With the retreat of the Chesterian sea to the south, the area now occupied by the Illinois basin was exposed to subaerial erosion. In most of this area a linear drainage system of southwestward-flowing streams developed, which eroded valleys as much as 450 ft. deep and 20 mi. wide. The topography of the eroded Chesterian surface strongly influenced initial Pennsylvanian sedimentation when the eroded valleys were filled with fine-to-coarse quartz sand, siltstone, and dark gray shale. A detailed study of the Chesterian-Pennsylvanian unconformity in the subsurface of northeastern Wayne County, Illinois, revealed extensive rotational slumping of Chesterian strata along some steep (at least 34° slope) valley walls. Displaced blocks of Chesterian strata penetrated by drill holes range in thickness from 10 ft. to 125 ft., and some may exceed 660 ft. in maximum dimension. Maximum observed displacement is 170 ft. Failure of Clore and Menard shales, which are rich in expandable clay minerals, allowed downward displacement of overlying strata capped by the Negli Creek Limestone Member of the Kinkaid Formation. Slumping of strata along valley walls where the valleys cut through the northeast regional escarpment of the Negli Creek Limestone produced the greatest known concentration of Chesterian slump blocks within the Illinois basin.