AN EARLY WISCONSINAN TILL IN SOUTHERN MINNESOTA?

A thin, friable surface till forms a band east of the late Wisconsinan Des Moines lobe, ranging in width from 25 to 30 kilometers. With an average thickness of about 2 meters, it reaches 4 meters at its thickest point, and is also absent in places. The till drapes over a stream-dissected landscape with at least 35 meters of relative relief, underlain mostly by firm pre-Illinoian tills. Wood recovered from the till has been dated as >45370 BP.

In general, the "friable till" has less clay than either the Sheldon Creek till (the easternmost Des Moines lobe till) or the pre-Illinoian tills, but its texture is variable and changes according to the underlying materials. In contrast to the Sheldon Creek member, Pierre shale is not present. The "friable till" is generally less oxidized (10YR 5/5 ) than the underlying tills, which are oxidized to 6 or 8 chromas in places. In one gravel pit, the underlying oxidized, leached gravel is deformed and incorporated into the "friable till," which proves the latter is a glacial, rather than a mudflow deposit.

The radiocarbon date shows that the till is pre-late Wisconsinan, unless the wood is recycled. The till is younger than both the stream erosion that has dissected the pre-Illinoian tills, and the highly weathered outwash which occupies the valley bottoms in part of this dissected area. The new till could thus be either Illinoian or early Wisconsinan in age.