GRAIN-SIZE VARIATION AMONG MORAINES OF A RETREATING LAURENTIDE ICE SHEET LOBE

During the Last Glacial Maximum, the Lake Michigan Lobe of the Laurentide Ice Sheet covered much of south-eastern Wisconsin. As this lobe retreated to the east, into the Lake Michigan basin, parallel moraines were deposited documenting the changing edge of the Ice Sheet. Racine County contains multiple moraines from this deposition, ranging from the LGM moraine in the west to closely-spaced recessional moraines in the east. Building off prior research in Racine and Kenosha County on a Lake Border Moraine, sampling was extended north along the moraine crest and west to older moraines within Racine County. Samples were taken, from West to East, from the oldest Darien Moraine comprised of the New Berlin Member of the Holy Hill Formation to the respectively younger Valparaiso, Tinley, and Lake Border Moraines comprised of the Oak Creek Formation. At locations within Racine County Parks and with land usage histories showing little or no recent agricultural use, shallow samples (<1 m) were taken from roughly 20 locations. Initially, 5 have been identified as likely of Holocene fluvial origin and the other 15 likely have classified as till or other glacial deposits. For the glacial samples less then 2mm in size, the bulk of the material, a Malvern laser particle size analyzer was used to obtain grain size distributions. In general, many of the samples exhibit bimodal grain size distributions, common in glacial deposits. The grain size averages are generally in fine silt and smaller, but considerable spread in the grain size distribution is common. The initial grain size analysis has revealed that there is extreme variation both among and within the till deposits, with little spatial coherence. This variation likely reflects the various depositional environments that can exist at the front of a receding ice sheet.