The Influence of Bedrock Topography on the Origin of a Mid-Pleistocene Epoch Glacial Lake in Rock County, Southwest Minnesota

Abstract

A circular, closed depression 1.9 miles (3 kilometers) in diameter that was formerly occupied by a shallow lake is located in the glaciated landscape of northern Rock County, southwest Minnesota. The depression is partly framed by Sioux Quartzite and is situated above a bedrock swale on a broad, quartzite-supported upland that is thinly mantled by pre-Wisconsinan glacial deposits. The quartzite-supported upland has been a positive topographic feature since at least the Late Cretaceous Epoch; near the depression, its discontinuous cover of unconsolidated sediment consists of glacial till, outwash deposits, and loess that aggregate to a total preserved thickness between 1 and 82 feet (less than 1 to 25 meters). The depression originated from the melting out of a buried, tabular ice mass that was isolated near the stagnating margin of a mid-Pleistocene Epoch continental glacier. The ice mass was buried in outwash and then further buried by a thin till deposited when the glacier readvanced. The ice mass became isolated and was slow to melt because of its position in a bedrock swale on a topographic high that was near a dynamically fluctuating glacier margin; its relative thickness and protected location in the swale were key factors in its transient preservation. The closed depression that formed upon final melting of the ice mass has survived in the post-glacial landscape because of its location on a geomorphically persistent bedrock upland where the erosive energy of post-glacial and modern streams has been minimal. It has been speculated that this bedrock-framed, geographically unique circular depression may be a deeply eroded meteorite impact structure. No supporting evidence for this speculation has been discovered in the field or laboratory.