GLACIAL HISTORY OF WISCONSIN – AN OVERVIEW

All of Wisconsin was glaciated during the Quaternary Period other than the Driftless Area in southwestern Wisconsin. Researchers have formalized till lithostratigraphic units (Mickelson et al., 1984; Attig et al., 1988) and expanded our understanding of the glacial history of Wisconsin, as summarized by Syverson and Colgan (2004). This overview paper provides a context for other talks in the session.

Till characteristics provide evidence for lobate ice flow from the northwest (silty, calcareous), the Superior lowland (red with abundant rift lithologies), and the northeast (calcareous). Till units pre-dating the Wisconsin Glaciation are quite weathered and lack original glacial landforms. The oldest till units (Pierce and Marathon Fms.) display reversed remanent magnetization, so they are likely > 460,000 yrs old (Baker et al., 1983; Syverson et al., 2005). Red till from the penultimate “Illinoian” glaciation (River Falls Fm.) is present in western Wisconsin.

Ice activity during the Late Wisconsin Glaciation (~30,000-10,000 yrs ago) did the most to shape the Wisconsin landscape. Impressive hummocky moraines and drumlin fields formed during this time. The Green Bay Lobe dammed the Wisconsin River near the Baraboo Hills and formed Glacial Lake Wisconsin. Catastrophic drainage of this lake eroded the Wisconsin Dells. As ice wasted back across drainage divides, large ice-dammed lakes formed in the Superior (L. Duluth), Green Bay (L. Oshkosh), and Lake Michigan (L. Chicago) lowlands. Readvancing glaciers altered lake levels, eroded silt and clay from these lake beds, and deposited stone-poor, fine-grained till units near the shores of Lakes Michigan and Superior (e.g. Oak Creek, Kewaunee, and Miller Creek Fms.). Red till in the Green Bay and Lake Michigan lowlands (Kewaunee Fm.) indicates water drainage (and iron contamination) from the Superior region. Ice-wedge polygons and ice-wedge casts reflect permafrost conditions in Wisconsin during the Late Wisconsin Glaciation (Clayton et al., 2001).

The Two Creeks Forest was growing in northeastern Wisconsin by 12,000 ¹⁴C yrs ago, and ice last advanced out of the Superior lowland ~9,900 ¹⁴C yrs ago. Lake levels in the Superior, Green Bay, and Lake Michigan lowlands fluctuated as new outlets opened and isostatic rebound altered outlet elevations.