MAPPING THE ELEVATION OF THE MINNESOTA RIVER VALLEY BOULDER PAVEMENT: A PROXY RECORD OF THE GEOMETRY AND SLOPE OF THE BASE OF THE DES MOINES LOBE

Boulder Pavements are planar concentrations of glacially modified clasts, which can occur between or within glacial units. Throughout the Minnesota River Valley an aerially extensive striated and faceted boulder pavement frequently crops out at the base of the Late Wisconsinan age New Ulm till – a deposit of the Des Moines lobe of the Laurentide ice sheet. The pavement is one stone thick, consisting of boulders that are striated and faceted and striae have a preferred orientation to the south-southeast (flow direction of the Des Moines lobe). Individual boulders are relatively flat lying, and the pavement as a whole has a gentle gradient. In general striae and facets are found only on the upper surface of the clasts.

Although the method of clast concentration is enigmatic, the Minnesota River Valley boulder pavement appears to have formed under the direct influence of the Des Moines lobe (e.g. boulder emplacement and clast modification: Cotter et al, 1992). Thus it can be assumed that the boulder surfaces of the Minnesota River Valley boulder pavement approximates the elevation of the base of the Des Moines lobe during at least a portion of the latest Wisconsinan advance. The goal of this project was to map the location, elevation, clast lithologies and striae direction of boulder pavement exposures. Topographic maps and a GPS unit were used to determine elevation and outcrop locations and a brunton compass was used to determine striae direction.

Boulder pavement outcrop elevation increases from the southeast to the northwest, indicating that the base of the Des Moines lobe had a slight gradient to the southeast. An overwhelming majority of the boulder pavement clasts were granites but gneiss, schist, and quartzite clasts were also present. All the striae mapped had a southeast orientation, which was the flow direction of the Des Moines lobe in the Minnesota River Valley.

Additional research will focus on increasing the resolution of the Boulder Pavement record of the base of the Des Moines lobe. Research for this study was funded by a grant from the N.S.F.-R.E.U. Program (NSF-EAR 0640575)