10Be exposure dates from pre-Illinoian glacial erratics deposited near the southern terminus of the Laurentide Ice Sheet

The remnants of multiple ice sheet expansions beyond the southern limits of the Wisconsinan glaciation (ca. 75−12 ka, Marine Isotope Stage [MIS]-4 and MIS-2) and the Illinoian glaciation (ca. 191−125 ka, MIS-6) are present in north-central USA. Sioux Quartzite-rich glacial erratics in northeast Kansas, USA, have marked the southernmost extent of the Laurentide Ice Sheet during the Quaternary (38°−39°N). However, the timing of pre-Illinoian ice advances remains uncertain. To date deglaciation, we measured cosmogenic 10Be in five quartzite boulders on hilltops near Wamego, Kansas (39.14°N, 96.26°W). Apparent exposure ages (248.3−74.7 ka) are younger than previous pre-Illinoian retreat evidence. These dates, coupled with exhumation histories, suggest complex geomorphological processes revealing the boulders are from a deposit, supporting their origin as lag deposits from a once thick, weathered till deposited at least several hundred thousand years ago.