Abstract

The area described completes the detailed mapping of the glacial geology of the Green Bay lobe. The drift belongs to two substages of the Wisconsin stage of glaciation. The major topographic features due to glaciation are the result of the Cary substage. Above this drift lies the till of a later substage, tentatively named Valders because its relation to the Mankato drift of Minnesota is unknown. Separating the two drifts is the Forest Bed which indicates a retreat of the ice margin far enough to free from ice both the Straits of Mackinac and the entire St. Lawrence valley. The advance of the Valders ice buried the two creeks Forest Bed with lake deposits formed in front of the ice. The age of this deposit by radiocarbon dating is about 11,400 years. The Valders till is red and is high in clay and silt because the ice readvanced over red lake clays formed during the Two Creeks interval of recession. Valders till smoothed and almost obliterated Cary land forms but was so thin over wide areas that it is no longer identifiable. These areas were not nunatacks. When the Valders ice receded, lakes were again formed in front of it. Four outlets of Lake Oshkosh in the Green Bay lowland across the Niagara cuesta to Lake Chicago in the Lake Michigan basin are described. Opening of the last outlet caused the two lakes to merge into Lake Algonquin. Ice was not far away then since varved clays accumulated in this lake. Further recession of the ice margin uncovered an outlet to the northeast which caused the water level to fall about 350 feet below that of the present. Evidence of this low level, now named Lake Chippewa, is described from this area. Rise of the land to the northeast caused the water to rise to the level of Lake Nipissing about 20 feet above the present. Erosion of the modern outlet of lake Huron caused the fall to present conditions.

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