Core samples taken from the deeper parts of Lake Michigan show a complete sequence of clay deposits from glacial to present time, whereas samples taken from depths less than 350 feet show that a part of this deep water sequence is missing and replaced by a shallow-water deposit of sand and shells, overlain by normal deep-water clays. The shells are of species of gastropods and pelecypods whose living representatives inhabit water 1–15 feet deep.
The level of this low-water stage is placed at 350 feet below the present lake surface, and the time of the low-water stage at post-Algonquin and pre-Nipissing. This low-level Lake Michigan drained through the Straits of Mackinac into a low-level Lake Huron, as was proposed by G. M. Stanley in 1936.
Features of the low-water stage in the Lake Michigan basin are named Southern Lake Chippewa, Grand Haven River, Lake Chippewa, and Mackinac River. The low-water stage in the Lake Huron basin is named Lake Stanley.