Geological History
Horicon Marsh was created by the Green Bay lobe of the Wisconsin glaciation during the Pleistocene era. The glacier, during its advance created many drumlins (a type of knoll) in the region, many of which have become the islands of Horicon Marsh. The marsh and surrounding Dodge County have the highest concentration of drumlins in the world.
During the glacier's retreat, a moraine was created, forming a natural dam holding back the waters from the melting glacier and forming Glacial Lake Horicon. The Rock River slowly eroded the moraine, and the lake drained. As the levels of silt, clay and peat accumulated in the former lake's basin, the Horicon Marsh was formed.
The Horicon Marsh State Wildlife Area is one of nine units of the Ice Age National Scientific Reserve system, being considered to contain unique, representative evidence of the Ice Age of the Pleistocene era.
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