Abstract
The geomorphic history of two fingertip drainage basins in the Iowan region of northeast Iowa was reconstructed from borehole observations. A multicyclic land surface (a residual upland with Yarmouth-Sangamon paleosols on Kansan till, a lower erosion surface with Late Sangamon paleosols, and still lower Iowan erosion surfaces in till without paleosols) is cloaked by Wisconsin loess. The upper two of these three levels were buried about 29,000 years B.P. Successively lower portions of the Iowan surfaces are overlain by successively younger subdivisions of the loess, indicating time-transgressive burial owing to continuing erosion. Burial was completed around 18,300 years B.P. Locally thick loess on top of the Iowan surfaces is indicative of spatially different rates of deposition. Iowan surfaces served as the source of sand in the loess. Modifications since the close of loess deposition around 14,000 years B.P. were inferred from valley fills. They include a major gully cycle which terminated around 6,200 years B.P., followed by massive sheet erosion. Anthropogenic influences after white settlement are evidenced by an uppermost increment of valley fill that is locally breached by discontinuous gullies.
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