GENERAL STRATIGRAPHY OF THE REGION. 
83 
it belongs to the Pottsville formation. It seems probable that the 
latter is true arc! that it is the first member laid down on the eroded 
surface. 
ROCKS BELOW TIIE POTTSVILLE FORMATION. 
UNCONFORMITY BETWEEN THE MISSISSIPPIAN AND THE PENNSYLVANIAN 
SERIES. 
In the Appalachian region south of the Steubenville quadrangle 
the Pocono sandstone is overlain by the great Greenbrier (“lower 
Carboniferous") limestone, which first appears in the section in 
the southern half of the Steubenville quadrangle as a thin wedge. 
This increases in thickness rapidly to the south until in the Tennessee 
region it attains a maximum of more than 1,000 feet. 
In the northern half of the Appalachian region this great lime¬ 
stone mass is overlain by an immense body of red shale which reaches 
its maximum development of 5,000 to 8,000 feet in thickness in 
eastern Pennsylvania, where it is known as the Mauch Chunk shale. 
This shale does not appear to be present in the three quadrangles 
investigated. The nearest point of outcrop of the Greenbrier lime¬ 
stone and Mauch Chunk shale is in the Chestnut Ridge of Pennsyl¬ 
vania and northern West Virginia, where both formations are pres¬ 
ent, but with a maximum thickness of not more than 300 feet. From 
the numerous drill records in southwestern Pennsylvania it is pos¬ 
sible to trace the red shale to the northwest as far as Washington, Pa., 
and the Greenbrier limestone a short distance beyond. Beyond these 
points there are no traces of the formations, either in outcrop or in 
drill records. 
From recent studies® in the northern Appalachian coal field it 
has been demonstrated that the absence of these formations and also 
of a large part of the lower portion of the Pottsville is due to a great 
unconformity which exists on the western side of the Appalachian 
coal field between the Mississippian and Pennsylvanian series of 
rocks. After the deposition of the Mauch Chunk shale, which was 
probably laid down throughout the Appalachian province, the rocks 
on the northwestern side of the province were elevated above sea 
level, forming a land mass in what is now northwestern Pennsylvania, 
Ohio, Kentucky, and Tennessee. Erosion was active on this land 
area and most, if not all, of the Mauch Chunk shale and the Green¬ 
brier limestone was removed. This area continued above water level 
until at least two-thirds of the Pottsville formation was deposited in 
« White, David, Fossil floras of the Pottsville formation in the Southern Anthracite 
field, Pennsylvania : Twentieth Ann. Rept. U. S. Geol. Survey, pt. 2, 1000, pp. 751-930 ; 
Deposition of the Appalachian Pottsville, Bull. Geol. Soc. America, vol. 15, 1004, pp. 
267-282. 
