The Valley and Ridge Province in the east is composed of folded and faulted rocks
that range in age from late Precambrian to early Mississippian. The late Precambrian Catoctin
Formation is technically in the Blue Ridge Province, but we only have limited outcrops
exposed on the Blue Ridge Mountains. From the Blue Ridge Mountains westward for about
20 miles is the Great Valley. This relatively flat, agriculturally rich region is composed of
complexly folded and faulted Cambrian and Ordovician limestone and dolomite with one
prominent Ordovician shale (the Martinsburg Shale). The Great Valley ends at North
Mountain and from here to the Allegheny Front, a distance of about 50 miles, are a series of
northeast-trending mountains and valleys. The rocks in this part of the Valley and Ridge range
in age from late Ordovician to early Mississippian. The valleys are primarily composed of
less-resistant shale and siltstone, while the mountain ridges are mainly resistant sandstone and
limestone. The structural geology of the Valley and Ridge is complex with extensive thrust
faults and folds that contribute to the repetition of all the rock formations. In addition, three
major allochthonous thrust sheets have displaced the surface and subsurface rocks westward in
the order of 30 to 50 miles. The interested reader may wish to examine recent geological
maps and reports noted in the Survey's publications list.
The Appalachian Plateau Province covers the western two-thirds of the State where the
rock formations are relatively flat, except for several distinct folds and faults on the eastern
side of the Province. The oldest rocks are located in these eastern fold sequences and range in
age from late Ordovician up through the Mississippian. The majority of the Appalachian
Plateau is comprised of Pennsylvanian and Permian strata and is where all the minable
coal is located. The rocks exposed in the northern part of the Plateau are younger than those
exposed in the southern part. This is also reflected in the age of the minable coal seams;
younger to the north and older in the south. Although some natural gas has been obtained
from the Valley and Ridge Province, by far the majority of natural gas (perhaps 95 to 98
percent) and all the oil comes from the Plateau (see also the sections on coal and oil and gas
on this web site). The boundary between the two provinces, the Allegheny Front, is a
complex and rather abrupt change in the topography, stratigraphy, and structure. This
boundary extends southwestward across the eastern part of the State, passes through Virginia,
and reenters the State in the southeast in Monroe County.
(by Peter Lessing, July 1996)
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