INTERPRETATION OF THE INTERLOBATE MARGINS OF LATE WISCONSIN ICE LOBES IN INDIANA: 100 YEARS LATER

Leverett and Taylor’s epic 1915 work, The Pleistocene of Indiana and Michigan and the History of the Great Lakes (U.S. Geological Survey Monograph 53) remains a foundation by which all surficial mapping in the region is based. Their observations were keen and well-documented; however, their interpretations were not always correct. We are only able to make this judgment with the modern advantages of detailed topographic data like LiDAR, air photos, and various databases such as soils and water-well records. Modern analytical techniques allow us to differentiate subtle (or not so subtle) textural and mineralogical differences in glacial sediments, as well as determine the depositional age of the sediments. Our contemporary knowledge of glacial processes, along with larger and more detailed data sets permit us to improve upon and reinterpret some of the conclusions made regarding the interlobate zones in Indiana.

Leverett conceptualized a single, great advance of coalesced ice from the Lake Michigan, Saginaw, and Huron-Erie basins. Using this concept, he interpreted a series of laterally contemporaneous moraines across the region. Studies in west-central Indiana confirm that the Lake Michigan and Huron-Erie ice lobes had alternating advances, contrary to Leverett’s conclusion that “neither … persisted after the other had withdrawn or melted away.” (p. 29) Detailed mapping of sediment characteristics, morphosequences, and stratigraphic relationships in northern Indiana has revealed the extent of the Saginaw Lobe, as seen by Leverett, to be much more constrained along its western and eastern borders, the Maxinkuckee and Packerton Moraines, respectively. Current studies are working to refine the textural and mineralogical characteristics of the glacial sediments and the chronology of their deposition to define the ultimate margins of the ice lobes in the interlobate region. Initial results reveal uncertainties with regards to correlating subsurface units (e.g., tills) between separate lobes based on clay mineralogy and particle size, as tills from all three lobes are dominated by illite and silt.