MAPPING PRE-WISCONSIN GLACIAL AND PERIGLACIAL DEPOSITS IN GREEN COUNTY, SOUTHERN WISCONSIN

The Wisconsin Geological & Natural History Survey (WGNHS) is currently mapping Green County in south-central Wisconsin at the 1:100,000 scale. Green County bridges two distinct landscapes found in Wisconsin: the Driftless Area, which has no evidence of glacial sediment, and the glaciated majority of the state. Green Co is the western-most location in Wisconsin of glacial sediment deposited directly by a pre-Wisconsin-age ice sheet (Illinoian; correlated to MIS 6). Little research has been conducted on reconstructing the age, thickness, and nature of these sediments in south-central Wisconsin. However, OSL dates from Rockford, IL, constrain the age of this sediment to ~150,000-128,000 years old.

Map units in the glaciated region of Green County follow the WGNHS Lexicon of Stratigraphic Units; map units in the unglaciated portion have been developed based on lithology, landscape position and geomorphic process. Lithology is characterized using soil data, exposures and outcrops, and sediment cores, while landforms are classified using lidar DEM-derived hillshade rasters and satellite imagery. Glacial landforms deposited during the Illinoian glaciation include morainal sediment (restricted to far NE Green County), flutes (SE Rock County), and hummocky topography on top of preexisting dendritic topography.

In June 2023, a 10-cm-diameter rotosonic core was collected on the floodplain of the Pecatonica River near Browntown in southwestern-most Green County, WI (42.568627° N, -89.811533 °E). This southeast-flowing river was dammed by MIS 6 ice advancing southwest out of the Lake Michigan lowland, causing an extensive lake to fill dendritic valleys in the Driftless Area in southern Wisconsin. A continuous core was collected down to the bedrock surface at ~32 m depth. This core, along with shallower 4.5-cm-diameter Geoprobe cores collected in similar depositional settings, will provide new insight into Illinoian glaciations in the region.