The Baraboo Ranges and Devil's Lake Gorge

A Geological Tour

Keith Montgomery
Department of Geography-Geology
University of Wisconsin - Marathon County

 

Table of contents:

  1. A description of the ranges
  2. Other features of interest
  3. A tour of the area
  4. The geological evolution of the ranges
  5. Sources of information
A description of the ranges:

The Baraboo Ranges (North and South) are located in south-central Wisconsin and comprise an oval ring of hills, approximately 30 miles long and 10 miles wide. The ranges are composed of a metamorphic rock, the Baraboo Quartzite, and owe their existence to the great strength of this material. While the forces of erosion (e.g. weathering, rivers, glaciers) stripped off overlying rock layers in the past 350 million years or so, the quartzite proved extremely resistant to this assault and forms this distinctive ring of hills in the present landscape of Wisconsin. Remnants of these other layers form fairly small, isolated hills (or, buttes) throughout the area of the Baraboos.

The underlying geologic structure (that is, the manner in which the quartzite and other rock layers are lying) explains both the overall oval form of the ranges and the fact that the South Range is by far the wider and most continuous of the two ranges. As a geologic cross-section of the ranges shows, each range represents the "limb" of a syncline or elongate basin. The gentler "dip" of the southern limb provides the quartzite there with a wider surface exposure. The limbs of the syncline are pinched together in the east. In the west, the limbs curve gently together, forming a western range, and there the layers of rock dip towards the east, or inwards towards the center of the main syncline.

The later (younger) rocks that once buried this syncline lie largely horizontally, and their remnants form isolated, often flat-capped hills called buttes (see again the geologic cross-section). Examples of these remnants, as shown on the cross-section, are Pine Bluff, in the center of the syncline, and Gibraltar Rock, to the southeast.

Other features of note:

To add further interest to this landscape there is the presence of the terminal moraine of the final regional advance of the Wisconsinan, or last, glaciation. This hummocky moraine belt, which demarcates the furthermost extent of ice cover during the last glaciation, lies draped across both ranges and almost bisects the ranges from north to south (see location on block diagram). To the west of the moraine is the famous "Driftless Area" of Wisconsin, an area that remained, almost inexplicably, unglaciated throughout the Ice Age, even as surrounding areas were overrun by ice several times in the past 2 million years.

In the South Range there is Devil's Lake gorge. It is generally believed that the Wisconsin River flowed through here prior to that last glaciation (see map). This gorge, now plugged by the Wisconsinan Moraine at each end and occupied by Devil's Lake, is one of Wisconsin's most impressive sights. The talus, or quartzite blocks, that mantles the sides of the gorge is also very impressive as we will see.

The relatively narrow North Range is cut by two gorges through which the Baraboo River flows. These are the Upper and Lower Narrows (see again the block diagram).

Finally, there is the beautiful Parfrey's Glen, a narrow canyon cut into the south slope of the South Range just east of the Devil's Lake gorge. Durward's Glen is another example.
 

A tour of the area:

Before continuing, you might like to see what the gorge and its features look like. In addition you should visit Parfrey's Glen. To do this, go to the clickable map (give it time to load fully, please. This might take a minute or so).
 

The geological evolution of the ranges:

This description raises a number of interesting questions about the evolution of this landscape (its rocks, structures, and appearance). The following questions are generally in chronological order.

Sources of information:

The primary source of geological information about the Baraboo area is the "Geology of the Baraboo District, Wisconsin", published in 1970 by the Wisconsin Geological and Natural History Survey as Information Circular #14. The contributors to this volume are I.W.D. Dalziel and R.H. Dott ("A description and field guide incorporating structural analysis of the Precambrian rocks and sedmimentologic analysis of the Paleozoic strata"), and R.F. Black ("Summary of the glacial geology").
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 


 

The Baraboo Quartzite is a metamorphosed sandstone. Therefore when was the original sandstone laid down and from what rocks did its grains come?

This question touches upon an important concept in geology termed the "rock cycle." As erosion (or other processes in different circumstances) breaks down pre-existing rocks then the resultant materials are recycled into the stuff of new rocks in various ways.

In this case the quartz grains that formed the sandstone appear to have come largely from erosion of the Penokean Mountains to the north. These mountains formed about 1.85 billion years ago in the Early Proterozoic Eon, and the sandstone seems to have been laid down in the subsequent 200 million years. About 2 miles due west of Devil's Lake, where HWY13 cuts through the South Range, one can see a spectacular example of ripples formed in the original sandstone by the running water that laid it down.

If you wish to know more about the Penokean Mountains then visit the Rib Mountain geological tour.

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The sandstone would have been originally horizontal (just like the later sedimentary rocks). However, the quartzite and some other rocks have been compressed and folded into a syncline. When did this happen?

First, recall that ripple marks (and other structures) in the sandstone itself tell us that significant changes in angle have occurred since the sand was originally deposited. The changes appear to have happened about 1.65 billion yeas ago (near the middle of the Proterozoic). At this time there was an episode of mountain building (an orogeny) to the south and this may have provided the necessary force for the folding and metamorphosis in the Baraboo region.

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The later rocks seem to have buried the syncline at one time. When and how did these later rocks come to form? And when and how did they come to be largely removed?

Following the folding and metamorphosis in this region there followed a long period of erosion in this landscape. Then, about 570 million years ago (at the start of the Cambrian Period) sea level rose and came to submerge most, but not all, of Wisconsin (see unconformity description and explanation). This continental sea existed for much of the Cambrian, Silurian, and Devonian (i.e. for about 200 million years -- see time scale). Various sediments and precipitates formed in, or at the margins of this sea. In Parfrey's Glen we can see from the rocks that the Baraboo Hills formed islands in this sea and supplied the material for at least the some of the local sediments. Across the state, sediments and other materials subsequently formed beds of shale, limestone and dolomite, and sandstone. A geologic map of Wisconsin shows their present distribution.

The distribution of these rocks around the Baraboo Hills shows that they certainly covered these ranges at one time (see geological cross-section). However, the youngest of these rocks in all of Wisconsin is only about 250 million years old. Therefore, when the sea receded at around this time, these rocks were laid bare to the elements and it is 250 million of attack from weathering, rivers, wind, and glaciers that have removed them from much of the state.

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When and how was the Devil's Lake Gorge formed? And the lake?

There is no clear answer to this question. Recall that the Devil's Lake gorge is one of several gaps that cut the ranges. The gorge is certainly the product of river erosion. It is possible that some of this erosion took place in the time interval between folding and uplift, and the Paleozoic deposition. Evidence for this is the (limited) occurrence of Paleozoic materials in the southwestern end of the Devil's Lake gorge (Messenger Creek basin) and the filling of some erosional potholes by such material. It is also possible that further erosion occurred between the Paleozoic deposition and the present day.

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How does Parfrey's Glen and the potholes fit into this history?

Parfrey's Glen is cut through early Paleozoic rocks that lap against the South Range. Therefore it is post-Paleozoic. It could well be as recent as the Pleistocene (maybe meltwater from the ice sheet cut it) if one thinks that a longer period of rock fall would have lowered the angle of its walls by now. Certainly the present creek lacks the power to have had anything to do with it.

The potholes are a mystery. They are certainly the product of stream erosion, but are only found at higher elevations (although often below the rim of the gorge, as at the Potholes Trail). It is likely that they represent river erosion during an early stage of erosion in the Proterozoic (Pre-Cambrian).

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How significant has glaciation been in this history of deposition and erosion?

Very significant, but also very recent. It has created a suite of landforms and deposits around the eastern half of the ranges that contrasts with the unglaciated western half. Also, glaciation has dammed the Devil's lake gorge. However, most of the landscape we see today is the product of earlier erosion.

It is speculated that the Wisconsin River once flowed through the Lower Narrows and Devil's Lake gorge and was diverted around the eastern end of the Baraboo Hills at the end of the glaciation.

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When and how was the talus produced?

The talus is most abundant in the area outside of glaciation -- most spectacularly in the Devil's lake gorge. Angular debris such as this is produced by mechanical weathering of the quartzite. The talus today seems very stable and little, if any is being produced. It is likely that it was produced by frost action during glacial phases of the Pleistocene.

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Since July 15 1999.