Late Pleistocene Shorelines and Stratigraphic Relations in the Lake Michigan Basin

Abstract

Recent investigations in the Ontario, Huron, and Erie basins and in the Glacial Grand River demonstrate that the entire Glenwood Stage (640 ft above tide [a.t.]), and at least an early portion of the Calumet Stage (620 ft a.t.), of Glacial Lake Chicago are pre-Twocreekan. Glenwood and Calumet shoreline features can be traced from Manistee, Michigan, southward around the south end of Lake Michigan and northward to Two Rivers, Wisconsin. The presence of Glenwood shore features on the red drift between Muskegon and Manistee, Michigan, and between Milwaukee and Two Rivers, Wisconsin, demonstrates a pre-Twocreekan age for these drift deposits. Stratigraphic relations in Manitowoc County, Wisconsin, strongly suggest that the Glenwood deposits there are younger than the type Valders till at Valders, Wisconsin, and that the type Valders deposits are pre-Twocreekan in age. Furthermore, the red-clay till north of Manistee, Michigan, and Two Rivers, Wisconsin, where it overlies the Two Creeks forest bed, is post-Glenwood in age and thus is not correctable with the type Valders till, as previously assumed. The use of the term “Valders” for the post-Glenwood till is rejected and the new name “Two Rivers till” is proposed for the Manitowoc County area. The term “Valders” is restricted to the type till at Valders, Wisconsin, and to those tills which are unequivocally correctable with the type deposits. It is suggested, although the name is not extended, that the lower red till at the Two Creeks type locality and the red till occurring as far south as Milwaukee, Wisconsin, are correctable with the type Valders deposits. The red till occurring between Whitehall and Manistee, Michigan, is also correlatable with the type-Valders till as well as with the Port Huron drift in Michigan. Frye and others (1968) defined the base of the Valderan Substage as the contact of the Valders till on the Two Creeks forest bed. Having concluded in this study that the Valders till does not overlie the forest bed (but correlates with the till which underlies it), the continued use of the term “Valderan” for the time represented in part by the till over the Two Creeks forest bed is confusing and the term “Valderan” should be abandoned. As traced northward from the area of horizontality at the southern end of Lake Michigan, the Glenwood shoreline features rise from 640 ft a.t. to a 660 ft a.t. platform which continues at this level to its northern limit where it is buried by deposits of the Two Rivers advance. Data for intermediate levels between the Glenwood and Calumet is sparse, and no definite conclusions can be drawn concerning the attitude, uplift, and correlations of the intermediate shorelines. The Calumet shoreline features remain horizontal throughout the area investigated.