PLEISTOCENE GEOLOGY OF PART OF CENTRAL SOUTH DAKOTA1

Abstract

The area around Pierre, South Dakota, is bisected by the trench of the Missouri River, which forms a boundary between a region mostly covered by glacial deposits that overlie Pierre shale on the east and a region directly underlain by shale on the west. The earliest Pleistocene deposits are remnants of stream alluvium derived from the region to the west. These are now perched on some of the highest knobs in the area and bear witness to an inversion of topography caused, in part, by a lowering of base level when the Missouri River trench was formed marginal to the glacier of the Illinoian age. In the Wisconsin age, the glacier of the Iowan sub-age crossed the Missouri trench and in one place extended 20 miles farther west. The westward extension of each successive glacier was less than its predecessor. The glacier of the Tazewell sub-age reached the Missouri River, and in the course of its retreat built two massive end moraines in the area. Deposits of this substage cover most of the area east of the Missouri. Till of the Cary substage is restricted to the northeast corner of the area; the drift border of the glacier of the Mankato sub-age lies about 15 miles east of the area investigated in detail. Glacial drift of the several substages of the Wisconsin is locally mantled with as much as 30 feet of loess. The Tazewell-Cary interval is represented in this loess by a buried, immature soil profile.