Subsurface studies in the Northern Appalachian basin reveal several different histories of sedimentation (Cambro-Ordovician and Lower Silurian). Too few wells have been drilled to the Precambrian to reveal anything but very regional data on the Cambrian sediments. The Cambrian thickness varies from 13,000 feet on the outcrop in south-central Pennsylvania to 900 feet in northeastern Ohio and 0 feet in southwestern Ontario. The section is predominantly dolomite and sandstone. The Upper Cambrian isopach map shows a probable regional high in northeastern Ohio. This high seems to trend north-south. Two regional unconformities have been detected within this section.

Middle-Ordovician sedimentation marked a time of emergence with the Adirondack-Tazwell axis in the center of the basin forming a structural and facies barrier separating for the first time the Appalachian basin into two distinct basins of deposition. On the northwest flank of the Allegheny synclinorium, the Crawford arch came into existence along with the Olean embayment and Erie trough (Chatham Sag extension). Unlike the more pronounced Adirondack-Tazwell feature, the Crawford arch did not act as a facies barrier. The Middle Ordovician sediments encountered so far are argillaceous limestones. The exception to this being the dolomitized productive areas on the Findlay arch.

Upper Ordovician sedimentation shows regional east to west thinning with occasional interruptions across local highs. These sediments are predominantly shales with occasional layers of siltstones or sands. The subsequent Lower Silurian deposits also show regional thinning eastward with local variations. Lower Silurian production appears to be confined to within the 200-400-foot isopach interval. Variations within this isopach interval are critical as to the quality of production to be found. Detailed study reveals the productive sand bodies are deltaic rather than offshore bars or shoe-string sands.

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