THE SEARCH FOR EARLY WISCONSINAN GLACIATION IN MINNESOTA

Early geologists mapped early Wisconsinan tills and ice margins outside the late Wisconsinan margins in parts of the Midwestern United States. In recent decades, most of these tills have been reinterpreted as either late Wisconsinan or as Illinoian. But Rittenour's (2004) dating of braid-belt sediments in the lower Mississippi River valley makes clear that meltwater had entered the Mississippi River drainage more than 50,000 years ago. It seems likely then, that early Wisconsinan deposits should be present in Minnesota, which includes the northernmost part of the watershed. So where is the till? And how can one tell the age?

Let's start by looking at till surfaces that show late-Wisconsinan periglacial activity, outside the main end moraines. They are thus older than part of the late Wisconsinan. They are subdued rather than hummocky, and are covered with patchy eolian sand and silt. These till surfaces are: 1. The Wadena drumlin field of central Minnesota, 2. Deposits of the Des Moines lobe outside the Bemis moraine, 3. Superior-lobe till in Minnesota and Wisconsin outside the St Croix moraine, and 4. A clay-poor till east of the extra-Bemis deposits in southeastern Minnesota.

1. The Wadena drumlin field itself is probably early Late Wisconsinan because the western part of the drumlin field appears to be collapsed by melting of pre-drumlin ice. 2. The extra-Bemis Des Moines lobe (and an equivalent area of the James lobe in South Dakota) is dated in the 20,000s B.P. The oldest date, about 40,000 B.P., is still much younger than the oldest Mississippi River braid belt. The equivalent till in the James lobe overlies a deep weathering profile, with no intervening till. 3. No radiometric ages are known from the extra-St. Croix till. 4. The clay-poor till east of the extra-Bemis deposits produced a wood date of >45,370 B.P., and a log from mudflow sediment in Rochester suggests that periglacial activity was active in the area before 46,610 B.P.

Deposits that have been traditionally mapped Illinoian and pre-Illinoian in Minnesota are probably indeed pre-Wisconsinan. Their upper surfaces, where not strongly eroded, preserve a weathering profile of depth and intensity comparable to the Sangamon geosol. The conclusion is that there may have been extensive early Wisconsinan glaciation from a northeast source but not from a northwest source.