A systematic study of the Mississippian-Pennsylvanian unconformity in the southern part of the Illinois basin, using both surface and subsurface data, indicates that many factors were involved in the geologic history of the erosional surface. Criteria are presented for the recognition of the position of the unconformity in the subsurface and the rocks above and below the contact are compared and contrasted. An areal geologic map of the pre-Pennsylvanian formations shows three major features: beveling of younger formations at the edges of the Illinois basin; beveling of small structures in the basin; and long, deep, narrow channels as features of the erosional surface. Several isopach maps indicate the configuration of the channel systems and the upland surface between cha nels.

The geologic history of the unconformity started with uplift and withdrawal of the sea in late Chester time followed by a long period of subaerial erosion. A peneplain was formed across beveled minor structures and older Chester and Lower Mississippian formations at the structurally higher basin borders. After peneplanation and a subsequent second uplift a new cycle of erosion began that resulted in the incision of deep channels in the peneplain surface. In early Pennsylvanian time the topography was buried by detritus laid down by aggrading streams which later merged with coarse sediment advancing westward from the Appalachian geosyncline. During and after Pennsylvanian time the unconformity was warped, folded, and faulted to its present attitude.