Ecological interpretations of Pliocene and Pleistocene stratigraphy in the Great Plains region

In the Great Plains the succession of gross ecological conditions through Neogene and Quaternary time may be interpreted from the stratigraphy, geomorphological history, buried soils, and fossil mollusks and plants. A mild humid climate prevailed over a late-mature erosional topography in earliest Neogene. Progressive reduction of topographic relief and lowering of the regional water table, accompanied by an almost uniform drying of the climate, followed. Semiarid, rigorous conditions existed on the constructional plain, temporarily at erosional equilibrium, at the end of the Tertiary. A sharp reversal of climatic trend accompanied by stream incision and minor alluviation marked the beginning of the Pleistocene. The trend toward humidity culminating in the Kansan, was followed by a return to the trend pulsating and irregular but none the less distinct-of progressive desiccation. The Recent climate is judged to approach in dryness and rigor that which existed on the late Tertiary plain.