The Cache Valley, a prominent physiographic feature connecting the Ohio and Mississippi river valleys across southern Illinois, at one time probably carried discharge from the ancestral Ohio River. The valley is now occupied by two small streams. Core samples from 27 boreholes in the Cache Valley and 25 on the adjoining uplands were described and analyzed in terms of clay mineralogy and texture. The Holocene surficial deposits of the Cache Valley contain predominantly silt with high percentages of expandable and kaolinite + chlorite clay minerals in the clay fraction, a characteristic of the local upland provenance. The surfcial deposits are underlain by predominantly sandy sediment containing a high percentage of illite (35%-70%) relative to the expandable and kaolinite + chlorite clay minerals, a characteristic of Ohio River outwash. Clay content of sediment at depth at the mouths of valleys tributary to the Cache Valley suggests that these deposits accumulated within quiet water impounded by sediment dams within the Cache Valley as the ancestral Ohio River aggraded during late Wisconsinan time. Physical relations between tributary deposits and main valley deposits, plus radiocarbon dates on organic material recovered from the boreholes, suggest that the Ohio River abondoned the Cache Valley after 25ka, but before 8 ka.