Cool fall weather provided the perfect backdrop to Geological History of the Preserve and Madison Lakes on November 13th. 31 guests wandered through the Preserve under an overcast sky with Philip Fauble, geologist with the Department of Natural Resources and a passionate teacher of local geology. Philip began our hike with a crash-course on geoscience; before we could understand how the rocks in the Preserve reached their current form, we needed to learn a bit about how rocks move, their timescales, and the idiosyncratic geological history of the Madison lakes. We learned that the majority of the Preserve is comprised of Ordovician- and Cambrian-age bedrock, with much of it belonging to the Tunnel City Group rock layer, defined by its very fine-grained quartzite sand. Different glacial movements across the area cut into this bedrock throughout the last couple hundred thousand years, both creating the basins in which the Madison lakes could form and exposing the outcroppings which we observed with Philip.
Our hike took us to the Preserve’s highest point (the bluff in Eagle Heights Woods) where we studied glacial erratics—large boulders cut and picked up by glaciers and deposited by way of glacial recession, often hundreds of kilometers away from its source rock. Philip estimated the erratics on Eagle Heights Woods bluff were 2.4-billion-year-old gneiss from southern Ontario! Other highlights included Philip showing us evidence of the ancient seas which once covered the land we know call Wisconsin, including the presence of stromatolites in our rocks – fossilized microbial mats which thrived in the shallow waters of Cambrian and Ordovician Wisconsin. We express our deep gratitude to Philip for filling in on such short notice – we hope he’ll join us again on a geologic excursion in the Preserve!
Report written by Cole Roecker, photos by Signe Holtz.
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