ILLINOIS STATE GEOLOGICAL SURVEY

The map depicts the geology of the surficial deposits of the Oak Hill Quadrangle. On this map, surficial deposits consist of the unconsolidated and residual, alluvial, or glacial deposits which occur at or near the surface of the earth. The map also depicts where lithified strata (bedrock) crop out or are near the surface, but it does not depict the occur-rence of loess, which blankets most of the quadrangle. The only previous detailed surficial maps which include this area are by Horberg (1950), who differentiated “Illinoian ” and “Wisconsin ” drift, glacial outwash, alluvium, and bedrock on his 1:126,720 scale map of the Peoria County area, and Anderson and Hunter (1965), who differentiated “Illinoian” glacial till and outwash deposits, “Wisconsinan ” glacial till, outwash, and valley train deposits, sand dunes, slackwater deposits, and alluvium on their 1:63,360 scale map of Peoria County. The Oak Hill Quadrangle is located about nine miles west of the edge of the Illinois River Valley (fig. 1). Most of the quadrangle is just west of the terminal Buda Moraine (fig. 1) of the Wisconsin Episode glaciation. The physiography of the quadrangle is dominated by broad uplands separated by narrow (Jubilee Creek and Hickory and Nixon Runs) to wider stream valleys (Kickapoo and West Fork of the Kickapoo) and their associated tributaries. The upland areas are dominated by glacial till overlain by loess (eolian sedi-ments). Quaternary sediments were deposited on the bedrock dur-ing the last two episodes of glaciation. Pre-Illinois Episode sediments are present within Peoria County (Udden 1912, Wanless 1957, Stumpf and Weibel 2005), but have not been found within the Oak Hill Quadrangle. During the Illinois glacial episode, glaciers generally advanced westerly and retreated to the east across the area (Lineback 1979). These glaciers eroded the existing landscape and deposited the diamicton of the widespread Glasford Formation in the Oa