Abstract

A remnant alluvial fill of early Pleistocene age exposed in Pumpkin Creek Valley, Banner and Morrill counties, Nebraska, has yielded fossils of Mammuthus meridionalis (Nesti) and Equus sp. cf. E. scotti Gidley. Younger fill remnants of trunk and tributary streams allow a refinement of earlier views on the development of the Pumpkin Creek drainage basin during the Quaternary Period. Ancestral Pumpkin Creek both shifted to the north and entrenched its valley several times during the Quaternary Period leaving alluvial fills at three levels or more south of the present creek. Piracy of the headwaters of ancestral Pumpkin Creek took place after the last of these erosional events, probably in middle or late Pleistocene time.

Asymmetrical distribution of alluvial fills occurs in Pumpkin Creek Valley, along the north side of parts of the North Platte River in western Nebraska, and along streams east of the Black Hills. The asymmetrical distribution of these deposits may have been due to structural warping, which could have caused these streams to shift their courses laterally and to entrench their valleys repeatedly.

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