The post-Beekmantown and pre-Martinsburg rocks of the central Appalachians comprise numerous formations. For the purpose of regional stratigraphic analysis, these rocks can be subdivided into two major units, the Chazy Group at the bottom and the Chambersburg Group above.

The major depocenter during Chazy deposition was located in central Pennsylvania. A medial high divided it into a shallower western part and a more deeply subsiding eastern part. Another broad depocenter existed in central West Virginia. Areas surrounding that depocenter received irregularly varying amounts of sediments. With few exceptions the limestone facies of the Chazy covers the area southeast of the broad depocenter in central West Virginia. Northwestward, dolomite and clastic facies are present, but limestone remains the most important lithology. Areas with higher clastic and dolomite ratios appear to be rather limited.

During the Chambersburg deposition, a depoaxis existed along the Appalachian Valley. Along that axis two separate depocenters existed in western Virginia. Maximum deposition occurred toward the southwest. An axis of thinning (the Adirondack axis) bordered the Appalachian Valley depoaxis on the west along its northern part. However, this positive area died out toward the south, and, therefore, the "Tazewell axis" of thinning appears to have been nonexistent. A large broad depocenter (Allegheny synclinorium) also extended from central West Virginia into eastern Kentucky. Carbonates (almost all limestone) are the most important Chambersburg facies, but facies with higher clastic ratios occur in parts of the Appalachian Valley of Virginia, an isolated area in West Virginia, and a narrow b lt in Ohio.

The Trenton Limestone has produced oil in the Rose Hill and Ben Hur fields in Lee County, Virginia. The Trenton-Black River limestone section has produced gas in numerous old fields in west-central New York. Gas shows have been reported from Middle Ordovician carbonates in West Virginia. Additional hydrocarbon reserves should be found in the Middle Ordovician carbonates of the central Appalachians. Areas of local dolomitization and fracture porosity related too structure are considered to be the most promising prospective areas.

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