding to the
cross section shown in Fig. 18.
Illustration: FIG. 2.
Sketch of a valley at the stage of development corresponding to the
cross section shown in Fig. 20.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XX.
FIG. 1.
Sketch of a part of a valley at the stage of development corresponding
to the cross section shown in Fig 21.
Illustration: FIG. 2.
Sketch of a section of the Baraboo valley.]
_Transportation and Deposition._
Sediment is carried by streams in two ways: (1) by being rolled along
the bottom, and (2) by being held in suspension. Dissolved mineral
matter (which is not sediment) is also carried in the water. By means of
that rolled along the bottom and carried in suspension, especially the
former, the stream as already stated abrades its bed.
The transporting power of a stream of given size varies with its
velocity. Increase in the declivity or the volume of a stream increases
its velocity and therefore its transportive power. The transportation
effected by a stream is influenced (1) by its transporting power, and (2)
by the size and amount of material available for carriage. Fine material
is carried with a less expenditure of energy than an equal amount of
coarse. With the same expenditure of energy therefore a stream can carry
a greater amount of the former than of the latter.
Since the transportation effected by a stream is dependent on its
gradient, its size, and the size and amount of material available, it
follows that when these conditions change so as to decrease the carrying
power of the river, deposition will follow, if the stream was previously
fully loaded. In other words, a stream will deposit when it becomes
overloaded.
Overloading may come about in the following ways: (1) By decrease in
gradient, checking velocity and therefore carrying power; (2) by
decrease in amount of water, which may result from evaporation,
absorption, etc.; (3) by change in the shape of the channel, so that the
friction of flow is increased, and therefore the force available for
transportation lessened; (4) by lateral drainage bringing in more
sediment than the main stream can carry; (5) by change in the character
of the material to which the stream has access; for if it becomes finer,
the coarse material previously carried will be dropped, and the fine
taken; and (6) by the checking of velocity when a stream flows into a
body of standing water.
_Topographic forms resulting from stream deposition._--The topographic
forms resulting from stream deposition are various. At the bottoms of
steep slopes, temporary streams build _alluvial cones_ or _fans_. Along
its flood-plain portion, a stream deposits more or less sediment on its
flats. The part played by deposition in building a river flat has
already been alluded to. A depositing stream often wanders about in an
apparently aimless way across its flood plain. At the bends in its
course, cutting is often taking place on the outside of a curve while
deposition is going on in the inside. The valley of the Baraboo
illustrates this process of cutting and building. Fig. 2, Plate XX, is
based upon the features of the valley within the city of Baraboo.
Besides depositing on its flood-plain, a stream often deposits in its
channel. Any obstruction of a channel which checks the current of a
loaded stream occasions deposition. In this way "bars" are formed. Once
started, the bar increases in size, for it becomes an obstacle to flow,
and so the cause of its own growth. It may be built up nearly to the
surface of the stream, and in low water, it may become an island by the
depression of the surface water. In some parts of its course, as about
Merrimac, the Wisconsin river is marked by such islands at low water,
and by a much larger number of bars.
At their debouchures, streams give up their loads of sediment. Under
favorable conditions deltas are built, but delta-building has not
entered into the physical history of this region to any notable extent.
_Rejuvenation of Streams._
After the development of a base-level plain, its surface would suffer
little change (except that effected by underground water) so long as it
maintained its position. But if, after its development, a base-level
plain were elevated, the old surface in a new position would be subject
to a new series of changes identical in kind with those which had gone
before. The elevation would give the established streams greater fall,
and they would reassume the characteristics of youth. The greater fall
would accelerate their velocities; the increased velocities would entail
increased erosion; increased erosion would result in the deepening of
the valleys, and the deepening of the valleys would lead to the
roughening of the surface. But in the course of time, the _rejuvenated_
streams would have cut their valleys as low as the new altitude of the
land permitted, that is, to a new base-level. The process of deepening
would then stop, and the limit of vertical relief which the streams were
capable of developing, would be attained. But the valleys would not stop
widening when they stopped deepening, and as they widened, the
intervening divides would become narrower, and ultimately lower. In the
course of time they would be destroyed, giving rise to a new level
surface much below the old one, but developed in the same position which
the old one occupied when it originated; that is, a position but little
above sea level.
If at some intermediate stage in the development of a second base-level
plain, say at a time when the streams, rejuvenated by uplift, had
brought half the elevated surface down to a new base-level, another
uplift were to occur, the half completed cycle would be brought to an
end, and a new one begun. The streams would again be quickened, and as a
result they would promptly cut new and deeper channels in the bottoms of
the great valleys which had already been developed. The topography which
would result is suggested by the following diagram (Fig. 22) which
illustrates the cross-section which would be found after the following
sequence of events: (1) The development of a base-level, A A; (2)
uplift, rejuvenation of the streams, and a new cycle of erosion half
completed, the new base-level being at B B; (3) a second uplift,
bringing the second (incomplete) cycle of erosion to a close, and by
rejuvenating the streams, inaugurating the third cycle. As represented
in the diagram, the third cycle has not progressed far, being
represented only by the narrow valley C. The base-level is now 2-2, and
the valley represented in the diagram has not yet reached it.
[Illustration: Fig. 22.--Diagram (cross-section), illustrating the
topographic effect of rejuvenation by uplift.]
[Illustration: Fig. 23.--Normal profile of a valley bottom in a
non-mountainous region.]
The rejuvenation of a stream shows itself in another way. The normal
profile of a valley bottom in a non-mountainous region is a gentle
curve, concave upward with gradient increasing from debouchure to
source. Such a profile is shown in Fig. 23. Fig. 24, on the other hand,
is the profile of a rejuvenated stream. The valley once had a profile
similar to that shown in Fig. 23. Below B its former continuation is
marked by the dotted line B C. Since rejuvenation the stream has
deepened the lower part of its valley, and established there a profile
in harmony with the new conditions. The upper end of the new curve has
not yet reached beyond B.
[Illustration: Fig. 24.--Profile of a stream rejuvenated by uplift.]
_Underground Water._
In what has preceded, reference has been made only to the results
accomplished by the water which runs off over the surface. The water
which sinks beneath it is, however, of no small importance in reducing a
land surface. The enormous amount of mineral matter in solution in
spring water bears witness to the efficiency of the ground water in
dissolving rock, for since the water did not contain the mineral matter
when it entered the soil, it must have acquired it below the surface. By
this means alone, areas of more soluble rock are lowered below those of
less solubility. Furthermore, the water is still active as a solvent
agent after a surface has been reduced to so low a gradient that the
run-off ceases to erode mechanically.
CHAPTER IV.
EROSION AND THE DEVELOPMENT OF STRIKING SCENIC FEATURES.
The uplift following the period of Paleozoic deposition in south central
Wisconsin, inaugurated a period of erosion which, with some
interruptions, has continued to the present day. The processes of
weathering began as soon as the surface was exposed to the weather, and
corrasion by running water began with the first shower which fell upon
it. The sediment worn from the land was carried back to the sea, there
to be used in the building of still younger formations.
The rate of erosion of a land surface depends in large measure upon its
height. As a rule, it is eroded rapidly if high, and but slowly if low.
It is not known whether the lands of central Wisconsin rose to slight or
to great heights at the close of the period of Paleozoic sedimentation.
It is therefore not known whether the erosion was at the outset rapid or
slow. If the land of southern Wisconsin remained low for a time after
the uplift which brought the Paleozoic sedimentation to a close,
weathering would have exceeded transportation and corrasion. A large
proportion of the rainfall would have sunk beneath the surface, and
found its way to the sea by subterranean routes. Loosening of material
by alternate wetting and drying, expansion and contraction, freezing and
thawing, and by solution, might have gone on steadily, but so long as
the land was low, there would have been little run-off, and that little
would have flowed over a surface of gentle slopes, and transportation
would have been at a minimum. On the whole, the degradation of the land
under these conditions could not have advanced rapidly.
If, on the other hand, the land was raised promptly to a considerable
height, erosion would have been vigorous at the outset. The surface
waters would soon have developed valleys which the streams would have
widened, deepened and lengthened. Both transportation and corrasion
would have been active, and whatever material was prepared for
transportation by weathering, and brought into the valleys by side-wash,
would have been hurried on its way to the sea, and degradation would
have proceeded rapidly.
_Establishment of drainage._--Valleys were developed in this new land
surface according to the principles already set forth. Between the
valleys there were divides, which became higher as the valleys became
deeper, and narrower as the valleys widened. Ultimately the ridges were
lowered, and many of them finally eliminated in the manner already
outlined. The distance below the original surface and that at which the
first series of new flats were developed is conjectural, but it would
have depended on the height of the land. So far as can now be inferred,
the new base-plain toward which the streams cut may have been 400 or 500
feet below the crests of the quartzite ridges. It was at this level that
the oldest base-plain of which this immediate region shows evidence, was
developed.
Had the quartzite ranges not been completely buried by the Paleozoic
sediments, they would have appeared as ridges on the new land surface,
and would have had a marked influence on the development of the drainage
of the newly emerged surface. But as the ranges were probably completely
buried, the drainage lines were established regardless of the position
of the hard, but buried ridges. When in the process of degradation the
quartzite surfaces were reached, the streams encountered a formation far
more resistant than the surrounding sandstone and limestone. As the less
resistant strata were worn away, the old quartzite ridges, long buried,
again became prominent topographic features. In this condition they were
"resurrected mountains."
If, when erosion on the uplifted surface of Paleozoic rocks began, a
valley had been located directly over the buried quartzite ridge, and
along its course, it would have been deepened normally until its bottom
reached the crest of the hard formation. Then, instead of sinking its
valley vertically downward into the quartzite, the stream would have
shifted its channel down the slope of the range along the junction of
the softer and harder rock (Fig. 25). Such changes occasioned by the
nature and position of the rock concerned, are known as _adjustments_.
[Illustration: Fig. 25.--Diagram illustrating the hypothetical case of a
stream working down the slope of the quartzite range. The successive
sections of the valley are suggested by the lines ae, be, ce and de.]
Streams which crossed the quartzite ridges on the overlying strata might
have held their courses even after their valleys were lowered to the
level of the quartzite. Such streams would have developed narrows at the
crossing of the quartzite. In so far as there were passes in the
quartzite range before the deposition of the Paleozoic beds, they were
filled during the long period of sedimentation, to be again cleared out
during the subsequent period of erosion. The gap in the South range now
occupied by the lake was a narrows in a valley which existed, though
perhaps not to its present depth, before the Potsdam sandstone was
deposited. It was filled when the sediments of that formation were laid
down, to be again opened, and perhaps deepened, in the period of erosion
which followed the deposition of the Paleozoic series.
During the earliest period of erosion of which there is positive
evidence, after the uplift of the Paleozoic beds, the softer formations
about the quartzite were worn down to a level 400 or 500 feet below the
crests of the South quartzite range. At this lower level, an approximate
plain, a peneplain, was developed, the level of which is shown by
numerous hills, the summits of which now reach an elevation of from
1,000 to 1,100 feet above the sea. At the time of its development, this
peneplain was but little above sea level, for this is the only elevation
at which running water can develop such a plain. Above the general level
of this plain rose the quartzite ranges as elongate monadnocks, the
highest parts of which were fully 500 feet above the plain. A few other
points in the vicinity failed to be reduced to the level of the
peneplain. The 1,320 foot hill (d, Plate XXXVII), one and one-half miles
southeast of the Lower narrows, and Gibraltar Rock (e, same Plate), two
miles southeast of Merrimac, rose as prominences above it. It is
possible that these crests are remnants of a base-level plain older than
that referred to above. If while the quartzite remained much as now, the
valleys in the sandstone below 1,000 or 1,100 feet were filled, the
result would correspond in a general way to the surface which existed in
this region when the first distinctly recognizable cycle of erosion was
brought to a close. Above the undulating plain developed in the
sandstone and limestone, the main quartzite ridge would have risen as a
conspicuous ridge 400 to 500 feet.
This cycle had not been completed, that is, the work of base-leveling
had not been altogether accomplished, when the peneplain was elevated,
and the cycle, though still incomplete, brought to a close. By the
uplift, the streams were rejuvenated, and sunk their valleys into the
elevated peneplain. Thus a new cycle of erosion was begun, and the
uplifted peneplain was dissected by the quickened streams which sank
their valleys promptly into the slightly resistant sandstone. At their
new base-level, they ultimately developed new flats. This cycle of
erosion appears to have advanced no farther than to the development of
wide flats along the principal streams, such as the Wisconsin and the
Baraboo, and narrow ones along the subordinate water courses, when it
was interrupted. Along the main streams the new flats were at a level
which is now from 800 to 900 feet above the sea, and 700 to 800 feet
below the South quartzite range. It was at this time that the plains
about Camp Douglas and Necedah, already referred to, were developed.
During this second incomplete cycle, the quartzite ranges, resisting
erosion, came to stand up still more prominently than during the first.
The interruption of this cycle was caused by the advent of the glacial
period which disturbed the normal course of erosion. This period was
accompanied and followed by slight changes of level which also had their
influence on the streams. The consideration of the effects of glaciation
and of subsequent river erosion are postponed, but it may be stated that
within the area which was glaciated the post-glacial streams have been
largely occupied in removing the drift deposited by the ice from the
preglacial valleys, or in cutting new valleys in the drift. The streams
outside the area of glaciation were less seriously disturbed.
At levels other than those indicated, partial base-levels are suggested,
and although less well marked in this region, they might, in the study
of a broader area, bring out a much more complicated erosion history. As
already suggested, one cycle may have preceded that the remnants of
which now stand 1,000-1,100 feet above sea level, and another may have
intervened between this and that marked by the 800 to 900 foot level.
From the foregoing it is clear that the topography of the region is, on
the whole, an erosion topography, save for certain details in its
eastern portion. The valleys differ in form and in size, with their age,
and with the nature of the material in which they are cut; while the
hills and ridges differ with varying relations to the streams, and with
the nature of the material of which they are composed.
_Striking Scenic Features._
In a region so devoid of striking scenery as the central portion of the
Mississippi basin, topographic features which would be passed without
special notice in regions of greater relief, become the objects of
interest. But in south central Wisconsin there are various features
which would attract attention in any region where the scenery is not
mountainous.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXI.
Cleopatra's Needle. West Bluff of Devil's Lake.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXII.
Turk's Head. West Bluff of Devil's Lake.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXIII.
Devil's Doorway. East of Devil's Lake.]
On the bluffs at Devil's lake there are many minor features which are
sure to attract the attention of visitors. Such are "Cleopatra's Needle"
(Plate XXI), "Turk's Head" (Plate XXII), and the "Devil's Doorway"
(Plate XXIII).
These particular forms have resulted from the peculiar weathering of the
quartzite. The rock is affected by several systems of vertical or nearly
vertical joint planes (cracks), which divide the whole formation into a
series of vertical columns. There are also horizontal and oblique planes
of cleavage dividing the columns, so that the great quartzite pile may
be said to be made up of a series of blocks, which are generally in
contact with one another. The isolated pillars and columns which have
received special names have been left as they now stand by the falling
away of the blocks which once surrounded them. They themselves must soon
follow. The great talus slopes at the base of the bluffs, such as those
on the west side of the lake and on the East bluff near its southeast
corner, Plate XXIV, are silent witnesses of the extent to which this
process has already gone. The blocks of rock of which they are composed
have been loosened by freezing water, by the roots of trees, and by
expansion and contraction due to changing temperature, and have fallen
from their former positions to those they now occupy. Their descent,
effected by gravity directly, is, it will be noted, the first step in
their journey to the sea, the final resting place of all products of
land degradation.
_The Baraboo bluffs._--Nowhere in southern Wisconsin, or indeed in a
large area adjacent to it, are there elevations which so nearly approach
mountains as the ranges of quartzite in the vicinity of Baraboo and
Devil's lake. So much has already been said of their history that there
is need for little further description. Plate IV gives some idea of the
appearance of the ranges. The history of the ranges, already outlined,
involves the following stages: (1) The deposition of the sands in
Huronian time; (2) the change of the sand to sandstone and the sandstone
to quartzite; (3) the uplift and deformation of the beds; (4) igneous
intrusions, faulting, crushing, and shoaring, with the development of
schists accompanying the deformation; (5) a prolonged period of erosion
during which the folds of quartzite were largely worn away, though
considerable ridges, the Huronian mountains of early Cambrian times,
still remained high above their surroundings; (6) the submergence of the
region, finally involving even the crests of the ridges of quartzite;
(7) a protracted period of deposition during which the Potsdam sandstone
and several later Paleozoic formations were laid down about, and finally
over, the quartzite, burying the mountainous ridges; (8) the elevation
of the Paleozoic sea-bottom, converting it into land; (9) a long period
of erosion, during which the upper Paleozoic beds were removed, and the
quartzite re-discovered. Being much harder than the Paleozoic rocks, the
quartzite ridges again came to stand out as prominent ridges, as the
surrounding beds of relatively slight resistance were worn away. They
are "resurrected" mountains, though not with the full height which they
had in pre-Cambrian time, for they are still partially buried by younger
beds.
_The narrows in the quartzite._--There are four narrows or passes in the
quartzite ridges, all of which are rather striking features. One of them
is in the South range, one in the North range near its eastern end,
while the others are in an isolated area of quartzite at Ablemans which
is really a continuation of the North range. Two of these narrows are
occupied by the Baraboo river, one by Narrows creek, and the fourth by
Devil's lake.
From Ablemans to a point several miles east of Baraboo, the Baraboo
river flows through a capacious valley. Where it crosses the North
range, six miles or more north of east of Baraboo, the broad valley is
abruptly constricted to a narrow pass with precipitous sides, about 500
feet high (c, Plate XXXVII). This constriction is the Lower narrows,
conspicuous from many points on the South range, and from the plains to
the north. Beyond the quartzite, the valley again opens out into a broad
flat.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXIV.
Talus slope on the east bluff of Devil's lake.]
Seen from a distance, the narrows has the appearance of an abrupt notch
in the high ridge (Plate IV). Seen at closer range, the gap is
still more impressive. It is in striking contrast with the other narrows
in that there are no talus accumulations at the bases of the steep
slopes, and in that the slopes are relatively smooth and altogether free
from the curious details of sculpture seen in the other gaps where the
slopes are equally steep.
The Upper narrows of the Baraboo at Ablemans (b, Plate II) is in some
ways similar to the Lower, though less conspicuous because less deep.
Its slopes are more rugged, and piles of talus lie at their bases as at
Devil's lake. This narrows also differs from the Lower in that the
quartzite on one side is covered with Potsdam conglomerate, which
overlies the truncated edges of the vertical layers of quartzite with
unconformable contact. So clear an example of unconformity is not often
seen. Potsdam sandstone is also seen to rest against the quartzite on
either side of the narrows (Fig. 26), thus emphasizing the unconformity.
The beauty and interest of this narrows is enhanced by the quartzite
breccia ( which appears on its walls.
[Illustration: Fig. 26.--A generalized diagrammatic cross-section at the
Upper narrows, to show the relation of the sandstone to the quartzite.]
One and one-half miles west of Ablemans (a, Plate II) is the third
pass in the north quartzite ridge. This pass is narrower than the
others, and is occupied by Narrows creek. Its walls are nearly vertical
and possess the same rugged beauty as those at Ablemans. As at the Upper
narrows, the beds of quartzite here are essentially vertical. They are
indeed the continuation of the beds exposed at that place.
The fourth narrows is across the South range (i, Plate II). It is not
now occupied by a stream, though like the others it was cut by a stream,
which was afterwards shut out from it. Because of its depth, 600 feet,
and the ruggedness of its slopes, and because of its occupancy by the
lake, this pass is the center of interest for the whole region. So much
has already been said concerning it in other portions of this report
that further description is here omitted. The manner in which the pass
was robbed of its stream will be discussed later.
The history of these several narrows, up to the time of the glacial
period may now be summarized. Since remnants of Potsdam sandstone are
found in some of them, it is clear that they existed in pre-Cambrian
time,[6] and there is no reason to doubt that they are the work of the
streams of those ancient days, working as streams now work. Following
the pre-Cambrian period of erosion during which the notches were cut,
came the submergence of the region, and the gaps were filled with sand
and gravel, and finally the ridges themselves were buried. Uplift and a
second period of erosion followed, during which the quartzite ranges
were again exposed by the removal of the beds which overlay them, and
the narrows cleaned out and deepened, and again occupied by streams.
This condition of things lasted up to the time when the ice invaded the
region.
[6] It is not here asserted that these notches were as deep
as now, in pre-Cambrian time. It is, however, certain that
the quartzite was deeply eroded, previous to the deposition
of the Potsdam sandstone.
_Glens._--No enumeration of the special scenic features of this region
would be complete without mention of Parfrey's and Dorward's glens (a
and b, Plate XXXVII, and Plate XXV). Attention has already been
directed to them as illustrations of young valleys, and as places where
the Potsdam conglomerate is well shown, but they are attractive from the
scenic point of view. Their frequent mention in earlier parts of this
report makes further reference to them at this point unnecessary.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXV.
In Dorward's Glen. The basal conglomerate of the Potsdam formation is
shown at the lower right-hand corner, and is overlain by sandstone.
(Photograph furnished by Mr. Wilfred Dorward).]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXVI.
Natural bridge near Denzer.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXVII.
Navy Yard. Dalles of the Wisconsin.]
Pine Hollow (k, Plate II) is another attractive gorge on the south flank
of the greater quartzite range. The rock at this point is especially
well exposed. This gorge is beyond the drift-covered portion of the
range, and therefore dates from the pre-glacial time.
The Pewit's nest, about three miles southwest of Baraboo (m, Plate II),
is another point of interest. Above the "nest," Skillett creek flows
through a narrow and picturesque gorge in the Potsdam sandstone. The
origin of this gorge is explained elsewhere.
_Natural Bridge._--About two miles north and a little west of the
village of Denzer (Sec. 17, T. 10 N., R. 5 E.), is a small natural
bridge, which has resulted from the unequal weathering of the sandstone
(see Plate XXVI). The "bridge" is curious, rather than beautiful or
impressive.
_The Dalles of the Wisconsin._--The _dalles_ is the term applied to a
narrow canyon-like stretch of the Wisconsin valley seven miles in
length, near Kilbourn City (see frontispiece). The depth of the gorge is
from 50 to 100 feet. The part above the bridge at Kilbourn City is the
"Upper dalles;" that below, the "Lower dalles." Within this stretch of
the valley are perhaps the most picturesque features of the region.
The sides of the gorge are nearly vertical much of the way, and at many
points are so steep on both sides that landing would be impossible.
Between these sandstone walls flows the deep and swift Wisconsin river.
Such a rock gorge is in itself a thing of beauty, but in the dalles
there are many minor features which enhance the charm of the whole.
One of the features which deserves especial mention is the peculiar
crenate form of the walls at the banks of the river. This is perhaps
best seen in that part of the dalles known as the "Navy Yard." Plate
XXVII. The sandstone is affected by a series of vertical cracks or
joints. From weathering, the rock along these joints becomes softened,
and the running water wears the softened rock at the joint planes more
readily than other parts of its bank, and so develops a reentrant at
these points. Rain water descending to the river finds and follows the
joint planes, and thus widens the cracks. As a result of stream and rain
and weathering, deep reentrant angles are produced. The projections
between are rounded off so that the banks of the stream have assumed the
crenate form shown in Plate XXVIII, and Frontispiece.
When this process of weathering at the joints is carried sufficiently
far, columns of rock become isolated, and stand out on the river bluffs
as "chimneys" (Plate XXVIII). At a still later stage of development,
decay of the rock along the joint planes may leave a large mass of rock
completely isolated. "Steamboat rock" (Plate XII) and "Sugar
bowl" (Plate XXIX) are examples of islands thus formed.
The walls of sandstone weather in a peculiar manner at some points in
the Lower dalles, as shown on Plate XXX. The little ridges stand out
because they are harder and resist weathering better than the other
parts. This is due in part at least to the presence of iron in the more
resistant portions, cementing them more firmly. In the process of
segregation, cementing materials are often distributed unequally.
The effect of differences in hardness on erosion is also shown on a
larger scale and in other ways. Perhaps the most striking illustration
is _Stand rock_ (Plate XXXI), but most of the innumerable and
picturesque irregularities on the rock walls are to be accounted for by
such differences.
Minor valleys tributary to the Wisconsin, such as _Witch's gulch_ and
_Cold Water canyon_ deserve mention, both because of their beauty, and
because they illustrate a type of erosion at an early stage of valley
development. In character they are comparable to the larger gorge to
which they are tributary. In the downward cutting, which far exceeds the
side wear in these tributary canyons, the water has excavated large bowl
or jug-like forms. In Witch's gulch such forms are now being excavated.
They are developed just below falls, where the water carrying debris,
eddies, and the jugs or pot-holes are the result of the wear effected by
the eddies. The "Devil's jug" and many similar hollows are thus
explained.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXVIII.
Chimney Rock. Dalles of the Wisconsin. Cross-bedding well shown in
foreground near bottom.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXIX.
An Island in the Lower Dalles.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXX.
View in lower Dalles showing peculiar honeycomb weathering.]
_The mounds and castle rocks._--In the vicinity of Camp Douglas and over
a large area to the east, are still other striking topographic forms,
which owe their origin to different conditions, though they were
fashioned by the same forces. Here there are many "tower" or "castle"
rocks, which rise to heights varying from 75 to 190 feet above the
surrounding plain. They are remnants of beds which were once continuous
over the low lands above which the hills now rise. In Plates XVII and
XVIII the general character of these hills is shown. The rock of
which they are composed is Potsdam sandstone, the same formation which
underlies most of the area about Baraboo. The effect of the vertical
joints and of horizontal layers of unequal hardness is well shown.
Rains, winds, frosts, and roots are still working to compass the
destruction of these picturesque hills, and the talus of sand bordering
the "castle" is a reminder of the fate which awaits them. These hills
are the more conspicuous and the more instructive since the plain out of
which they rise is so flat. It is indeed one of the best examples of a
base-level plain to be found on the continent.
The crests of these hills reach an elevation of between 1,000 and 1,100
feet. They appear to correspond with the level of the first peneplain
recognized in the Devil's lake region. It was in the second cycle of
erosion, when their surroundings were brought down to the new
base-level, that these hills were left. West of Camp Douglas, there are
still higher elevations, which seem to match Gibraltar rock.
The Friendship "mounds" north of Kilbourn City, the castellated hills a
few miles northwest of the same place, and Petenwell peak on the banks
of the Wisconsin (Plate XXXII), are further examples of the same class
of hills. All are of Potsdam sandstone.
In addition to the "castle" rocks and base-level plain about Camp
Douglas, other features should be mentioned. No other portion of the
area touched upon in this report affords such fine examples of the
different types of erosion topography. In the base-level plain are found
"old-age" valleys, broad and shallow, with the stream meandering in a
wide flood-plain. Traveling up such a valley, the topography becomes
younger and younger, and the various stages mentioned earlier in the
text, and suggested in Plate XIX, Figs. 1 and 2, and Plate XX, Fig. 1,
are here illustrated.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXXI.
Stand Rock. Upper end of the Upper Dalles.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXXII.
Petenwell Peak.]
CHAPTER V.
THE GLACIAL PERIOD.
The eastern part of the area with which this report deals, is covered
with a mantle of drift which, as already pointed out, has greatly
modified the details of its topography. To the consideration of the
drift and its history attention is now turned.
_The drift._--The drift consists of a body of clay, sand, gravel and
bowlders, spread out as a cover of unequal thickness over the rock
formations beneath. These various classes of material may be confusedly
commingled, or they may be more or less distinctly separated from one
another. When commingled, all may be in approximately equal proportions,
or any one may predominate over any or all the others to any extent.
It was long since recognized that the materials of the drift did not
originate where they now lie, and that, in consequence, they sustain no
genetic relationship to the strata on which they rest. Long before the
drift received any special attention from geologists, it was well known
that it had been transported from some other locality to that where it
now occurs. The early conception was that it had been drifted into its
present position from some outside source by water. It was this
conception of its origin which gave it the name of _drift_. It is now
known that the drift was deposited by glacier ice and the waters which
arose from its melting, but the old name is still retained.
Clearly to understand the origin of the drift, and the method by which
it attained its present distribution, it may be well to consider some
elementary facts and principles concerning climate and its effects, even
at the risk of repeating what is already familiar.
_Snow fields and ice sheets._--The temperature and the snowfall of a
region may stand in such a relation to each other that the summer's heat
may barely suffice to melt the winter's snow. If under these
circumstances the annual temperature were to be reduced, or the fall of
snow increased, the summer's heat would fail to melt all the winter's
snow, and some portion of it would endure through the summer, and
through successive summers, constituting a perennial snow-field. Were
this process once inaugurated, the depth of the snow would increase from
year to year. The area of the snow-field would be extended at the same
time, since the snow-field would so far reduce the surrounding
temperature as to increase the proportion of the annual precipitation
which fell as snow. In the course of time, and under favorable
conditions, the area of the snow-field would attain great dimensions,
and the depth of the snow would become very great.
As in the case of existing snow fields the lower part of the snow mass
would eventually be converted into ice. Several factors would conspire
to this end. 1. The pressure of the overlying snow would tend to
compress the lower portion, and snow rendered sufficiently compact by
compression would be regarded as ice. 2. Water arising from the melting
of the surface snow by the sun's heat, would percolate through the
superficial layers of snow, and, freezing below, take the form of ice.
3. On standing, even without pressure or partial melting, snow appears
to undergo changes of crystallization which render it more compact. In
these and perhaps other ways, a snow-field becomes an ice-field, the
snow being restricted to its surface.
Eventually the increase in the depth of the snow and ice in a snow-field
will give rise to new phenomena. Let a snow and ice field be assumed in
which the depth of snow and ice is greatest at the center, with
diminution toward its edges. The field of snow, if resting on a level
base, would have some such cross-section as that represented in the
diagram, Fig. 27.
When the thickness of the ice has become considerable, it is evident
that the pressure upon its lower and marginal parts will be great. We
are wont to think of ice as a brittle solid. If in its place there were
some plastic substance which would yield to pressure, the weight of the
ice would cause the marginal parts to extend themselves in all
directions by a sort of flowing motion.
[Illustration: Fig. 27.--Diagrammatic cross-section of a field of ice
and snow (c) resting on a level base A-B.]
Under great pressure, many substances which otherwise appear to be
solid, exhibit the characteristics of plastic bodies. Among the
substances exhibiting this property, ice is perhaps best known. Brittle
and resistant as it seems, it may yet be molded into almost any
desirable form if subjected to sufficient pressure, steadily applied
through long intervals of time. The changes of form thus produced in ice
are brought about without visible fracture. Concerning the exact nature
of the movement, physicists are not agreed; but the result appears to be
essentially such as would be brought about if the ice were capable of
flowing, with extreme slowness, under great pressure continuously
applied.
In the assumed ice-field, there are the conditions for great pressure
and for its continuous application. If the ice be capable of moving as a
plastic body, the weight of the ice would induce gradual movement
outward from the center of the field, so that the area surrounding the
region where the snow accumulated would gradually be encroached upon by
the spreading of the ice. Observation shows that this is what takes
place in every snow-field of sufficient depth. Motion thus brought about
is glacier motion, and ice thus moving is glacier ice.
Once in motion, two factors would determine the limit to which the ice
would extend itself: (1) the rate at which it advances; and (2) the rate
at which the advancing edge is wasted. The rate of advance would depend
upon several conditions, one of which, in all cases, would be the
pressure of the ice which started and which perpetuates the motion. If
the pressure be increased the ice will advance more rapidly, and if it
advance more rapidly, it will advance farther before it is melted. Other
things remaining constant, therefore, increase of pressure will cause
the ice-sheet to extend itself farther from the center of motion.
Increase of snowfall will increase the pressure of the snow and ice
field by increasing its mass. If, therefore, the precipitation over a
given snow-field be increased for a period of years, the ice-sheet's
marginal motion will be accelerated, and its area enlarged. A decrease
of precipitation, taken in connection with unchanged wastage would
decrease the pressure of the ice and retard its movement. If, while the
rate of advance diminished, the rate of wastage remained constant, the
edge of the ice would recede, and the snow and ice field be contracted.
The rate at which the edge of the advancing ice is wasted depends
largely on the climate. If, while the rate of advance remains constant,
the climate becomes warmer, melting will be more rapid, and the ratio
between melting and advance will be increased. The edge of the ice will
therefore recede. The same result will follow, if, while temperature
remains constant, the atmosphere becomes drier, since this will increase
wastage by evaporation. Were the climate to become warmer and drier at
the same time, the rate of recession of the ice would be greater than if
but one of these changes occurred.
If, on the other hand, the temperature over and about the ice field be
lowered, melting will be diminished, and if the rate of movement be
constant, the edge of the ice will advance farther than under the
earlier conditions of temperature, since it has more time to advance
before it is melted. An increase in the humidity of the atmosphere,
while the temperature remains constant, will produce the same result,
since increased humidity of the atmosphere diminishes evaporation. A
decrease of temperature, decreasing the melting, and an increase of
humidity, decreasing the evaporation, would cause the ice to advance
farther than either change alone, since both changes decrease the
wastage. If, at the same time that conditions so change as to increase
the rate of movement of the ice, climatic conditions so change as to
reduce the rate of waste, the advance of the ice before it is melted
will be greater than where only one set of conditions is altered. If,
instead of favoring advance, the two series of conditions conspire to
cause the ice to recede, the recession will likewise be greater than
when but one set of conditions is favorable thereto.
Greenland affords an example of the conditions here described. A large
part of the half million or more square miles which this body of land is
estimated to contain, is covered by a vast sheet of snow and ice,
thousands of feet in thickness. In this field of snow and ice, there is
continuous though slow movement. The ice creeps slowly toward the
borders of the island, advancing until it reaches a position where the
climate is such as to waste (melt and evaporate) it as rapidly as it
advances.
The edge of the ice does not remain fixed in position. There is reason
to believe that it alternately advances and retreats as the ratio
between movement and waste increases or decreases. These oscillations in
position are doubtless connected with climatic changes. When the ice
edge retreats, it may be because the waste is increased, or because the
snowfall is decreased, or both. In any case, when the ice edge recedes
from the coast, it tends to recede until its edge reaches a position
where the melting is less rapid than in its former position, and where
the advance is counterbalanced by the waste. This represents a condition
of equilibrium so far as the edge of the ice is concerned, and here the
edge of the ice would remain so long as the conditions were unchanged.
When for a period of years the rate of melting of the ice is diminished,
or the snowfall increased, or both, the ice edge advances to a new line
where melting is more rapid than at its former edge. The edge of the ice
would tend to reach a position where waste and advance balance. Here its
advance would cease, and here its edge would remain so long as climatic
conditions were unchanged.
If the conditions determining melting and flowage be continually
changing, the ice edge will not find a position of equilibrium, but will
advance when the conditions are favorable for advance, and retreat when
the conditions are reversed.
Not only the edge of the ice in Greenland, but the ends of existing
mountain glaciers as well, are subject to fluctuation, and are delicate
indices of variations in the climate of the regions where they occur.
_The North American ice sheet._--In an area north of the eastern part of
the United States and in another west of Hudson Bay it is believed that
ice sheets similar to that which now covers Greenland began to
accumulate at the beginning of the glacial period. From these areas as
centers, the ice spread in all directions, partly as the result of
accumulation, and partly as the result of movement induced by the weight
of the ice itself.
The ice sheets spreading from these centers came together south of
Hudson's bay, and invaded the territory of the United States as a single
sheet, which, at the time of its greatest development, covered a large
part of our country (Plate XXXIII), its area being known by the extent
of the drift which it left behind when it was melted. In the east, it
buried the whole of New England, most of New York, and the northern
parts of New Jersey and Pennsylvania. Farther west, the southern margin
of the ice crossed the Ohio river in the vicinity of Cincinnati, and
pushed out over the uplands a few miles south of the river. In Indiana,
except at the extreme east, its margin fell considerably short of the
Ohio; in Illinois it reached well toward that river, attaining here its
most southerly latitude. West of the Mississippi, the line which marks
the limit of its advance curves to the northward, and follows, in a
general way, the course of the Missouri river. The total area of the
North American ice sheet, at the time of its maximum development, has
been estimated to have been about 4,000,000 square miles, or about ten
times the estimated area of the present ice-field of Greenland.
Within the general area covered by the ice, there is an area of several
thousand square miles, mainly in southwestern Wisconsin, where there is
no drift. The ice, for some reason, failed to cover this _driftless
area_ though it overwhelmed the territory on all sides.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXXIII.
The North American Ice Sheet, at the time of maximum development.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXXIV.
View from the north of the Owl's Head, a hill two miles north of east of
Merrimac, which has been shaped by the ice. The side to the left is the
stone side.]
Plate II shows the limit of ice advance in the area here described. The
region may have been affected by the ice of more than one glacial epoch,
but the chief results now observable were effected during the last, and
the others need not be considered.
_The Work of Glacier Ice._
As the edge of an ice sheet, or as the end of a glacier, retreats, the
land which it has previously covered is laid bare, and the effects which
the passage of the ice produced may be seen. In some cases one may
actually go back a short distance beneath the ice now in motion, and see
its mode of work and the results it is effecting. The beds of living
glaciers, and the beds which glaciers have recently abandoned, are found
to present identical features. Because of their greater accessibility,
the latter offer the better facilities for determining the effects of
glaciation.
The conspicuous phenomena of abandoned glacier beds fall into two
classes, (1) those which pertain to the bed rock over which the ice
moved, and (2) those which pertain to the drift left by the ice.
_Erosive work of the ice._--_Effect on topography._--The leading
features of the rock bed over which glacier ice has moved, are easily
recognized. Its surface is generally smoothed and polished, and
frequently marked by lines (striae) or grooves, parallel to one another.
An examination of the bottom of an active glacier discloses the method
by which the polishing and scoring are accomplished.
The lower surface of the ice is thickly set with a quantity of clay,
sand, and stony material of various grades of coarseness. These earthy
and stony materials in the base of the ice are the tools with which it
works. Thus armed, the glacier ice moves slowly forward, resting down
upon the surfaces over which it passes with the whole weight of its
mass, and the grinding action between the stony layer at the base of the
ice and the rock bed over which it moves, is effective. If the material
in the bottom of the ice be fine, like clay, the rock bed is polished.
If coarser materials, harder than the bed-rock, be mingled with the
fine, the rock bed of the glacier will be scratched as well as polished.
If there are bowlders in the bottom of the ice they may cut grooves or
gorges in the underlying rock. The grooves may subsequently be polished
by the passage over and through them of ice carrying clay or other fine,
earthy matter.
All these phases of rock wear may be seen about the termini of receding
glaciers, on territory which they have but recently abandoned. There can
thus be no possible doubt as to the origin of the polishing, planing and
scoring.
There are other peculiarities, less easily defined, which characterize
the surface of glacier beds. The wear effected is not confined to the
mere marking of the surface over which it passes. If prominences of rock
exist in its path, as is often the case, they oppose the movement of the
ice, and receive a corresponding measure of abrasion from it. If they be
sufficiently resistant they may force the ice to yield by passing over
or around them; but if they be weak, they are likely to be destroyed.
As the ice of the North American ice sheet advanced, seemingly more
rigid when it encountered yielding bodies, and more yielding when it
encountered resistant ones, it denuded the surface of its loose and
movable materials, and carried them forward. This accumulation of earthy
and stony debris in the bottom of the ice, gave it a rough and grinding
lower surface, which enabled it to abrade the land over which it passed
much more effectively than ice alone could have done. Every hill and
every mound which the ice encountered contested its advance. Every
sufficiently resistant elevation compelled the ice to pass around or
over it; but even in these cases the ice left its marks upon the surface
to which it yielded. The powerful pressure of pure ice, which is
relatively soft, upon firm hills of rock, which are relatively hard,
would effect little. The hills would wear the ice, but the effect of the
ice on the hills would be slight. But where the ice is supplied with
earthy and stony material derived from the rock itself, the case is
different. Under these conditions, the ice, yielding only under great
pressure and as little as may be, rubs its rock-shod base over every
opposing surface, and with greatest severity where it meets with
greatest resistance. Its action may be compared to that of a huge
"flexible-rasp" fitting down snugly over hills and valleys alike, and
working under enormous pressure.
The abrasion effected by a moving body of ice under such conditions
would be great. Every inch of ice advance would be likely to be attended
by loss to the surface of any obstacle over or around which it is
compelled to move. The sharp summits of the hills, and all the angular
rugosities of their surfaces would be filed off, and the hills smoothed
down to such forms as will offer progressively less and less resistance.
If the process of abrasion be continued long enough, the forms, even of
the large hills, may be greatly altered, and their dimensions greatly
reduced. Among the results of ice wear, therefore, will be a lowering of
the hills, and a smoothing and softening of their contours, while their
surfaces will bear the marks of the tools which fashioned them, and will
be polished, striated or grooved, according to the nature of the
material which the ice pressed down upon them during its passage. Figs.
28 and 29 show the topographic effects which ice is likely to produce by
erosion. Plate XXXIV is a hill two miles northeast of Merrimac, which
shows how perfectly the wear actually performed corresponds to that
which might be inferred.
[Illustration: Fig. 28.--A hill before the ice passes over it.]
A rock hill was sometimes left without covering of drift after having
been severely worn by the ice. Such a hill is known as a _roche
moutonnee_. An example of this type of hill occurs three miles north of
east of Baraboo at the point marked z on Plate XXXVII. This hill,
composed of quartzite, is less symmetrical than those shown in Figs. 28
and 29. Its whole surface, not its stoss side only, has been smoothed
and polished by the ice. This hill is the most accessible, the most
easily designated, and, on the whole, the best example of a _roche
moutonnee_ in the region, though many other hills show something of the
same form.
[Illustration: Fig. 29.--The same hill after it has been eroded by the
ice. A the stoss side. B the lee side.]
It was not the hills alone which the moving ice affected. Where it
encountered valleys in its course they likewise suffered modification.
Where the course of a valley was parallel to the direction of the ice
movement, the ice moved through it. The depth of moving ice is one of
the determinants of its velocity, and because of the greater depth of
ice in valleys, its motion here was more rapid than on the uplands
above, and its abrading action more powerful. Under these conditions the
valleys were deepened and widened.
Where the courses of the valleys were transverse to the direction of ice
movement, the case was different. The ice was too viscous to span the
valleys, and therefore filled them. In this case it is evident that the
greater depth of the ice in the valley will not accelerate its motion,
since the ice in the valley-trough and that above it are in a measure
opposed. If left to itself, the ice in the valley would tend to flow in
the direction of the axis of the valley. But in the case under
consideration, the ice which lies above the valley depression is in
motion at right angles to the axis of the valley. Under these
circumstances three cases might arise:
(1) If the movement of the ice sheet over the valley were able to push
the valley ice up the farther slope, and out on the opposite highland,
this work would retard the movement of the upper ice, since the
resistance to movement would be great. In this case, the thickness of
the ice is not directly and simply a determinant of its velocity. Under
these conditions the bottom of the valley would not suffer great
erosion, since ice did not move along it; but that slope of the valley
against which the ice movement was projected would suffer great wear
(Fig. 30). The valley would therefore be widened, and the slope
suffering greatest wear would be reduced to a lower angle. Shallow
valleys, and those possessing gentle slopes, favor this phase of ice
movement and valley wear.
[Illustration: Fig. 30.--Diagram showing effect on valley of ice moving
transversely across it.]
(2) The ice in the valley might become stationary, in which case it
might serve as a bridge for the upper ice to cross on (Fig. 31). In this
case also the total thickness of ice will not be a determinant of its
velocity, for it is the thickness of the moving ice only, which
influences the velocity. In this case the valley would not suffer much
wear, so long as this condition of things continued. Valleys which have
great depth relative to the thickness of the ice, and valleys whose
slopes are steep, favor this phase of movement.
(3) In valleys whose courses are transverse to the direction of ice
movement, transverse currents of ice may exist, following the direction
of the valleys. If the thickness of the ice be much greater than the
depth of the valley, if the valley be capacious, and if one end of it be
open and much lower than the other, the ice filling it may move along
its axis, while the upper ice continues in its original course at right
angles to the valley. In this case the valley would be deepened and
widened, but this effect would be due to the movement along its course,
rather than to that transverse to it.
[Illustration: Fig. 31--Diagram to illustrate case where ice fills a
valley (C) and the upper ice then moves on over the filling.]
If the course of a valley were oblique to the direction of ice movement,
its effect on the movement of ice would be intermediate between that of
valleys parallel to the direction of movement, and those at right angles
to it.
It follows from the foregoing that the corrasive effects of ice upon the
surface over which it passed, were locally dependent on pre-existent
topography, and its relation to the direction of ice movement. In
general, the effort was to cut down prominences, thus tending to level
the surface. But when it encountered valleys parallel to its movement
they were deepened, thus locally increasing relief. Whether the
reduction of the hills exceeded the deepening of the valleys, or whether
the reverse was true, so far as corrasion alone is concerned, is
uncertain. But whatever the effect of the erosive effect of ice action
upon the total amount of relief, the effect upon the contours was to
make them more gentle. Not only were the sharp hills rounded off, but
even the valleys which were deepened were widened as well, and in the
process their slopes became more gentle. A river-erosion topography,
modified by the wearing (not the depositing) action of the ice, would be
notably different from the original, by reason of its gentler slopes and
softer contours (Figs. 28 and 29).
_Deposition by the ice. Effect on topography._--On melting, glacier ice
leaves its bed covered with the debris which it gathered during its
movement. Had this debris been equally distributed on and in and beneath
the ice during its movement, and had the conditions of deposition been
everywhere the same, the drift would constitute a mantle of uniform
thickness over the underlying rock. Such a mantle of drift would not
greatly alter the topography; it would simply raise the surface by an
amount equal to the thickness of the drift, leaving elevations and
depressions of the same magnitude as before, and sustaining the same
relations to one another. But the drift carried by the ice, in whatever
position, was not equally distributed during transportation, and the
conditions under which it was deposited were not uniform, so that it
produced more or less notable changes in the topography of the surface
on which it was deposited.
The unequal distribution of the drift is readily understood. The larger
part of the drift transported by the ice was carried in its basal
portion; but since the surface over which the ice passed was variable,
it yielded a variable amount of debris to the ice. Where it was hilly,
the friction between it and the ice was greater than where it was plain,
and the ice carried away more load. From areas where the surface was
overspread by a great depth of loose material favorably disposed for
removal, more debris was taken than from areas where material in a
condition to be readily transported was meager. Because of the
topographic diversity and lithological heterogeneity of the surface of
the country over which it passed, some portions of the ice carried much
more drift than others, and when the ice finally melted, greater depths
of drift were left in some places than in others. Not all of the
material transported by the ice was carried forward until the ice
melted. Some of it was probably carried but a short distance from its
original position before it lodged. Drift was thus accumulating at some
points beneath the ice during its onward motion. At such points the
surface was being built up; at other points, abrasion was taking place,
and the surface was being cut down. The drift mantle of any region does
not, therefore, represent simply the material which was on and in and
beneath the ice of that place at the time of its melting, but it
represents, in addition, all that lodged beneath the ice during its
movement.
The constant tendency was for the ice to carry a considerable part of
its load forward toward its thinned edge, and there to leave it. It
follows that if the edge of the ice remained constant in position for
any considerable period of time, large quantities of drift would have
accumulated under its marginal portion, giving rise to a belt of
relatively thick drift. Other things being equal, the longer the time
during which the position of the edge was stationary, the greater the
accumulation of drift. Certain ridge-like belts where the drift is
thicker than on either hand, are confidently believed to mark the
position where the edge of the ice-sheet stood for considerable periods
of time.
Because of the unequal amounts of material carried by different parts of
the ice, and because of the unequal and inconstant conditions of
deposition under the body of the ice and its edge, the mantle of drift
has a very variable thickness; and a mantle of drift of variable
thickness cannot fail to modify the topography of the region it covers.
The extent of the modification will depend on the extent of the
variation. This amounts in the aggregate, to hundreds of feet. The
continental ice sheet, therefore, modified the topography of the region
it covered, not only by the wear it effected, but also by the deposits
it made.
In some places it chanced that the greater thicknesses of drift were
left in the positions formerly marked by valleys. Locally the body of
drift was so great that valleys were completely filled, and therefore
completely obliterated as surface features. Less frequently, drift not
only filled the valleys but rose even higher over their former positions
than on either side. In other places the greater depths of drift,
instead of being deposited in the valleys, were left on pre-glacial
elevations, building them up to still greater heights. In short, the
mantle of drift of unequal thickness was laid down upon the rock
surface in such a manner that the thicker parts sometimes rest on hills
and ridges, sometimes on slopes, sometimes on plains, and sometimes in
valleys.
[Illustration: Fig. 32.--Diagrammatic section showing relation of drift
to underlying rock, where the drift is thick relative to the relief of
the rock. a and b represent the location of post-glacial valleys.]
These relations are suggested by Figs. 32 and 33. From them it will be
seen that in regions where the thickness of the drift is great, relative
to the relief of the underlying rock, the topography may be completely
changed. Not only may some of the valleys be obliterated by being
filled, but some of the hills may be obliterated by having the lower
land between them built up to their level. In regions where the
thickness of the drift is slight, relative to the relief of the rock
beneath, the hills cannot be buried, and the valleys cannot be
completely filled, so that the relative positions of the principal
topographic features will remain much the same after the deposition of
the drift, as before (Fig. 33).
[Illustration: Fig. 33.--Diagrammatic section showing relation of drift
to underlying rock where the drift is thin relative to the relief of the
underlying rock.]
In case the pre-glacial valleys were filled and the hills buried, the
new valleys which the surface waters will in time cut in the drift
surface will have but little correspondence in position with those
which existed before the ice incursion. A new system of valleys, and
therefore a new system of ridges and hills, will be developed, in some
measure independent of the old. These relations are illustrated by Fig.
32.
Inequalities in the thickness of drift lead to a still further
modification of the surface. It frequently happened that in a plane or
nearly plane region a slight thickness of drift was deposited at one
point, while all about it much greater thicknesses were left. The area
of thin drift would then constitute a depression, surrounded by a higher
surface built up by the thicker deposits. Such depressions would at
first have no outlets, and are therefore unlike the depressions shaped
by rain and river erosion. The presence of depressions without outlets
is one of the marks of a drift-covered (glaciated) country. In these
depressions water may collect, forming lakes or ponds, or in some cases
only marshes and bogs.
DIRECTION OF ICE MOVEMENT.
The direction in which glacier ice moved may be determined in various
ways, even after the ice has disappeared. The shapes of the rock hills
over which the ice passed, the direction from which the materials of the
drift came, and the course of the margin of the drift, all show that the
ice of south central Wisconsin was moving in a general southwest
direction. In the rock hills, this is shown by the greater wear of their
northeast ("stoss") sides (Plate XXXIV). From the course of the drift
margin, the general direction of movement may be inferred when it is
remembered that the tendency of glacier ice on a plane surface is to
move at right angles to its margin.
For the exact determination of the direction of ice movement, recourse
must be had to the striae on the bed-rock. Were the striated rock surface
perfectly plane, and were the striae even lines, they would only tell
that the ice was moving in one of two directions. But the rock surface
is not usually perfectly plane, nor the striae even lines, and between
the two directions which lines alone might suggest, it is usually
possible to decide. The minor prominences and depressions in the rock
surface were shaped according to the same principles that govern the
shaping of hills (Fig. 29) and valleys (Fig. 30); that is, the stoss
sides of the minor prominences, and the distal sides of small
depressions suffered the more wear. With a good compass, the direction
of the striae may be measured to within a fraction of a degree, and thus
the direction of ice movement in a particular place be definitely
determined. The striae which have been determined about Baraboo are shown
on Plate II.
_Effect of topography on movement._--The effect of glaciation on
topography has been sketched, but the topography in turn exerted an
important influence on the direction of ice movement. The extreme degree
of topographic influence is seen in mountain regions like the Alps,
where most of the glaciers are confined strictly to the valleys.
As an ice sheet invades a region, it advances first and farthest along
the lines of least resistance. In a rough country with great relief,
tongues or lobes of ice would push forward in the valleys, while the
hills or other prominences would tend to hold back or divide the onward
moving mass. The edge of an ice sheet in such a region would be
irregular. The marginal lobes of ice occupying the valleys would be
separated by re-entrant angles marking the sites of hills and ridges.
If the ice crossed a plane surface above which rose a notable ridge or
hill, the first effect of the hill would be to indent the ice. The ice
would move forward on either side, and if its thickness became
sufficiently great, the parts moving forward on either side would again
unite beyond it. A hill thus surrounded by ice is a nunatak. Later, as
the advancing mass of ice became thicker, it might completely cover the
hill; but the thickness of ice passing over the hill would be less than
that passing on either side by an amount equal to the height of the
hill. It follows that as ice encounters an isolated elevation, three
stages in its contest with the obstruction may be recognized: (1) the
stage when the ridge or hill acts as a wedge, dividing the moving ice
into lobes, Fig. 34; (2) the nunatak stage, when the ice has pushed
forward and reunited beyond the hill, Fig. 35; (3) the stage when the ice
has become sufficiently deep to cover the hill.
[Illustration: Fig. 34.--Diagrammatic representation of the effect of a
hill on the edge of the ice.]
After the ice has disappeared, the influence of the obstruction might be
found in the disposition of the drift. If recession began during the
first stage, that is, when the ice edge was separated into lobes, the
margin of the drift should be lobate, and would loop back around the
ridge from its advanced position on either side. If recession began
during the second stage, that is, when the lobes had become confluent
and completely surrounded the hill, a _driftless area_ would appear in
the midst of drift. If recession began during the third stage, that is,
after the ice had moved on over the obstruction, the evidence of the
sequence might be obliterated; but if the ice moved but a short distance
beyond the hill, the thinner ice over the hill would have advanced less
far than the thicker ice on either side (Fig. 35), and the margin of
the drift would show a re-entrant pointing back toward the hill, though
not reaching it. All these conditions are illustrated in the Devil's
lake region.
[Illustration: Fig. 35.--Same as Fig. 34, when the ice has advanced
farther.]
_Limit of the Ice._
The region under description is partly covered with drift, and partly
free from it. The limit of the ice, at the time of its maximum expansion
is well defined at many points, and the nature and position of the drift
limit are so unique as to merit attention (see Plates II and XXXVII).
They illustrate many of the principles already discussed.
The ice which covered the region was the western margin of the Green Bay
lobe (Fig. 36) of the last continental ice sheet. Its limit in this
region is marked by a ridge-like accumulation of drift, the _terminal
moraine_, which here has a general north-south direction. The region
may have been affected by the ice of more than one epoch, but since the
ice of the last epoch advanced as far to the west in this region as that
of any earlier epoch, the moraine is on the border between the glaciated
country to the east, and the driftless area to the west (Plates I and
II). That part of the moraine which lies west of the Wisconsin river
follows a somewhat sinuous course from Kilbourn City to a point a short
distance north of Prairie du Sac. The departures from this general
course are especially significant of the behavior of glacier ice.
[Illustration: Fig. 36.--Map showing relations of lobes of ice during
the Wisconsin ice epoch, to the driftless area.]
In the great depression between the quartzite ranges, the moraine bends
westward, showing that the ice advanced farther on the lowlands than on
the ridges. As the moraine of this low area approaches the south range,
it curves to the east. At the point southwest of Baraboo where the
easterly curve begins to show itself, the moraine lies at the north base
of the quartzite range; but as it is traced eastward, it is found to lie
higher and higher on the slope of the range, until it reaches the crest
nearly seven miles from the point where the eastward course was assumed.
At this point it crosses the range, and, once across the crest, it turns
promptly to the westward on the lower land to the south. Here the ice
advanced up the valley between the East bluff (east of the lake) and the
Devil's nose (Plate XXXVII), again illustrating the fact that lowlands
favor ice advance. The valley between the Devil's nose and the East
bluff is a narrow one, and the ice advanced through it nearly to the
present site of the lake. Meanwhile the restraining influence of the
"nose" was making itself felt, and the margin of the ice curved back
from the bottom of the bluff near Kirkland, to the top of the bluff at
the end of the nose. Here the edge of the ice crossed the point of the
nose, and after rounding it, turned abruptly to the west. Thence its
edge lay along the south slope of the ridge, descending from the crest
of the ridge at the nose, to the base of the ridge two miles farther
west. Here the ice reached its limit on the lowland, and its edge, as
marked by the moraine, turned southward, reaching the Wisconsin river
about a mile and a half above Prairie du Sac.
The course of the terminal moraine across the ridges is such as the
margin of the ice would normally have when it advanced into a region of
great relief. The great loop in the moraine with its eastern extremity
at k, Plate XXXVII, is explained by the presence of the quartzite
ridge which retarded the advancing ice while it moved forward on either
side. The minor loop around the Devil's nose is explained in the same
way. Both the main loop, and the smaller one on the nose, illustrate
the point made earlier in the text.
The narrow and curious loop at m, is of a slightly different origin,
though in principle the same. It is in the lee of a high point in the
quartzite ridge. The ice surmounted this point, and descended its
western slope; but the thickness of the ice passing over the summit was
so slight that it advanced but a short distance down the slope before
its force was exhausted, while the thicker ice on either side advanced
farther before it was melted.
_Glacial Deposits._
Before especial reference is made to the drift of this particular
region, it will be well to consider the character of drift deposits in
general. When the ice of the continental glacier began its motion, it
carried none of the stony and earthy debris which constitute the drift.
These materials were derived from the surface over which the ice moved.
From the method by which it was gathered, it is evident that the drift
of any locality may contain fragments of rock of every variety which
occurs along the route followed by the ice which reached that locality.
Where the ice had moved far, and where there were frequent changes in
the character of the rock constituting its bed, the variety of materials
in the drift is great. The heterogeneity of the drift arising from the
diverse nature of the rocks which contributed to it is _lithological
heterogeneity_--a term which implies the commingling of materials
derived from different rock formations. Thus it is common to find pieces
of sandstone, limestone, quartzite, granite, gneiss, schist, etc.,
intimately commingled in the drift, wherever the ice which produced it
passed over formations of these several sorts of rock. Lithological
heterogeneity is one of the notable characteristics of glacial
formations.
Another characteristic of the drift is its _physical heterogeneity_. As
first gathered from the bed of moving ice, some of the materials of the
drift were fine and some coarse. The tendency of the ice in all cases
was to reduce its load to a still finer condition. Some of the softer
materials, such as soft shale, were crushed or ground to powder, forming
what is known in common parlance as clay. Clayey (fine) material is
likewise produced by the grinding action of ice-carried bowlders upon
the rock-bed, and upon one another. Other sorts of rock, such as soft
sandstone, were reduced to the physical condition of sand, instead of
clay, and from sand to bowlders all grades of coarseness and fineness
are represented in the glacial drift.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXXV.
Cut in drift, showing its physical heterogeneity.]
Since the ice does not assort the material which it carries, as water
does, the clay, sand, gravel and bowlders will not, by the action of the
ice, be separated from one another. They are therefore not stratified.
As left by the ice, these physically heterogeneous materials are
confusedly commingled. The finer parts constitute a matrix in which the
coarser are embedded.
Physical heterogeneity (Plate XXXV), therefore, is another
characteristic of glacial drift. It is not to be understood that the
proportions of these various physical elements, clay, sand, gravel, and
bowlders, are constant. Locally any one of them may predominate over any
or all the others to any extent.
Since lithological and physical heterogeneity are characteristics of
glacial drift, they together afford a criterion which is often of
service in distinguishing glacial drift from other surface formations.
It follows that this double heterogeneity constitutes a feature which
can be utilized in determining the former extension of existing
glaciers, as well as the former existence of glaciers where glaciers do
not now exist.
Another characteristic of glacial drift, and one which clearly
distinguishes it from all other formations with which it might be
confounded, is easily understood from its method of formation. If the
ice in its motion holds down rock debris upon the rock surface over
which it passes with such pressure as to polish and striate the
bed-rock, the material carried will itself suffer wear comparable to
that which it inflicts. Thus the stones, large and small, of glacial
drift, will be smoothed and striated. This sort of wear on the
transported blocks of rock, is effected both by the bed-rock reacting on
the bowlders transported over it, and by bowlders acting on one another
in and under the ice. The wear of bowlders by bowlders is effected
wherever adjacent ones are carried along at different rates. Since the
rate of motion of the ice is different in different parts of the
glacier, the mutual abrasion of transported materials is a process
constantly in operation. A large proportion of the transported stone and
blocks of rock may thus eventually become striated.
From the nature of the wear to which the stones are subjected when
carried in the base of the ice, it is easy to understand that their
shapes must be different from those of water-worn materials. The latter
are rolled over and over, and thus lose all their angles and assume a
more or less rounded form. The former, held more or less firmly in the
ice, and pressed against the underlying rock or rock debris as they are
carried slowly forward, have their faces planed and striated. The
planation and striation of a stone need not be confined to its under
surface. On either side or above it other stones, moving at different
rates, are made to abrade it, so that its top and sides may be planed
and scored. If the ice-carried stones shift their positions, as they may
under various circumstances, new faces will be worn. The new face thus
planed off may meet those developed at an earlier time at sharp angles,
altogether unlike anything which water-wear is capable of producing. The
stone thus acted upon shows a surface bounded by planes and more or less
beveled, instead of a rounded surface such as water wear produces. We
find, then, in the shape of the bowlders and smaller stones of the
drift, and in the markings upon their surfaces, additional criteria for
the identification of glacier drift (Plate XXXVI).
The characteristics of glacial drift, so far as concerns its
constitution, may then be enumerated as, (1) its lithological, and (2)
physical heterogeneity; (3) the shapes, and (4) the markings of the stones
of the drift. In structure, the drift which is strictly glacial, is
unstratified.
In the broadest sense of the term, all deposits made by glacier ice are
_moraines_. Those made beneath the ice and back from its edge constitute
the _ground moraine_, and are distinguished from the considerable
marginal accumulations which, under certain conditions, are accumulated
at or near the margin. These marginal accumulations are _terminal
moraines_. Associated with the moraines which are the deposits of the
ice directly, there are considerable bodies of stratified gravel and
sand, the structure of which shows that they were laid down by water.
This is to be especially noted, since lack of stratification is
popularly supposed to be the especial mark of the formations to which
the ice gave rise.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,
PL. XXXVI.
Glaciated stones, showing both form and striae. (Matz.)]
These deposits of stratified drift lie partly beyond the terminal
moraine, and partly within it. They often sustain very complicated
relations both to the ground and terminal moraines.
The drift as a whole is therefore partly stratified and partly
unstratified. Structurally the two types are thoroughly distinct, but
their relations are often most complex, both horizontally and
vertically. A fuller consideration of these relations will be found on a
later page.
_The Ground Moraine._
The ground moraine constitutes the great body of the glacial drift.
_Bowlder clay_, a term descriptive of its constitution in some places,
and _till_, are other terms often applied to the ground moraine. The
ground moraine consists of all the drift which lodged beneath the ice
during its advance, all that was deposited back from its edge while its
margin was farthest south, and most of that which was deposited while
the ice was retreating. From this mode of origin it is readily seen that
the ground moraine should be essentially as widespread as the ice
itself. Locally, however, it failed of deposition. Since it constitutes
the larger part of the drift, the characteristics already enumerated
as belonging to drift in general are the characteristics of the
till. Wherever obstacles to the progress of the ice lay in its path,
there was a chance that these obstacles, rising somewhat into the
lower part of the ice, would constitute barriers against which debris in
the lower part of the ice would lodge. It might happen also that the
ice, under a given set of conditions favoring erosion, would gather a
greater load of rock-debris than could be transported under the changed
conditions into which its advance brought it. In this case, some part of
the load would be dropped and over-ridden. Especially near the margin of
the ice where its thickness was slight and diminishing, the ice must
have found itself unable to carry forward the loads of debris which it
had gathered farther back where its action was more vigorous. It will be
readily seen that if not earlier deposited, all material gathered by the
under surface of the ice would ultimately find itself at the edge of the
glacier, for given time enough, ablation will waste all that part of the
ice occupying the space between the original position of the debris, and
the margin of the ice. Under the thinned margin of the ice, therefore,
considerable accumulations of drift must have been taking place while
the ice was advancing. While the edge of the ice sheet was advancing
into territory before uninvaded, the material accumulated beneath its
edge at one time, found itself much farther from the margin at another
and later time. Under the more forcible ice action back from the margin,
the earlier accumulations, made under the thin edge, were partially or
wholly removed by the thicker ice of a later time, and carried down to
or toward the new and more advanced margin. Here they were deposited, to
be in turn disturbed and transported still farther by the farther
advance of the ice.
Since in its final retreat the margin of the ice must have stood at all
points once covered by it, these submarginal accumulations of drift must
have been made over the whole country once covered by the ice. The
deposits of drift made beneath the marginal part of the ice during its
retreat, would either cover the deposits made under the body of the ice
at an earlier time, or be left alongside them. The constitution of the
two phases of till, that deposited during the advance of the ice, and
that deposited during its retreat, is essentially the same, and there
is nothing in their relative positions to sharply differentiate them.
They are classed together as _subglacial till_.
Subglacial till was under the pressure of the overlying ice. In keeping
with these conditions of accumulation, the till often possesses a
firmness suggestive of great compression. Where its constitution is
clayey it is often remarkably tough. Where this is the case, the quality
here referred to has given rise to the suggestive name "hard pan." Where
the constitution of the till is sandy, rather than clayey, this firmness
and toughness are less developed, or may be altogether wanting, since
sand cannot be compressed into coherent masses like clay.
_Constitution._--The till is composed of the more or less comminuted
materials derived from the land across which the ice passed. The soil
and all the loose materials which covered the rock entered into its
composition. Where the ice was thick and its action vigorous, it not
only carried away the loose material which it found in its path, but,
armed with this material, it abraded the underlying rock, wearing down
its surface and detaching large and small blocks of rock from it. It
follows that the constitution of the till at any point is dependent upon
the nature of the soil and rock from which it was derived.
If sandstone be the formation which has contributed most largely to the
till, the matrix of the till will be sandy. Where limestone instead of
sandstone made the leading contribution to it, the till has a more
earthy or clayey matrix. Any sort of rock which may be very generally
reduced to a fine state of division under the mechanical action of the
ice, will give rise to clayey till.
The nature and the number of the bowlders in the till, no less than the
finer parts, depend on the character of the rock overridden. A hard and
resistant rock, such as quartzite, will give rise to more bowlders in
proportion to the total amount of material furnished to the ice, than
will softer rock. Shale or soft sandstone, possessing relatively slight
resistance, will be much more completely crushed. They will, therefore,
yield proportionately fewer bowlders than harder formations, and more
of the finer constituents of till.
The bowlders taken up by the ice as it advanced over one sort of rock
and another, possessed different degrees of resistance. The softer ones
were worn to smaller dimensions or crushed with relative ease and speed.
Bowlders of soft rock are, therefore, not commonly found in any
abundance at great distances from their sources. The harder ones yielded
less readily to abrasion, and were carried much farther before being
destroyed, though even such must have suffered constant reduction in
size during their subglacial journey. In general it is true that
bowlders in the till, near their parent formations, are larger and less
worn than those which have been transported great distances.
The ice which covered this region had come a great distance and had
passed over rock formations of many kinds. The till therefore contains
elements derived from various formations; that is, it is lithologically
heterogeneous. This heterogeneity cannot fail to attract the attention
of one examining any of the many exposures of drift about Baraboo at
road gradings, or in the cuts along the railway. Among the stones in the
drift at these exposures are limestone, sandstone, quartzite, diabase,
gabbro, gneiss, granite, schist, and porphyry, together with pieces of
flint and chert.
Such an array may be found at any of the exposures within the immediate
vicinity of Devil's lake. To the north, and a few miles to the south of
the Baraboo ranges, the quartzite from these bluffs, and the porphyry
from the point marked h in Plate II, are wanting, though other
varieties of porphyry are present. The ice moved in a general
west-southwest direction in this region, and the quartzite in the drift,
so far as derived from the local formation, is therefore restricted to a
narrow belt.
The physical heterogeneity may be seen at all exposures, and is
illustrated in Plate XXXV. The larger stones of the drift are
usually of some hard variety of rock. Near the Baraboo ranges, the local
quartzite often predominates among the bowlders,
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