Comparison of a generalized 250-foot contour map of the bedrock surface and the glacial map of central United States shows that the Pleistocene glacial lobes were closely controlled by bedrock lowlands even in areas of relatively thick drift and that positions of ice centers and deflection by adjoining lobes were of secondary importance. The implications of these relations in the study of glacial centers and possible flow units within glacial lobes are considered, and an interpretation of preglacial drainage, based on the bedrock-surface map, is proposed.
A wedge-shaped projection of early Wisconsin till into underlying Farmdale loess indicates that the ice advanced over periglacial frozen ground and that a crack, possibly formed by melting of a ground-ice wedge, became filled with till.
The classification proposes minor modifications in Fenneman's divisions and recognizes subdivisions of the Till Plains and Great Lakes sections, based largely on glacial features. The boundaries and characteristic features of the subdivisions are described, and their origin and relations to glacial features and bedrock topography are discussed.
Caliches with similar zonal profiles lie on the Pliocene Ogallala formation of the Llano Estacado and on three younger surfaces along the Pecos depression. Physiographic relations, the caliche profile, and other features indicate that the caliches were developed by soil-forming processes from parent-materials consisting largely of limestone gravels. The caliches and associated solution features provide a partial record of climatic changes since middle Pliocene time.
The relations of Yellowstone Tertiary volcanics to Tertiary faults in the Teton Range indicate that major movements probably occurred in early Pliocene time, following deposition of Miocene basic volcanics and associated sediments and prior to outpourings of plateau rhyolite, and that minor movements continued into Recent time. Erosion surfaces representing the late-mature stage Black Rock cycle are covered by the rhyolite. This relationship and the youthful geomorphic aspect of the rhyolite plateau indicate that the rhyolite and younger valley basalt are probably of early, or early middle, Pleistocene age. More precise dating of the rhyolite depends upon the age assigned to the Buffalo drift, which rests on these volcanics.
A generalized map of buried erosion surfaces in Illinois, compiled from detailed maps of the bedrock topography, provides regional perspective for recognition and correlation of erosion surfaces within the state. A correlation of the Lancaster peneplain of northwestern Illinois, the Calhoun peneplain of western Illinois, and the Ozark peneplain of northern Illinois is suggested; an independent lower surface which developed on the weak rocks of the Illinois basin is described and named the "Central Illinois peneplain"; and possible straths along major preglacial valleys are recognized and named the "Havana strath."
