LATE PLEISTOCENE ICE-FLOW DIRECTIONS AND THE AGE OF GLACIAL LANDSCAPES IN NORTHERN LOWER MICHIGAN

Abstract

Lineation (alignment) of certain landscape features, most of which are glacigenic, and till-fabric data were mapped and used to indicate glacier flow directions in northern lower Michigan. I also reexamine the limit of the Greatlakean (ca. 11,850 14C yr B.P.) ice advance in northern Michigan. My study assumes that reddish-brown (7.5YR and redder) till in the region was deposited by the last ice advance, the Greatlakean, since this color till covers uplands immediately down ice from an established Greatlakean stratigraphic marker bed. Landscape linearity and till-fabric data within the reddish-brown till landscapes of northwestern lower Michigan both show strong northwest-southeast alignment, indicating that Greatlakean ice advanced out of the Lake Michigan basin. Earlier (Port Huron, ca. 13,000 14C yr B.P.) ice deposited brown till of 10YR hue, as evidenced at a borrow pit where reddish-brown Greatlakean till overlies brown drift. Thus, the “7.5YR and redder” vs 10YR hue limit is interpreted as the outer limit of the Greatlakean advance in northern lower Michigan. By mapping this limit only a few tens of km southeast of its only stratigraphically confirmed position at the Cheboygan bryophyte bed, I imply that Greatlakean ice did not advance as far into northeastern lower Michigan as others previously have suggested. Port Huron landforms in brown till show similar alignment and till fabrics to Greatlakean landforms in red till, suggesting that Port Huron ice in northern Michigan (also) advanced primarily out of the Lake Michigan basin. As it advanced from the northwest, flowlines and till fabrics suggest that the Port Huron ice gradually curved, eventually flowing north-to-south in northeastern lower Michigan, nearer the Saginaw Lobe. [Key words: glacial geology, drumlins, landscape fabric, Greatlakean advance, Port Huron advance, till fabric, glacial dynamics.]