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There are
three limestone layers that form most caves in western Pennsylvania.
 | Loyalhanna Limestone. This generally flat-lying
limestone is of Mississippian age and is 40 to 60 feet thick in western
Pennsylvania which varies by locality. It occurs at the base of Mauch Chunck
Formation and just above the Pocono Formation. It is a very silaceous
limestone that is highly crossbedded (especially in the upper 25
feet). In occasional localities such as Somerset County, the
limestone may be red in color. A
relatively pure thin (up to 12 feet) layer of Deer Valley Limestone (which
indicates the bottom of the Mauch Chunk sequence) is often found at the top
of the Loyalhanna Limestone and is typically isolated to southern Somerset
County and throughout Fayette County, Pennsylvania. Loyalhanna
outcrops near the top of Chestnut and Laurel Ridges. Caves in
the Loyalhanna include Laurel Caverns (largest show cave in Pennsylvania) in
Fayette County, Bear Cave in Westmoreland County, and Shafer Run Cave in
Somerset County. |
| Greenbrier (or Wymp's Gap) Limestone. This limestone is of
Mississippian age and occurs within the Mauch Chuck formation. It is quite
pure and contains many fossils. Springs are common at the base of this
limestone layer. In many places this layer is covered by boulder fields of
blocks that have broken off from the overlying Pottsville Sandstone layers. Few limestone caves have been discovered in this unit but one
includes Lick Hole Cave in Somerset County. |
| Vanport Limestone. This limestone is of Pennsylvanian age and
is 10 to 25 feet thick. This generally flat-lying, high-calcium
limestone contains fossils such as crinoid stems. The upper part of this
layer includes iron deposits that affect compass readings. It
primarily outcrops in Lawrence, Butler, and Armstrong Counties. Vanport
caves are typically complex network mazes. Caves in the Vanport
include Harlansburg Cave in Lawrence County and Brady's Bend Cave in
Armstrong County. |
A number of
small shelter, talus, and fissure caves also occur, often in sandstone. The
Pottsville Sandstone is particularly rich in such caves.
| Pottsville Sandstone. This sandstone is of
Pennsylvanian age and occurs just above the Mauch Chunk formation. It has
two sublayers: the Homewood Sandstone and Connoquenessing Sandstone.
Both are massive cross-bedded layers containing plant fossils.
Well-known outcrops of Pottsville Sandstone include Elk Rocks in Fayette
County, Beam Rocks in Somerset County, and Wolf Rocks in Westmoreland
County. |
A fairly complete set of Pennsylvania
Geologic Maps is available online in Adobe Acrobat format from the
Pennsylvania Topographic and Geologic Survey.
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