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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.
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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.
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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.
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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.
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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.
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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.