The basal—that is, the lowest and oldest—units of the Kaskaskia consist of clean quartz sandstones eroded from the Appalachian orogenic belt to the east, the Ozark Dome in the center of the continent, and south from the Canadian Shield.
Beginning in the middle to late Devonian, black shales such as the Marcellus Formation, begin to dominate in the rocks, composed of detritus eroded from the uplifted Acadian highlands to the west; they also indicate widespread anoxic conditions across the sea floor.
By mid-Mississippian time however, shallow-water carbonate deposition resumed, until the final regression of the epeiric seas late in the period.
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