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