Geologic History of the Preserve and Madison Lakes – Nov 10, 2019

Dave Mickelson, in red jacket, explains the formation of the cliffs and caves along the east side of Eagle Heights Woods. Photo Dubielzig.

Back by popular demand, Geologist ​Dave Mickelson pictured for a group of more than twenty participants the glaciers that were in the Four Lakes area 25,000 years ago and their effects on the landscape. As the photos attest, It was a chilly and overcast fall day, but the walk with him from the Lake Mendota shore at Raymer’s Cove up to the vistas in Eagle Heights Woods warmed everyone internally, and Dave's fascinating stories of geological processes, backed up with maps, charts and handouts kept everyone's attention. 

​Throughout the walk, Dave made an effort to identify and describe the three classes of rocks which are all present in the Preserve: sedimentary, igneous, and metamorphic rocks. Through his provided descriptions, Dave was able to help attendees piece together a small part of the Preserve’s history. 
 
Sedimentary rock: Dave pointed out Cambrian sandstone which lays along the edge of Lake Mendota. It is approximated to be between 500 and 600 million years old and also probably has some calcium carbonate from organisms that became trapped between the layers of this rock. Another sedimentary rock found at the tops of the hills in Raymer’s Cove is a light-colored calcium-magnesium-carbonate rock known as dolomite. Interestingly, the dolomite can sometimes have gaps filled with a substance called “chert”. Chert is another sedentary, silicone-dioxide rock which is produced from microorganisms like algae. Dave mentioned a mechanism to explain why we sometimes find chert sandwiched between dolomite layers: imagine a decaying trilobite that will become integrated into the dolomite… trilobite decaying is an acidic process which causes the silicon based dolomite to crystalize  and aggregate in a particular way that allows spaces in the dolomite to form—where chert can eventually fill in!
 
Igneous rock: In the Preserve, igneous rocks are what geologists would call “erratic rocks” or a rock that is assumed to have been brought over via glaciers. A common igneous rock found in the Preserve is the basalt rock—a very dark-colored rock with high levels of iron and magnesium. 
 
Metamorphic rock: Like igneous rocks, metamorphic rocks are also considered erratic rocks in the Preserve. These rocks have a “gneissic texture” which refers to how the lighter and darker colored rocks appear to separate in a pattern— an arrangement which occurs when the rocks’ arrangement changes due to exposure to intense heating. 

Dave also told attendees how the Wisconsin landscape has been shaped by glaciers. With posters in hand which depicted the Wisconsin Glacier, Dave showed the terrain differences between the region that escaped glaciated (the “Driftless area”) and the regions covered by glaciers. Additional provided handouts illustrated the lasting geologic impressions left by the Wisconsin glacier in the Southern Wisconsin area: kettles, eskers, moraines, and drumlins galore are scattered across the land. 
 
Finally, Dave also distributed the free booklet, Landscapes of Dane County, Wisconsin, authored by him, and produced by the WI Geological and Natural History Survey, 2007. Report and Photos by Olympia Mathiaparanam.