Geologic History of the Area 

 Geologists believe 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 and the shallow sea drained as this land mass slowly rose above sea level. The area was then subjected to millions of years of erosion and repetitive glacial advances and retreats as the Earth went through numerous cycles of ice ages and warmer periods. 

Over 20,000 years ago the last ice age slowly advanced across the northern hemisphere of our planet causing the landforms and topography here in the Madison area to slowly change once more with time. The oceans provided the water which fell as snow and then was stored as ice in the massive and growing ice sheet. By the time the advancing glacier reached southern Wisconsin, the water level of the Earth's oceans had dropped to 350 to 400 feet below the present ocean water level. 

As a result, the coastal shorelines of the United States extended much farther out and were much lower then they presently are. All rivers with time gradually cut down the land surface along their path until they reach a gradient where the transport of sand diminishes to zero. At the maximum extent of the ice age the mouth of the Mississippi River would have been 350 to 400 lower than it is now. The mighty Mississippi would have been larger and more powerful then with the vast amounts of melting snow and ice each summer. Over thousands of years it cut down the soil and bedrock along its path all the way upstream past Minneapolis, Minnesota. The confluence of the Wisconsin River with the Mississippi would have also been about 350 feet lower than its present day elevation. Likewise, the confluence of the Yahara River with Rock River would have been about 300 to 350 feet lower than its present day elevation. In seeking this gradient and elevation, the pre-glacial Yahara River cut through the soils and bedrock in the Madison area over the thousands of years it took for the ice age to advance to the edge of Madison. This resulted in the formation of a system of bedrock hills and deep valleys in the Madison area. 

During this period in the past, the area where Lake Monona is now would have been a valley about 250 to 300 feet deeper than the present elevation of Lake Monona. This Pre-glacial Yahara River would have been much larger than it is now and the view for someone standing at the present lakeshore on Turville Point would have been like standing on the hills of Wyalusing State Park and looking down on the Wisconsin River today. The river in the deep valley would have been churning with vast amounts of water and sediment that flowed off the glacier each summer.

The red lines on the map mark the shorelines of present day Lake Mondota and Lake Monona

The picture at left is a photo of a three dimensional model of the pre-glacial Yahara River Valley buried beneath the existing glacial deposits in the Madison area. This river valley was formed to its maximum depth estimated to be about 250 to 300 feet below the present day Lake Mendota and Lake Monona water levels in the time period before the last glacier pushed across the Madison area. The pre-glacial Yahara River valley would have looked much like the present day deep valleys West of Madison in the un-glaciated part of Wisconsin. As the glacier advanced and retreated again bedrock hilltops were partially shaved off and the area was covered with varying amounts of glacial deposits. The deep pre-glacial valley was only partially filled in some areas as the glacier retreated creating today's lakes.

The three dimensional model 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 model are the shorelines of the present lakes Mendota and Monona. 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 leading edge of the glacier slowly advanced to the southwest of Madison ultimately filling in this deep river valley with ice and glacial deposits as it reached its maximum extent near Verona. Between about 14,000 and 20,000 years ago during the peak of the last ice age, the Madison area was buried under a vast sheet of ice. It is estimated that this ice was 1000 to 1600 feet thick in Madison. To the Southwest, this ice sheet tapered to zero near Verona. To the Northeast from Madison the great glacier gradually got thicker reaching an estimated thickness of over 10,000 feet in the Hudson Bay area of Canada. As this last ice age slowly ended starting about 12,000 years ago, the glacial lobe that covered the Madison area gradually retreated. The low lying areas of the undulating glacial topography left behind by the retreating glacier filled with melt water until this glacial lake found an exit to the southeast of Madison and into the Rock River. This huge lake was called Glacial Lake Yahara and was about 12 feet higher than the present elevation of Lake Monona. All of the lakes in the Madison area were interconnected then forming one huge lake that was dotted with many islands. With time, the Yahara River at the outlet of this lake cut down 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. 

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 as the glacier retreated. Many shallower areas of this glacial lake filled with silt, clay, and organic deposits as the glacier slowly retreated back into Canada and the lake drained to its present elevation. The extensive marsh and wetland areas surrounding Olin Turville Park were all filled with these sediments. However, in many areas these glaciolacustrine deposits are relatively shallow and lie over bedrock hills that were scrapped down by the glacier. At the Alliant Energy Center, the Coliseum was constructed on sandstone bedrock. in most of the Wingra Creek drainage area including the Quann Park and Willow Island areas this 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 soft glaciolacustrine deposits are much deeper as they cover former valleys in the pre-glacial hill and valley system that existed here before the glacier. Turville Bay and the marshland to the southwest of the bay consists of deep deposits of peat and muck soils on top of even deeper deposits of soft silts and clay as much as 80 feet deep (deeper than the bottom of the center of Lake Monona). Yet, along part of the south shoreline of Turville Bay sand and gravel barely covers the surface of hard bedrock just a few feet below the shoreline. The present Monona Bay and lands to the southwest past the west shore of Lake Wingra was also once a deep valley but now mostly filled with glacial deposits. Much of Madison's Isthmus was a long bedrock ridge that extended along the west shore of the pre-glacial Yahara River as the river made a sweeping S-turn through the Madison area. This bedrock ridge was partially shaved off by the glacier and is now covered with a relatively thin layer of glacial deposits. Sandstone bedrock is fairly close to the existing ground surface under much of the low lying central isthmus area east of the Capital hill. 

Geologists believe the series of events explained above happened after many years of research and analysis of the geology of Wisconsin. Water well boring logs which have been submitted by 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 Madison provide much of the raw data gathered and analyzed by geologists to piece together the geologic history of the area. 

The three dimensional map in the picture above was created in 1938 and based on the original geologic maps created by UW geologist, Frederick Turville Thwaites. Frederick 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. Frederick'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 a detailed biography of Frederick Turville Thwaites click on the following link to his biography on the Wisconsin Geological Survey website.

Frederick Turville Thwaites

 This geologic history essay was written by Ron Shutvet.