FRANK LEVERETT’S DESCRIPTION OF THE SAGINAW LOBE TERRAIN IN SOUTHERN MICHIGAN: HOW DOES IT STAND UP TODAY?

USGS Monograph 53 (1915) contains a detailed description of the Saginaw Lobe, the central lobe of the Late Wisconsin Laurentide Ice Sheet in Michigan and Indiana. The Saginaw Lobe lies between the Lake Michigan Lobe to the west and the Huron-Erie Lobe to the east. One of Leverett and Taylor’s (L & T’s) main objectives was to trace and correlate synchronous morainic systems from lobe to lobe and to track meltwater pathways, which they referred to as “gravel plains” or “lines of glacial drainage” downstream from their source. One hundred years later, the major conclusions of their work stand up quite well. For example, they recognized that the Saginaw Lobe had begun to retreat prior to the Lake Michigan and Huron-Erie Lobes, which then expanded into the footprint of the Saginaw Lobe.

L & T’s statewide 1: 1,000,000 scale surficial geologic map formed the basis for subsequent 1:500,000 scale maps by Martin (1955) and Farrand and Bell (1982). Although these later maps had the advantage of 1:24,000 and/or 1:62,000 scale topographic maps, soil surveys and air photos, the major ice-marginal positions were similar to those in Monograph 53.

Ongoing 1:24,000 surficial geology mapping in Barry and Calhoun Counties, the first detailed mapping of this area since L & T, provides a test of their work. Some features not recognized by Leverett, who mapped this area, such as the large drumlin field south of the Tekonsha Moraine, had already been incorporated into the Martin map. Conventional (30 m) and LiDAR DEMs, coupled with Rotosonic and Geoprobe drilling and coring, are leading to new interpretations of the sediment-landform assemblages between the Sturgis and Kalamazoo Moraines. The Sturgis Moraine was most likely constructed by a readvance that also formed the drumlins to the north. The final, active retreat of the ice front is marked by discontinuous ice-contact scarps and meltwater deposits, which partially bury the drumlins. Ice-marginal positions are most easily identified by coarse-grained subaerial fans deposited at tunnel valley portals. LiDAR DEMs show that L & T’s Tekonsha Moraine in most of this area is simply an upland and not a constructional ice-marginal landform. Despite these new clarifications based on detailed mapping, Monograph 53 remains a monumental achievement and the foundation of glacial geology in the Great Lakes region.