The Conemaugh Group "is stratigraphically defined as the rocks lying between the upper Freeport coal horizon and the Pittsburgh coal. The thickness of this interval ranges from 520 feet (158 m) in western Washington County to 890 feet (270 m) in southern Somerset County. A gradual eastward thickening of the Conemaugh is apparent" (Edmunds et al., 1998, p. 154). The Conemaugh is subdivided into a lower formation called the Glenshaw, and an upper formation called the Casselman. The division is made at the top of the Ames marine limestone. Mineable coals are uncommon in the Conemaugh.

Glenshaw Formation - The Glenshaw contains several widespread marine zones (see Figure 8.2). There are possibly as many as seven marine zones within the Glenshaw. The Glenshaw is thickest in Somerset and southern Cambria counties, where it reaches 400 to 420 ft (122 to 128 m). It is thinnest near the Ohio border where it is about 280 ft (85 m) thick (Edmunds et al., 1998).

The Brush Creek limestone facies is present within most of its outcrop belt in western Pennsylvania. An area of more clastic shale and siltstone occupies the limestone horizon in Washington, southern Allegheny, northern Westmoreland, southern Indiana, and Cambria Counties (Skema et al., 1991). The Brush Creek coal, which occurs below the marine zone, is typically thin and not mined over much of western Pennsylvania. The coal is sometimes mineable in portions of the south-central and southeastern sections of the bituminous coal fields of Pennsylvania.

Casselman Formation - "The thickness of the Casselman Formation ranges from 230 feet (70 m) in the extreme western part of the Appalachian Plateaus province to 485 feet (148 m) in southern Somerset County" (Edmunds, et al., 1998, p. 156). With the exception of the marine shales above the Ames limestone, and the Skelly horizon, which occurs about 30 to 60 ft (9 to 18 m) above the Ames marine zone, the Casselman is made up of exclusively fresh water rocks. Redbeds, which are regionally discontinuous, are scattered throughout the Casselman in the western portion of Pennsylvania. "Eastward they become thinner and fewer in number. This trend continues into eastern Somerset and Cambria Counties, where large areas of the Casselman Formation are completely devoid of red beds. Conversely, coals are nearly absent or very thin in the west but increase in quantity eastward. In Somerset County, a few coals are thick enough to mine" (Edmunds, et al., 1998, p. 156).

Very little coal is mined in the Casselman Formation.

References

Berryhill, H.L., Jr., S.P. Schweinfurth, and B.H. Kent, 1971. Coal-bearing Upper Pennsylvanian and Lower Permian Rocks, Washington Area, Pennsylvania. USGS Prof. Paper 621.

Edmunds, W.E., V.W. Skema, and N.K. Flint, 1998. Pennsylvanian. Part II. Stratigraphy and Sedimentary Tectonics. Geology of Pennsylvania, pp. 149-169.

Skema, V.W., C.H. Dodge and J.R. Shaulis, 1991. Lithologic character and correlation of marine units in the Conemaugh Group (Upper Pennsylvanian), western Pennsylvania (abstract). Geol. Soc. of America, Northeastern and Southeastern Section Meeting, v. 23, no. 1, p. 128.

Williams, E.G., 1960. Marine and fresh water fossiliferous beds in the Pottsville and Allegheny Groups of western Pennsylvania. Jour. of Paleontology, v. 34, pp. 908-922.