Drift Thickness

by Anthony H. Fleming and Marni D. Karaffa

Unconsolidated sediments of Pleistocene age underlie all of Allen County and range from about 30 ft to more than 300 ft in thickness (Fleming, 1994). Differences in thickness are attributable to as much as 200 ft of relief on the bedrock surface and to variations in the amount of glacial sediments deposited. Allen County experienced repeated glacial advances from both the north and east throughout the Ice Age, and deposits ranging in age from late Wisconsin to perhaps as old as pre-Illinoian are present. However, the bulk of the glacial deposits are associated with three glacial episodes of late Wisconsin age. The thickest deposits are in northern Allen County, where late Wisconsin sediments deposited by both northern source (Saginaw Lobe) and eastern source (Huron-Erie Lobe, Erie Lobe) ice overlie locally thick sections of pre-Wisconsin age (figs. 1 and 2).

Figure 1.
Highslide JS
Figure 1. Diagram showing origin and distribution of glacial lobes in the upper Midwest. Modified from Bleuer (1974).
  
Figure 2.
Highslide JS
Figure 2. Cross section showing the relationship of the Lagro Formation (Erie Lobe), Huntertown Formation (Saginaw Lobe), and Trafalgar Formation (Huron-Erie Lobe). Modified from Fleming (1994, Plate 3, Inset 1).


Deposits are considerably thinner in southern and eastern Allen County, and are almost entirely eastern source sediments deposited during the late Wisconsin. In addition to sediments deposited in direct association with glacial ice, a variety of postglacial deposits are also present, as are extensive bodies of glaciolacustrine sediments associated with several phases of ancestral Lake Erie.


References:

Bleuer, N. K., 1974, Geologic story of Pokagon State Park–legacy of Indiana's Ice Age: Indiana Geological Survey State Park Guide 1.

Fleming, A. H., 1994, The hydrogeology of Allen County, Indiana–a geologic and ground-water atlas: Indiana Geological Survey Special Report 57, 111 p.