During middle Paleozoic time, a broad shallow-water shelf, here named the Wabash platform, occupied nearly all but the southwest part of Indiana and extended at least as far as central and northern Illinois and western Ohio. This shelf was as predominant a sedimentational-structural feature as were the bordering protobasins in Iowa, Illinois, Michigan, and Ohio. The Middle Devonian rock record serves to show how intimately interrelated were the sedimentational histories of these major provinces.
At the time of the Tioga ash fall (late early Middle Devonian), a broad carbonate flat, which included both sabkha and lagoonal environments, covered the platform. During late Middle Devonian time, the shallow-water portion of the platform was narrowed somewhat, but shallow-water normal-marine environments broadly prevailed so that the present basin-separating arches were not at all as divisive as their present eroded crestal areas suggest.