Geology of the Lakeshore Preserve

Geology of Eagle Heights Woods

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David Mickelson is Emeritus Professor and Senior Scientist, Department of Geoscience, University of Wisconsin – Madison. Dave regularly conducts field trip for the Friends on the geology of the Preserve.

More than 500 million years ago all of southern Wisconsin was beneath a shallow sea. Sand delivered to the sea by rivers flowing from the continent to the North and East accumulated on this ocean bottom.    

At that time what is now Wisconsin was located about 15° south of the equator and was oriented so that what is now North was East. Of course, Wisconsin was not an island as depicted, but had the rest of the North American continent around it. From this position, it gradually moved north in a semi-circle, while turning 90° on its axis, and reached its current location about 10 million years ago. 

Living on the sandy ocean bottom were many forms of life that do not exist today. The most common one in the Cambrian Period (about 500 million years ago, click for more information) was the trilobite. This is Wisconsin’s state fossil and this photo shows a particularly well preserved example. 

This is a sample of Cambrian Age sandstone showing pieces of trilobites. It is much more common to find these fragments than a whole trilobite.    

The rock layers beneath Eagle Heights Hill are nearly flat lying. This slide shows a slice across Dane County with Lake Mendota in approximately the middle. All of these rock layers with the exception of the topmost layer shown in green are Cambrian Age.    

This geologic map shows the distribution of sandstone and limestone in the area of Eagle Heights. The layers shown in brown, yellow, and green are Cambrian Age rocks that are mostly sandstone. 

This is sandstone close to lake level at Raymer’s Cove, at the base of Eagle Heights Hill. The rock is quite soft and sand grains can easily be scratched off. These soft layers are slowly eroded by weathering, wave, and wind action. 

The area of younger Ordovician rocks on this photo (about 480 million years ago) that are mostly limestone is light blue. As you can see these rocks are only in the higher areas. In the Preserve, they are exposed only close to the top of the Hill in Eagle Heights Woods. The outcrops are mostly vegetation covered and severely weathered. In the Ordovician Period, many of the rivers that had delivered sand from the continent were flowing into other basins, and the sea covering what is now southern Wisconsin was a clear tropical sea much like the Florida Keys today. The sea bottom was dominated by algae that secreted calcium carbonate, and they grew in mounds. These are called stromatolites and they are common fossils in southern Wisconsin rocks of early Ordovician age.    

Stromatolites still exist today.  These are modern stromatolites along the southwest coast of Australia (Sharks Bay).    

This outcrop of weathered Ordovician limestone is on the north side of Eagle Heights Hill just downslope from the Indian Mounds trail.    

As the earth’s climate cooled over the last 70 million years, a series of glaciations began. The most recent of these reached its maximum extent about 30,000 years ago. 

The big lake west of the glacier is called glacial Lake Wisconsin. Madison is situated very close to where the edge of the glacier was from about 30,000 years ago ……    

.…until a little after 20,000 years ago. At that time climate began to warm and the edge of the glacier began to retreat. What are now the Madison lakes were joined together as one lake called glacial Lake Yahara (arrow.) The bigger lake to the east is called glacial Lake Scuppernong.    

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