LANDSCAPE DEVELOPMENT AS INDICATED BY BASIN MORPHOLOGY AND THE MAGNETIC POLARITY OF CAVE SEDIMENTS, CRAWFORD UPLAND, SOUTH-CENTRAL INDIANA

Paleomagnetic dates of sediment deposits in the Marengo Cave system an d four caves in Wyandotte Ridge have been combined with the morphometr ic analysis of surface drainage basins in their vicinity to elucidate the timing and significance of geomorphic events affecting karst lands cape development in the southern portion of the Crawford Upland in Cra wford County, Indiana, Wyandotte Cave exhibits two distinct levels sep arated by a 24 m vertical gap. It is located near the Ohio River, whic h acts as the local and regional control on base level, The magnetostr atigraphy suggests that the upper level of Wyandotte Cave was abandone d in the early to middle Pleistocene, following a drop in the regional base level that occurred no later than 0.78 Ma ago and accompanied th e expansion of the Ohio River drainage system, Commensurate with the t ime taken for the effects of a drop in base level to propagate up-basi n, it is likely the upper level in Marengo Cave was abandoned after th e upper level in Wyandotte Cave was vacated (that is, less than or equ al to 0.78 Ma ago). The smaller (7 m) separation between the upper and lower levels in Marengo Cave reflects the diminished influence that a drop in base level commonly has in locales relatively far removed fro m a basin outlet. Greater stability of the landscape in the vicinity o f Marengo Cave implies that there the subsurface and surface landforms are more mature than caves and drainage basins in close proximity to the Ohio River, Stability also facilitates the development of an exten sive, integrated drainage system in basins buffered by distance from t he full impact of base level lowering, In these basins, a further drop in base level will likely be absorbed by the subsurface portion of th e drainage network. Therefore, in the Crawford Upland, evolution towar d a karst plain will progress more rapidly in tributary drainage basin s that are relatively far removed from the Ohio River.