The ice that built the ground: the interlobate fans

THE WISCONSIN EPISODE — READ IN THE SURVEY RECORD

This ground was built where two lobes of the ice met: The Indiana Geological and Water Survey's account of the South Bend quadrangle (Antinao and Rupp, in the Indiana Journal of Earth Sciences) lays the county's northeast in the interplay of the Saginaw lobe, coming from the northeast, and the Michigan lobe, coming from the north. At the Last Glacial Maximum, about 24,000 years ago, the lobes stood against each other along a hinged interlobate area north and northeast of South Bend — this ground. As the Michigan lobe's fronts retreated between about 24,000 and 17,000 years ago, they left ice-contact sediment ramps and pitted, collapsed fan-moraine complexes, and the survey's sentence for the hinge is the ground's own description: "a sequence of stacked diamicton and sand and gravel fan heads appears at the hinged interlobate area north and northeast of South Bend." The ice-margin positions are marked by the fans, by kames and by ice-walled lake plains. The survey record gives this complex no single moraine name, and none is imported here: the named Valparaiso Moraine runs through LaPorte County and into Michigan, west of this ground, and is not its name.

The lowlands came after. Most of the low ground is post-glacial — valley-train outwash of about 18,000 years ago, its flow toward the south and west — with the ice margin retreating first from south to north near Mishawaka, then from southeast to northwest across the Kankakee valley; sand and gravel colluvial and alluvial fans and late-glacial wind-blown dunes followed, and the Holocene changes since have been comparatively minor. What the ice left, the federal survey measured. The United States Geological Survey's study of northeastern St. Joseph County (Bayless, Westjohn and Watson, Water-Resources Investigations Report 95-4041, 1995; a study area of 580 square kilometres) gives the unconsolidated deposits — outwash, till and lake sediment together — at from 15 metres to more than 70 metres thick. North and east of the St. Joseph River the drift is mostly sand and gravel, with laterally discontinuous clay-rich layers within it; south of the river clay dominates. Beneath the drift lies the conductive Ellsworth Shale. The outwash sand and gravel is the primary drinking-water source for the county's northeast — the aquifers the farms and then the subdivisions of this ground have drawn on ever since — and the same sand-and-gravel frame, as the Juday Creek entry below shows, decides where the ground's one creek gains water and where it loses it.

Sources:

1. Daniel M. Antinao and Jonathan Rupp, "Quaternary Geology of the Indiana Portion of the Eastern Extent of the South Bend 30- × 60-Minute Quadrangle," Indiana Journal of Earth Sciences (Indiana Geological and Water Survey) — the Saginaw and Michigan lobes, the hinged interlobate area north and northeast of South Bend, the stacked diamicton and sand and gravel fan heads, the 24-to-17-thousand-year retreat, the post-glacial outwash and the retreat directions.
2. E. R. Bayless, G. B. Westjohn and K. S. Watson, U.S. Geological Survey Water-Resources Investigations Report 95-4041 (1995) — northeastern St. Joseph County: unconsolidated deposits 15 to more than 70 metres thick, sand and gravel north and east of the river against clay south of it, the Ellsworth Shale beneath the drift, and the outwash as the northeast's primary drinking-water source.