Geologic History of the Area 

The Madison area was once a shallow sea closer to the equator. Over millions of years, changes in water levels in this sea left successive deposits of sandstone, limestone and shale. With the movement of Earth's tectonic plates, the Madison area gradually shifted to the north. The shallow sea drained and this land mass slowly rose above sea level. The area was then subjected to millions of years of erosion as the Earth went through numerous cycles of cooler and warmer periods. Each advancing ice sheet altered the topography and surface water drainage patterns of the land it pushed itself across. The higher elevations of bedrock hills were ground off. Valleys were filled in and new valleys were formed. Each advancing ice sheet  rearranged the deposits left behind by the ice sheet before it and deposited a terminal moraine of thick glacial deposits at its furthest advance. Click on the video below to view a time lapse of the ice sheet margin positions between 31,500 and 11,000 years ago. You can adjust the speed of the sequence if you like. There is no audio.

If you would prefer taking more time looking through the various ice margin positions, click on the above image to open a PDF containing the complete set of images.

The most recent ice age began about 100,000 years ago. In Canada, an ice sheet slowly grew for about 70,000 years and started advancing south into Wisconsin about 31,500 years ago. Around 30,000 years ago this growing ice sheet began to enter northeastern Dane County. As the ice sheet grew thicker and advanced further southward, the Earth's oceans lowered, as the oceans were the source of all the water for the growing ice sheet. By the time the ice sheet advanced to just northeast of the Madison area, there was a much larger pre-glacial river cutting a deep valley in a generally southeastward direction through the sandstone and limestone bedrock of the Madison area. The topography in the Madison area then looked  more like that of the Driftless Area that still exists today in the western portion of Dane County.

 About 28,000 years ago at the peak of the last ice age, the world's oceans were about 350 to 400 feet shallower than they are now.  At this time a pre-glacial Yahara River was flowing sweetly during the warmer months through a much deeper valley system that still exists today beneath Madison's lakes. The lowering ocean level caused the Mississippi River to cut its channel downward during each ice age. This caused the Rock River cut its bed downward while carrying high volumes of glacial meltwater to the Mississippi River. The pre-glacial Yahara River did the same as it carried its contents downstream to the Rock River. As it wandered through the Madison area, the pre-glacial Yahara River traveled through a valley that was over 200 feet below the existing surface of Lake Monona. Lake Monona today is just 74 feet deep.

Below are two topographic maps that show the depth to bedrock.  The topographic lines on the map are the depth to bedrock in feet from the existing ground surface to the buried bedrock surface at that location on the map.  When you click on each map below, a larger high resolution map will open. By zooming in on these maps you may notice that the existing streams and rivers do not always flow along the ancient pre-glacial valleys depicted on the maps. Instead the surface water followed the routes available after the glacial deposits altered the region's topography. The flow of glacial meltwater cut new valleys along its path which then slowly drained the large glacial lakes that formed behind many of the terminal and recessional moraines deposited in Wisconsin as the ice sheet receded.

Depth to Bedrock Map of the Madison Area

This is a cropped portion of the Depth to Bedrock Map of Wisconsin.

Click on the map to view a larger high resolution image.

Depth to Bedrock Map of Wisconsin

Click on the map to view a larger high resolution image.

 Click on the image above to open a larger high resolution image of the map.

The three dimensional map shown in the above picture is located at the University of Wisconsin Geology Museum here in Madison. It had been on display at the museum but is now in storage due to limited display space at the museum. The red outlines on the map are the shorelines of the present lakes Mendota and Monona. The bedrock ridge in the center of the map is beneath the Madison isthmus. The other colors on the map represent the various bedrock units consisting of sandstone, shale, and limestone exposed to weathering in the past but almost entirely covered by glacial deposits today.

The three dimensional map above was created in 1938 and based on geologic maps created by UW geologist, Fredrik Turville Thwaites. Fredrik graduated from the University of Wisconsin, majoring in geology. He later became a geology instructor at the University and was the first curator of the University of Wisconsin Geology Museum. Frederik's father, Rueben Gold Thwaites, was a famous historian and was for a number of years superintendent of the State Historical Society of Wisconsin. Reuben married Jessie Turvill, daughter of Henry and Mary Turvill. The Thwaites family moved onto the Turville farm in 1894 after selling their house on Langdon Street. They lived on the Turville farm in one of the many houses that were constructed on the property for the Turville family members. For a detailed biography of Frederik Turville Thwaites click on the following link to his biography on the Wisconsin Geological Survey website:

Fredrik Turville Thwaites

The topographic map below was created by Fredrik Turville Thwaites from geologic data collected between 1902 and 1907. This map was created in 1910 with some of the same data used to create the three dimensional topographic relief map shown earlier on this page. The map below shows the main pre-glacial Yahara River valley cutting across the middle of Madison's isthmus just southwest of the capitol building. However, the three dimensional topographic relief model created in 1938 shows the main pre-glacial Yahara River valley curving around the northeast end of the bedrock ridge that underlies the isthmus. It is likely that additional research after the map below was created formed the basis for the differences in the two maps. Additional geotechnical boring data from the Isthmus area probably lead to adjusting the main channel of the pre-glacial Yahara River valley from the southwest side of the capitol hill to the northeast end of the bedrock ridge on the east end of the isthmus. Some of the world's best geologists have made erroneous assumptions with the data before them, then, after analyzing additional data, arrive at a new theory as to how an area changed with time..

Drilling logs which have been submitted by water well drillers to the State for over the past 150 years as well as many geological surveys and opportune observations by geologists over the years at various construction sites across the Madison area provide much of the raw data gathered and analyzed by geologists to piece together the geologic history of the area. In recent years, the science of carbon dating and newer methods of dating soil and bedrock deposits have become more precise, further improving the ability to develop a more accurate description of past geology events and a timeline of the geology history of this area.

Pre-glacial Topo of Four Lakes Region by Fredrik Turville Thwaites B008-1910-plate37 orig download.pdf

Click on the pop-out button at the top right corner of the map to open the map in a separate larger window. Click on the - and + buttons at the bottom of the map to zoom out or in to view the map details more closely. 

 The Vast Ice Sheet Pushes over the Madison Area

Around 28,000 years ago, the ice sheet keep advancing and finally pushed over the Madison area, stopping just west of Verona at its furthest advance. The deep pre-glacial river valley that had previously cut through the Madison area was partially filled in with glacial deposits and the tops of bedrock hills were ground down. 

When the leading edge of the glacier had advanced to the location of its terminal moraine near Verona, it is estimated that this ice sheet was as much as 1000 feet thick in the Madison area. To the Northeast of Madison the ice sheet gradually became thicker reaching an estimated thickness of over 10,000 feet in the Hudson Bay area of Canada.

The ice sheet persisted for thousands of years after its furthest advance. It retreated somewhat and then advanced again over the Madison area multiple times until about 20,000 years ago when it began its final retreat to the northeast. During these successive retreats and advances of the ice sheet, meltwater was sometimes trapped between the ice sheet and the terminal moraine, causing temporary glacial lakes to form between the ice sheet and the terminal moraine. When the ice sheet retreated for the last time from the Madison area, a huge glacial lake flooded much of the Madison area to an elevation about 15 feet higher than present day Lake Monona at about 860 feet above the present sea level. 

As it was filling with meltwater, the glacial lake grew in size until it found an exit just south of Stoughton along the terminal moraine there. The escaping water started cutting a new channel for the Yahara River to the southeast towards the Rock River. All of the lakes in the Madison area were interconnected then, forming one huge irregularly shaped lake having several islands and peninsulas. With time, the Yahara River at the outlet of this lake cut through the glacial deposits along its path. In addition, the land slowly rebounded upward after the weight of the great ice sheet was removed. These two actions caused the one great lake to drain until it became the present chain of smaller lakes surrounded by extensive marshlands.

All of the marshland adjacent to the existing Madison lakes depicted in the map below was part of the huge Glacial Lake Yahara that existed here until it slowly drained to create the marshland we see today.

Glacial Geology of Dane County.pdf

Glacial Geology of Dane County, Wisconsin

Click on the pop-out button at the top right corner of the map to open the map in a seperate larger window. Click on the - and + buttons at the bottom of the map to zoom out or in to view the map details more closely. 

The undulating terrain of Olin Turville Park was once an island on the Glacial Lake Yahara. It consists of glacial moraine deposits that were left behind as the glacier retreated. To the west of this island  stratified layers of glaciolacustrine deposits of sand, silt, and clay settled out of the meltwater entering the lake. The extensive marshlands to the south and west of Olin Turville Park were all partially filled with these sediments. In some areas these glaciolacustrine  deposits are relatively shallow and lie over buried bedrock hills that were scrapped down by the glacier. In other areas where former valleys exist these deposits are much deeper.

At the Alliant Energy Center, the Coliseum was constructed on a  buried ridge of sandstone bedrock just below the former glacial lake bed. In the marshlands that exist just west and north of the Coliseum, the sandstone bedrock is only 30 to 40 feet below the ground surface. At the softball diamond at Olin Park the sandstone is only about 20 feet below home plate. In other locations the glaciolacustrine  deposits are much deeper as they have filled in the former valleys that existed here before being overrun by the glacier. The area southwest of the shoreline of Turville Bay contains deep deposits of peat and muck soils on top of even deeper deposits of silt and clay which continue to a depth of 60 feet or more. Yet, along part of the southeast shoreline of Turville Bay, sand and gravel barely covers the surface of bedrock just a few feet below the shoreline. 

The present Monona Bay and lands to the southwest past the west shore of Lake Wingra were also once a deep valley but now mostly filled with glacial deposits. The Dividing Ridge that once existed between lakes Wingra and Monona was likely an esker deposit of stratified sand and gravel layers that were formed beneath the ice sheet as it was melting. The Dividing Ridge and other deposits of stratified sands and gravel that exist between Lake Wingra and Lake Monona helped form a bowl-like shape to contain much of the sand, silt, and clay that entered this area from meltwater streams running off the ice sheet into this area.

It wasn't until about 13,000 years ago that the Madison area warmed enough during the summer months for the landscape to evolve from a treeless tundra to a land of lakes and evergreen forests. As the climate warmed further in southern Wisconsin, the evergreen forests were replaced by mixed hardwood forests, oak savannas, and extensive prairies.  During the same time, thick deposits of marl, muck, and peat finished filling in the shallower areas of the former glacial lake. By around 11,000 years ago, the glacial lake had drained to the level of today's Lake Monona which fluctuates between a record high of 848.52 feet above today's sea level and a set minimum of 842.2 feet above today's sea level.