Geologic History of the Preserve and the Madison Lakes—Oct 14

Dave Mickelson and Lillian Tong

We had perfect weather for a walk with beautiful fall colors! Dave Mickelson, Prof emeritus of Geology and Geophysics and leader this informative field trip, started with a brief parking-lot chat using maps he brought for illustration. See also Dave Mickelson's in-depth illustrated article on this website: Geology of Eagle Heights Woods.
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First from a cross-section illustration of the rock below us, we learned the history and mindboggling age of each layer all the way down to the very old bedrock.  A map showed Wisconsin at different time periods and the extent of the Wisconsin glacier as it advanced and receded multiple times. Yet another map showed the glacial moraines and drumlins distributed across Dane County.  From the pattern on the map, one could see where the glacier paused and left a moraine and where it scoured the land as it advanced and left drumlins parallel to its direction of movement.  Dave told us how researchers used a variety of techniques, such as radiocarbon dating to determine age, and well records and geophysical techniques to measure thickness of glacial deposits, and many other techniques to piece together the story of our place on earth. Our hike took us from Raymer’s Cove through a stand of bright yellow maples, across Lake Mendota Drive, and up into the Eagle Heights Woods. We could hear the birds as we entered the woods.  At the top, looking down at Lake Mendota, Dave had us imagine what it looked like before the glaciers, when it was a deep valley.  Our Madison lakes were not formed as kettles when blocks of glacial ice melted and left a hole, as were most of the Wisconsin lakes.  Our lakes lie in a former valley that didn't fill completely with glacial debris, luckily for us. The map Dave brought showed a complex valley with numerous side valleys.  Much of what is now Madison once was a larger glacial lake- Lake Yahara, before an outlet opened.  People lived on the shores of that ancient lake even when glaciers were still covering much of northern Wisconsin. They were mammoth and mastodon hunters. 

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Hiking from the shoreline cliffs up towards Eagle Heights Woods.

We looked at the limestone cap rock that didn’t erode as quickly as the sandstone below, and the erratics brought by the glaciers from far away.  These erratics were primarily volcanic or metamorphic in origin. One type was  rhyolite from volcanic eruptions with fast cooling of lava and ash. Another was granite, formed of the same materials but that cooled underground under high pressure. Some rocks had veins of quartz that filled cracks in the rock after it had cooled. We saw a large cone-shaped Effigy Mound at the highest point of Eagle Heights Woods and were lucky to have Diane Dempsey on the field trip who shared her expertise on the Late Woodland mound builders with us.  Also enriching the hike were people knowledgeable about birds and trees who were willing to share what they know.  Everyone on the field trip appreciated the generosity and expertise of Dave Mickelson who brought materials, planned our route, developed an engaging presentation, answered all our questions, and brought new meaning to the landscape we call home.
We returned to the Raymer’s Cove parking lot via a steeper but more direct route. Many were heard to exclaim that they knew about Picnic Point, but had no idea the campus had this gem in Eagle Heights Woods, and they planned to return again. Report by Lillian Tong, Friends field trip host, and photos by Doris Dubielzig.