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An exceptionally clear record of climatic changes has been preserved in the Quaternary and Tertiary soils and loess deposits of the central part of the United States. Major extinctions of mammals also coincide with regional breaks in sedimentation, perhaps at glacial maxima. The Quaternary stratigraphy is now well known in this region, with initial Quaternary glaciation succeeding end-Tertiary (Ogallala) alluviation and soil development.
The Quaternary ("Ice Age") succession of cutting-and-filling cycles that record episodes of glaciation and inter-glaciation begins just after the completion of the main calichecomplex of the end. Ogallala or end-Pliocene (circa 3.2 m.y. ago). The soils, such as the Sangamon soil-complex, unquestionably document times of equable temperature and abundant rainfall favorable for the growth of plains grasses and other vegetation, as well as for mammals. Unconformities cutting the soils record the onset of drought, followed by dominant colluviation, alluviation, and loess deposition. Since the outwash deposits rest on surfaces of major disconformity, the glacialmaximum is believed to be represented by this widespread erosion of the denuded plains. The retreating ice is represented by outwash and by widespread loess deposition transitional to the next period of interglaciation and significant soil development.
Thus, a terrace-cycle is easily recognizable, with the major soils and most-important erosional breaks being of greatest importance. Six couplet-episodes seem to represent progressively diminishing and weaker glaciations through the Quaternary to the present, and there are six valley-fills forming terraces (T-5 to T-1, plus the T-O or modern floodplain). The earlier valley-fills, represented in cuts through the terraces, are more complex than the later ones.
Similar climatic changes (associated with major extinctions), also with soils, unconformities, and loess deposits, characterize the Medial and Late Tertiary (Oligocene-Pliocene) of the Great Plains. The major breaks probably represent the Tertiary glaciations of the deep-sea cores.