Pleistocene erosion and alluviation along Smoky Hill River resulted in the formation of terrace deposits at three major levels. The following summary of Pleistocene events in the area of the field trip is drawn from the work of Frye and Leonard (1952) and Leonard and Berry (1961). Moderate initial dissection of the High Plains occurred during the Nebraskan Stage, during which time the streams deposited alluvium now preserved as a terrace remnant lying less than 100 feet below the upland surface in the Smoky Hills of southwestern Ellis County. Deep downcutting during Kansan time resulted in alluviation along channels now preserved mostly beneath sediments of Illinoisan age in the Pfeifer Terrace. Valley broadening and alluviation during Illinoisan time is reflected in widespread sand, gravel, and silt de- posits that form the upper part of Pfeifer Terrace. Remnants of a terrace of the same age are preserved along Saline River valley to the north. Renewed downcutting and alluviation during the Wisconsinan Stage produced the low river terraces. Post-Wisconsinan changes in- clude cutting of the present channel into Wisconsinan deposits or Cretaceous bedrock, and development of the present narrow flood plain. Widespread deposition of loess during the Illinoisan and Wisconsinan Stages has irreg- ularly veneered the uplands, pediments, and older terraces, thus masking the relationships among these features in some areas. STRUCTURE Structural dip at the top of the Dakota Formation is approximately 7 feet per mile to the northeast in the easternmost portion of the field-trip area, increasing westward in Ellis and Trego counties to approximately 10 feet per mile to the north or slightly east of north. Still farther to the west, in Gove County, the dip is approximately 13 feet per mile to the north and increases further to approximately 20 feet per mile to the northeast in Logan County (see Merriam, 1957b). Major structural features reflected at this datum in western Kansas include, from east to west, the Salina Basin, Cambridge Arch, Western Kansas Basin and the eastern flank of the Las Animas Arch (Fig. 2). According to Merriam (1963) the Salina Basin and Cambridge Arch are largely inherited from an earlier structural framework, the Western Kansas Basin is a Mesozoic feature, and the Las Animas Arch in Kansas is largely a post-Cretaceous feature. The Cambridge Arch lies outside the area of this field excursion. Paleozoic structures in the area that are not reflected at the top of the Dakota include the Central Kansas Uplift, which underlies our route in Ellsworth, Russell, Ellis, and Trego counties, and the Hugoton Embayment which embraces all of Kansas west of the Cambridge Arch and Central Kansas Uplift (Merriam, 1963, p. 178). 7