Influence of Nearshore Till Lithology on Lateral Variations in Coastline Recession Rate Along Southeastern Lake Michigan

Abstract

Retreat of coastal bluffs around margins of the Great Lakes is a continuing process. Despite the positive correlation which exists between lake level and recession rates, considerable lateral variation in rates is typical of many coastal areas, while causes of this spatial variation are not well understood. Detailed examination of a 10 km segment of Lake Michigan shore near the town of Glenn, Michigan, suggests that lateral variations in the lithology of Pleistocene drift are directly correlative with spatial variations in recession rates. High recession rates and concave shorelines occur along segments composed of either outwash sand or sandy till which contains gravel up to 2 cm in diameter as the coarest clast size. Conversely, low rates and the occurrence of convex shoreline segments are related to exposures of bouldery till. The exact nature of the relationship between bouldery till and the low recession rates is ambiguous. Features observed along this area suggest two non-mutally exclusive possibilities: 1) during erosion, bouldery till develops flat gently sloping wave-cut benches in the upper shore face; and 2) erosion of this coarse till results in the development of upper shoreface surfaces covered with large boulders. Both features may serve to attenuate wave energy in the nearshore zone. In either case, lateral varition in the recession rates near Glenn is greatly influenced by the lithology of glacial drift exposed in bluff faces and the nearshore areas.