THE SIGNIFICANCE OF GLACIODELTAIC DEPOSITS ALONG THE CREST OF THE LAKE BORDER MORAINE, NORTHWESTERN LOWER MICHIGAN

The map of moraines in Michigan has changed little since Leverett and Taylor’s (1915) seminal publication. We report on the Port Huron and Lake Border moraines in northwestern lower Michigan and show evidence for significant changes to their originally mapped locations. The Port Huron moraine is a large accumulation of mostly sand and gravel that reaches altitudes over 320 m, 80 m above the adjacent valley to the west. As currently mapped however, the Port Huron moraine abruptly ends on the north side of the Manistee River valley. Older maps suggest that it continues to the southwest through series of small hills to Lake Michigan at Ludington. To the east, the older Lake Border moraine, much of which exceeds 400 m altitude, is mapped in contact with the Valparaiso/Charlotte interlobate moraine at the latter’s northwest corner near Harrietta. It then curves southwest in a large reentrant that returns to the interlobate moraine 67 km to the south in Newaygo County.

Recent mapping of deposits indicates that the existing ice-margins represented by these moraines are likely incorrect. Much of the Lake Border and interlobate moraines are glaciodeltaic sediment. At the maximum extent of the Lake Border glacier, a high level lake was bounded by older interlobate deposits to the south and the Saginaw lobe to the east. As the Lake Border glacier retreated, large deltas formed west of the Lake Border moraine, but were supplied by water and sediment from Saginaw lobe. The Port Huron moraine did not dam a similarly high level proglacial lake and is predominantly composed of glaciofluvial deposits.

We present a more accurate position of the Port Huron moraine extending due south across the Manistee River where the Lake Border moraine is traditionally drawn. The Port Huron moraine then arcs to Lake Michigan in Oceana County where uplands are also composed of glaciofluvial deposits. The adjusted position of the Lake Border moraine is still in contact with the western edge of the interlobate moraine until the Lake-Newaygo County line where uplands composed of glaciodeltaic deposits are mapped. These reinterpreted positions also eliminate soil type disparities that cross the traditional moraine boundaries.