ANALYSIS OF THE HOMEGENEITY OF THE NEW ULM TILL OF THE DES MOINES LOBE OF THE LAURENTIDE ICE SHEET: IMPLICATIONS FOR A DEFORMING BED

The nature and dynamics of the Des Moines lobe of the Laurentide Ice Sheet have been the subject of much research. Some models have suggested that a deforming bed may have influenced the Des Moines lobe flow and geometry. Because some models for the flow of the Des Moines lobe are regional in extent, there is an implication of a sub- Des Moines lobe till layer that is behaving uniformly over an extensive area. This research project focused on the striated and faceted the boulder pavement which lies immediately below till deposited by the Des Moines lobe (New Ulm till), and the composition of the lowermost layer of the New Ulm till (till directly above the boulder pavement). The goal of this project was to determine if till composition is homogenous or if there are local differences in composition that may have been the result of local differences of the Des Moines lobe bed.

Eleven sample sights were chosen along the axis of the trough of the Des Moines Lobe (present Minnesota River Valley) from Browns Valley to Granite Falls, Minnesota. The Des Moines lobe occupied this trough, a pre-glacial topographic lowland, during the latest Wisconsinan. The geology of the trough in most localities includes a layer of shale rich till known as the New Ulm till, below which is a one stone thick striated and faceted boulder pavement. Most frequently the boulder pavement overlies the Granite Falls till but in places it lies on Cretaceous age shale.

Samples were prepared, dry sieved, and point counts were preformed on the coarse sand size fraction. Lithologies were identified and percentages were calculated for each sample. Lab results show that the lithology percentages vary between both a) samples from the same site and, b) samples from different sites. Thus it does not appear that the lowermost New Ulm till is homogeneous. Since this basal till horizon may have been deposited at any time the Des Moines lobe occupied the Minnesota River Valley no specific conclusion on the nature of Des Moines lobe flow can be made at this time. Future work will focus on the origin of this basal till layer.

Research for this study was funded by a grant from the N.S.F.-R.E.U Program (NSF-EAR 0640575).