A TALE OF TWO LOBES: THE PLEISTOCENE EVOLUTION OF INDIANA'S LARGEST INTERLOBATE LAKE BASIN

Lake Wawasee has the largest surface area of any natural lake in Indiana, while nearby Tippecanoe Lake is the deepest. They are among a group of large, deep lakes (and dozens of smaller ones) concentrated in a distinctly rugged, ~100 mi²area that defines the interlobate boundary between the Saginaw and Erie Lobes in Kosciusko County, north-central Indiana. Ongoing geologic mapping indicates that this more southerly part of the boundary experienced a sharply different history than the segment further to the northeast, which generally lacks large lakes, has more subdued topography, and in most places is characterized by the relatively simple onlap of large Erie Lobe fans into basins produced by the withdrawal of the Saginaw Lobe into Michigan.

Several lines of evidence indicate that extensive tracts of stagnant Erie Lobe ice were well established in these lake basins before the Saginaw Lobe arrived, and long before deposition of the large eastern fans (Topeka, Leesburg, Rochester) that followed. Individual ice blocks occupied basins as great as 10-12 mi²and were (minimally) hundreds of feet thick; moreover, they persisted for most of the late Wisconsin, greatly altering the behavior of subsequent glaciers and ultimately controlling the distributions and character of younger deposits of both the Saginaw and Erie Lobes.

The deposits from this earliest Erie Lobe event thicken southwestward into eastern Marshall and extreme western Kosciusko Counties, where they form an extensive region of Erie Lobe moraines and fans known as the Bourbon upland. As such, they seem to define a discrete southern edge of the interlobate area—one that lies significantly further north than suggested by historical interpretations. Moreover, these relations reinforce the growing recognition that the history of the interlobate area is not monolithic, but instead consists of a heterogeneous group of smaller regions, each defined by local ice margins competing for space.