EVIDENCE OF TWO BOULDER PAVEMENTS IN THE MINNESOTA RIVER VALLEY, WEST CENTRAL MINNESOTA

Boulder pavements are planar, single-stone thick concentrations of glacially modified clasts which occur between or within glacial units. An extensive boulder pavement occurs in the Minnesota River valley in west central Minnesota. Different models of boulder pavement formation have been suggested for this unique glacial deposit. Five sites located in tributaries of the Minnesota River valley in Chippewa and Yellow Medicine Counties were studied. Each site consisted of an outcrop of till with at least one horizon of faceted and striated boulders. Till stratigraphy and provenance was studied to determine if there were multiple boulder pavements and relative age, source and origin. Stratigraphic units were distinguished in the field on the basis of color, texture and clast lithology. The sediment above, below and surrounding the boulders was sampled and analyzed. Dry sieving and point counts were done on each sample. Results of this study indicate that two distinct boulder pavements are exposed in the Minnesota River valley. Till analyses indicate the boulder pavements were deposited during different glacial events. Because multiple horizons of boulder pavements occur in the Minnesota River valley and elsewhere (e.g. Saskatchewan and Brazil), depositional models for boulder pavements cannot call upon "overly" unique glacial conditions. Research for this study was funded by a grant from the N.F.S.- R.E.U. Program (NSR-EAR 9820249).