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.]