THE "PRE-ILLINOIAN" TILL OF SOUTHEASTERN MINNESOTA MAY ACTUALLY BE ILLINOIAN

The till east of the Bemis moraine in southeastern Minnesota has been considered pre-Illinoian since the early twentieth century. The only Illinoian drift recognized in Minnesota was the red drift (till and ice-contact sand and gravel) of the Hampton moraine, which contains abundant clasts from the Superior basin. Glaciations in Minnesota typically include a Labradorean, or northeastern source (Rainy and Superior provenances) and a Keewatin, or northwestern source (Winnipeg and Riding Mountain provenances.) The Labradorean domain was more extensive in Minnesota and Illinois during the Illinoian glaciation than in the Wisconsinan glaciation. It is likely that the Keewatin domain was equally extensive.

Recent work in the Rochester area suggests that the till of the Browerville Formation, first recognized in central Minnesota, is the surface till over much of southeastern Minnesota. The type Browerville Formation is the uppermost till below the sandy till of the Wadena drumlin field, which is believed to be the earliest Wisconsinan till. No numerical dates are available at this time; wood from the Browerville till is radiocarbon-dead. The simplest interpretation of its age is Illinoian, because otherwise Minnesota had no major northwestern-source glaciation during the Illinoian.

Correlation between the type Browerville Formation and the surface till in southeastern Minnesota is by texture and by grain counts in the 1-2 millimeter size fraction. The units share a distinctive suite of marine Cretaceous clasts, dominated by limestone rather than shale. They also share a similar abundance of Paleozoic carbonate clasts. The till under the Browerville Formation in southeastern Minnesota, called the Rose Creek till, is similar to the Browerville Formation, but a bit sandier and it contains a smaller proportion of the calcareous marine Cretaceous clast assemblage. They are separated by a layer of silty sediment and a paleosol that seems to represent less than a full interglacial period. The truly pre-Illinoian till under the Rose Creek till is highly oxidized, deeply leached, and relatively thin and patchy in many places.