Date of this Version
AMERICAN JOURNAL OF SCIENCE, VOL. 281, APRIL, 1981, P. 375·389
A stream bank along a tributary of Salt Creek in east·central Nebraska displays a sequence of sediments deposited along an eastward flowing stream during part of the approach and subsequent retreat of the last ice sheet to expand into this area during the Kansan glaciation. The section exposes a series of beds of calcareous silt which contain fossil·rich lenses. Sediment characteristics and the habitat requirements of 31 species of snails recovered suggest that the silts accumulated on an aggrading floodplain and probably were derived from erosion of unweathered cal· careous till and calcareous wind·blown silts deposited during an earlier ice advance from outwash a short distance to the west. Aggradation of this stream is attributed to a rise of base level that accompanied deposition of outwash along the Missouri River from an advancing glacier.
A disconformity separated these fine·grained sediments from a fining upward sequence of outwash which probably was deposited along the ice margin during the retreatal phase. Glacial ice surely buried the area when its margin advanced to about 12 km west of the site, but erosion by meltwater currents removed any till that may have been left in this location. Non·glacial alluvium overlies the outwash. Post-glacial dissection created a valley a little less deep than that of the modern stream, which is partly filled with pre-Wiconsinan alluvial sediments and by Peoria (Wisconsinan) loess.
The molluscan assemblage from the floodplain silts contains a few species that indicate a Kansan age. The vegetation was dominantly grassland with patches of woodland and shrubby areas that included a few spruce trees. The environment indicated by the fossils suggests a climate a little cooler and more moist than the the present one, perhaps comparable to that in west-central Minnesota or eastern South Dakota today.