Stone Mountain is a

stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostra ...

ridge A ridge or a mountain ridge is a geographical feature consisting of a chain of mountains or hills that form a continuous elevated crest for an extended distance. The sides of the ridge slope away from the narrow top on either side. The line ...

that is located in central

Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

, trending northwest of the

Jacks Mountain Jacks Mountain is a stratigraphic ridge in central Pennsylvania, United States, trending southeast of the Stone Mountain ridge and Jacks Mountain Anticline. The ridge line separates Kishacoquillas Valley from the Ferguson and Dry Valleys. Jacks ...

ridge and Jacks Mountain

Anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...

. The ridge line separates

Kishacoquillas Valley The Kishacoquillas Valley, known locally as both Kish Valley and Big Valley, is an enclosed anticlinal valley in the Ridge-and-valley Appalachians of Central Pennsylvania, and is located in Mifflin and Huntingdon counties. Geography The valle ...

from the Seven Mountains area. Stone Mountain lies in, and the ridge line forms part of the border between, Mifflin and

Huntingdon Huntingdon is a market town in the Huntingdonshire district in Cambridgeshire, England. The town was given its town charter by King John in 1205. It was the county town of the historic county of Huntingdonshire. Oliver Cromwell was born there ...

counties. Pennsylvania Route 305 crosses the ridge and follows a valley along a fault line at near

Greenwood Furnace State Park Greenwood Furnace State Park is a Pennsylvania state park in Jackson Township, Huntingdon County, Pennsylvania in the United States. The park is near the historic iron making center of Greenwood Furnace. The park includes the ghost town of Gre ...

. Allensville Road also crosses the ridge crest north of Allensville, and Barrville Road passes through a

wind gap A wind gap (or air gap) is a gap through which a waterway once flowed that is now dry as a result of stream capture. A water gap is a similar feature, but one in which a waterway still flows. Water gaps and wind gaps often provide routes which ...

west of Barrville. There are no

water gap A water gap is a gap that flowing water has carved through a mountain range or mountain ridge and that still carries water today. Such gaps that no longer carry water currents are called wind gaps. Water gaps and wind gaps often offer a prac ...

s carrying rivers through the ridge. The Saddler Creek passes the southern foot of the ridge near the town of Mill Creek.

Geology

Just below the ridge crest lies the contact between the older

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

Juniata Formation The Ordovician Juniata Formation is a mapped bedrock unit in Pennsylvania and Maryland. It is a relative slope-former occurring between the two prominent ridge-forming sandstone units: the Tuscarora Formation and the Bald Eagle Formation in the ...

on the southeast slope, and the younger more erosion resistant

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

Tuscarora Formation The Silurian Tuscarora Formation — also known as Tuscarora Sandstone or Tuscarora Quartzite — is a mapped bedrock unit in Pennsylvania, Maryland, West Virginia, and Virginia, USA. Description The Tuscarora is a thin- to thick-bedded fine-gr ...

geologic layer forming the steeper northwest slope and the crest. The Bald Eagle Formation crops out on the Kishacoquillas Valley-facing slopes of Stone Mountain, forming a steep topographic bench below the Juniata Formation. Below the Bald Eagle Formation, the uppermost sandstone layer of the underlying

Reedsville Formation The Ordovician Reedsville Formation is a mapped surficial bedrock unit in Pennsylvania, Maryland, Virginia, West Virginia, and Tennessee, that extends into the subsurface of Ohio. This rock is a slope-former adjacent to (and stratigraphically bel ...

contain

brachiopods Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, wh ...

and other marine

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

. Across the Kishacoquillas Valley, on the opposite side of the Jacks Mountain anticline, the same rock layers are repeated in reverse order on

References

Geographic Names Information System
Feature ID: 1193474. * USGS Allensville and Mc Alevys Fort 7.5-Minute Quadrangles * {{cite journal, url= http://www.dcnr.state.pa.us/topogeo/openfile/allensville.pdf, archive-url= https://web.archive.org/web/20110609185454/http://www.dcnr.state.pa.us/topogeo/openfile/allensville.pdf, url-status= dead, archive-date= June 9, 2011, author1=McElroy, Thomas A. , author2=Hoskins, Donald M. , name-list-style=amp , year=2007, title=Bedrock Geology of the Allensville Quadrangle, Huntingdon and Mifflin Counties, Pennsylvania, publisher=

Pennsylvania Geological Survey The Pennsylvania Geological Survey or Bureau of Topographic and Geologic Survey (BTGS), is a geological survey enacted by the Pennsylvania General Assembly "to serve the citizens of Pennsylvania by collecting, preserving, and disseminating impartia ...

Landforms of Mifflin County, Pennsylvania Landforms of Snyder County, Pennsylvania Ridges of Huntingdon County, Pennsylvania Landforms of Union County, Pennsylvania Ridges of Pennsylvania