A COMPARISON ANALYSIS OF WASHBOARD MORAINES IN CENTRAL MINNESOTA AND DES MOINES LOBE TILL

Washboard moraines are composed primarily of till with occasional lenses of moderately sorted sands and gravels; they are oriented transverse to ice flow; and are generally 3-6 meters high with variable widths and lengths. Washboard moraine complexes can extend over large areas (~10 to ~1000 km²) of relatively flat terrain and tend to be located adjacent to topographic highs. A comparison study of two washboard moranic complexes located to the north and south of the Minnesota River Valley (the former position of the Des Moines Lobe in Minnesota) addressed the implications that these complexes have on Des Moines Lobe ice dynamics. Field sample sites were located and mapped using aerial photographs and GIS images. At each of the sites, a hand auger was used to obtain a sample of the till at a depth of approximately five feet. These samples were then analyzed to calculate their sand, silt and clay ratios and determine their dominant lithology. Data from each of the samples, in one moranic complex, were compared against each other and to the lithologic composition of the underlying till. The findings of this study, based upon field and lab work, indicate that the northern complex is composed of predominantly clay-rich till that becomes coarser toward the margin, where as the southern complex is composed predominantly of sandy till that becomes finer toward the margin. The lithologic comparison of the two study areas suggests that the shale content in each complex decreases toward the margin; this correlates to a model of shale content for the underlying till. These findings suggest that the Des Moines Lobe till is a single, unified till sheet that is variable in composition along its margins. Research for this study was funded by a grant from the N.S.F.-R.E.U. Program (NSF-EAR 0353621).