Physical Description and Significance of the Tills at the Dobmeier Pit Near Park River, North Dakota
This study was conducted to describe an exposure of tills and correlate them with the known sequence of tills elsewhere. The Dobmeier pit is a large spring discharge site approximately 3 miles west of Park River, North Dakota (Edinburg Quadrangle, T157N, R56W, Sec. 23, 1/4SE, 1/4NW, 1/4SE). It is between 50 and 75 meters across and 15 to 20 meters deep. It is located on the east edge of the Edinburg Moraine and the south side of the Park River valley. Exposed at the base is a cross-bedded sand and gravel unit. Above is a compact gray till, which is separated by a relatively thin, discontinuous sand lens. Overlying the gray till is a much less compact, yellow brown till. Above the upper till are lenses of cross-bedded sand then a tan, jointed, silty unit; above this the A and B soil horizons are developed in shaley sand and gravel. The pit walls were photographed, sketched and samples collected at 1 meter intervals in the tills and at either side of the contact of the gray and yellow-brown till. Colors of the samples were described using the Munsell Soil Color Chart. Texture was determined by the NDGS hydrometer and sieve method. The coarse sand fraction was then divided into four lithologic groups; shale, carbonate, crystalline, and other. The lower till is very dark grayish-brown to dark olive gray in color. The normalized texture is 34+5% sand, 45+5% silt, and 21+3% clay. The normalized coarse sand lithology is 51+7% shale, 24+6% carbonate, and 26+5% crystalline grains. The upper till is yellowish-brown to brownish-yellow and olive brown in color. The normalized texture is 40+20% sand, 45+21% silt, and 15+ 1% clay. The normalized coarse sand lithology is 47+7% shale, 26+6% carbonate, and 27+1% crystalline grains. The results of this analysis were compared to previous descriptions of tills of the region. The lower unit compares well with the Dahlen Formation. The till of the Dahlen Formation was deposited by a glacier that moved in from the northwest in Late Wisconsinan time, about 12,000 years BP. Stratigraphically, the upper till is most likely the Falconer Formation of which the Edinburg Moraine marks the western extent. The Falconer Formation was deposited in latest Wisconsinan time by a readvance of the same glacier that deposited the Dahlen Formation before 11,000 years BP.
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