The glacial record in regions surrounding the Driftless Area

ABSTRACT The Driftless Area of southwestern Wisconsin is defined by its lack of glacial deposits; therefore, the geographic limits of the Driftless Area are delimited by glacial deposits of various ages. The northern boundary of the Driftless Area is marked by the patchy distribution of pre-Illinoian (pre–marine oxygen isotope stage [MIS] 8) deposits that contain clasts from the north and northwest. The western margin is also delimited by pre–MIS 8 sediments that extend roughly coincident with the Mississippi River. This boundary is best identified by the topographic feature of the Bridgeport moraine near the confluence of the Mississippi and Wisconsin rivers and by the presence of patchy till underlying MIS 2 loess on upland surfaces in southwesternmost Wisconsin. The southeastern margin is delimited by extensive Illinoian (MIS 6–8) deposits with eroded glacial landforms evident in modern light detection and ranging (LiDAR)–derived digital elevation models. The eastern margin is marked by well-preserved glacial landforms and sediment associated with the MIS 2 glaciation. The Green Bay Lobe of the Laurentide Ice Sheet deposited prominent moraines that dammed numerous ice-proximal lakes. The most significant of these was glacial Lake Wisconsin, which formed when the Green Bay Lobe dammed the Wisconsin River at the east end of the Baraboo Hills. The most recent episode of permafrost in the Driftless Area lasted from ca. 33 to 14 ka; a suite of permafrost-related features that formed sometime during this time period can be found throughout the Driftless Area.