Farmdale Glaciation in Northern Illinois and Southern Wisconsin

Abstract

The upper (Pecatonica) drift of the Pecatonica River basin and adjacent areas of northern Illinois and southern Wisconsin is believed to offer the best record in North America of a late Farmdale substage, Wisconsin stage of glaciation. The thinness of the drift and lack of end moraines, coupled with glacially deformed bedrock, indicate a brief but intensive glaciation by a glacial lobe moving from the North Bay, Ontario, area across Rock River and westward against the drainage of Pecatonica basin. Evidence favoring assignment of the Pecatonica drift to the Farmdale substage include: (1) its areal position beyond the outer margins of the Labradorean Iowan, Tazewell, and Cary drift sheets, (2) its strati-graphic position just above deeply weathered Illinoian drift, (3) a paleoregosol locally preserved in the upper part of the drift where it lies beneath Iowan loess, (4) the fact that the Farmdale loess which is widespread beyond the Pecatonica drift border has not been found either above or below the Pecatonica drift (thus indicating contemporaneous deposition), and (5) radiocarbon age of from 29,000 to about 31,000 B.P.