PITTED TERRAIN IN MUSKEGON COUNTY, MICHIGAN: IMPLICATIONS FOR GLACIER LOBES AND LAKES DURING THE LATE WISCONSIN DEGLACIATION

Quaternary mapping in Muskegon County, Michigan has identified a complex terrain of pitted plains and isolated uplands in the northeastern portion of the county. This area was noted briefly by Leverett in Monograph 53 (p. 223). Depressions occur within plains or in gently sloping surfaces. Depressions range in size from small nearly circular forms tens of meters in diameter to larger polygonal and irregular forms over 3000 meters in their longest dimension. The depths of depressions range from 3-40 meters and larger depressions contain lakes and wetlands. Depressions are elongate or form elongate chains. The upland areas between depressions are locally flat, but gently slope over the county from ~ 216 m in the east to ~198 m in the west (slope is 1-2 m/km). This is similar to the elevations and slopes of outwash plains and Glacial Lake Chicago plain in southern Muskegon County where depressions are absent. Some uplands are completely surrounded by depressions. Sediments in the area are similar to those in southern Muskegon County that have been mapped as outwash sand and gravel, and lacustrine sand, silt, and clay. Clay and diamicton underlies sand in most of the area at depths of 10-50 meters, and occurs at shallow depth (<10 m) near Twin Lake, Michigan as a small upland ridge.

Depressions probably reflect areas where stagnant glacier ice was covered by outwash or glaciolacustrine sediments during deglaciation. Some of the smaller uplands may have formed in ice-walled lakes within stagnant ice, whereas other uplands formed as bays in a larger proglacial lake. The upland composed of diamicton and lake sediments near Twin Lake is probably a moraine or ice-thrust ridge of the Lake Border moraine system. Ice most likely was left by a stagnant Lake Michigan lobe following an advance, perhaps a surge event. Ice was then partially buried by Glacial Lake Chicago sediment and glaciofluvial sediment coming from the east or the downwasting Saginaw lobe. The thickness of stagnant ice must have been at least 20-40 m thick near the ice margin based on the depth of the depressions. An alternative hypothesis is that an earlier westward advance of the Saginaw lobe could have left stagnant ice that was later over-ridden by the Lake Michigan lobe. More detailed mapping and subsurface information will be needed to test these various multiple working hypotheses.