STRUCTURE AND DEPOSITION OF THE BLUE MOUNDS, WEST-CENTRAL MINNESOTA

The Blue Mounds are made up of dozens of individual sand and gravel-cored ridges that trend generally northwest to southeast in Pope County of west central Minnesota. This unique landform lies atop the Alexandria Moraine and was, at least in part, deposited by the retreating Des Moines lobe. The Blue Mounds have been interpreted as glacial stagnation ridges, eskers, crevasse fills, and interlobate glaciofluvial deposits. The goal of my study was to analyze a portion of the Blue Mounds in detail in an attempt to shed light on the depositional environments and the origin of this enigmatic glacial landform. Stratigraphic sections in two of the mounds were cleared, measured, described and sampled. Seven stratigraphic sections were studied in the first mound in a variety of orientations and elevations. One stratigraphic section was described in the second mound. Sediment samples were analyzed to determine grain size distribution and sand grain lithologies, an indicator of provenance. Stratigraphic sections reveal an array of sedimentary structures including bedding, laminations, faults, deformation features, and crossbeds, sometimes in wedge sets. Some areas contain no structures. Grain sizes range from above 2 mm to finer than 0.063 mm and sorting in most samples is poor. Lithologic analysis through point counts reveals low carbonate percentage as well as low shale percentage which may be due to leaching and sorting at the time of deposition. Des Moines lobe sediments are characterized by the presence of shale, the content of which was also low in the samples. The complex sedimentary structures of the Blue Mounds indicate varying current directions and variable strength of flow. Most data suggests decreasing energy over time in a fluvial or glaciofluvial environment. Thus the Blue Mounds may be eskers or crevasse fills. Research for this study was funded by a grant from the N.S.F.-R.E.U. Program (NSF-EAR 0353621).