Jacks 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: lithost ...

ridge A ridge is a long, narrow, elevated geomorphologic landform, structural feature, or a combination of both separated from the surrounding terrain by steep sides. The sides of a ridge slope away from a narrow top, the crest or ridgecrest, wi ...

which is located in central

Pennsylvania Pennsylvania, officially the Commonwealth of Pennsylvania, is a U.S. state, state spanning the Mid-Atlantic (United States), Mid-Atlantic, Northeastern United States, Northeastern, Appalachian, and Great Lakes region, Great Lakes regions o ...

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

, trending southeast of the

Stone Mountain Stone Mountain is a quartz monzonite dome Inselberg, monadnock and the site of Stone Mountain Park, east of Atlanta, Georgia. Outside the park is the city of Stone Mountain, Georgia. The park is the most visited tourist site in the state of Ge ...

ridge and Jacks Mountain

Anticline In structural geology, an anticline is a type of Fold (geology), fold that is an arch-like shape and has its oldest Bed (geology), beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex curve, c ...

. 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. History John Armst ...

from the Ferguson and Dry Valleys. Jacks Mountain lies in Mifflin,

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

, Snyder, and Union Counties, and the ridge line forms part of the border between Huntingdon and Mifflin Counties.

History

This mountain was named for Jack Armstrong, an eighteenth-century

fur trade The fur trade is a worldwide industry dealing in the acquisition and sale of animal fur. Since the establishment of a world fur market in the early modern period, furs of boreal ecosystem, boreal, polar and cold temperate mammalian animals h ...

r. During the autumn of 1743, Armstrong confiscated the horse of Mushemeelin, a Delaware Indian from Shamokin who was in debt to Armstrong. Mushemeelin and two Delaware companions tracked Armstrong to the

Juniata River The Juniata River () is a tributary of the Susquehanna River, approximately long,U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map , accessed August 8, 2011 in central Pennsylvania. The river is ...

narrows and, in 1744, killed the trader and his two servants. The site was thereafter known as "Jack's Narrows".James H. Merrell, ''Into the American Woods: Negotiators on the Pennsylvania Frontier'' (New York: Norton, 1999; ), 42–45. U.S. Route 22 (US 22), the William Penn Highway, and the former Pennsylvania Main Line, now the

Norfolk Southern Railway The Norfolk Southern Railway is a Class I freight railroad operating in the Eastern United States. Headquartered in Atlanta, the company was formed in 1982 with the merger of the Norfolk and Western Railway and Southern Railway. The comp ...

Pittsburgh Line The Pittsburgh Line is the Norfolk Southern Railway's primary east–west artery in its Pittsburgh Division and Harrisburg Division across the U.S. state of Pennsylvania and is part of the Keystone Corridor, Amtrak-Norfolk Southern's combine ...

, follow the

through the Jacks Narrows

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 pract ...

between Mapleton and Mount Union.

U.S. Route 322 U.S. Route 322 (US 322) is a , east–west United States Highway, traversing Ohio, Pennsylvania, and New Jersey. The road is a spur route, spur of U.S. Route 22, US 22 and one of the original highways from 1926. A portion of it at one time was c ...

and the former

Milroy Branch Milroy may refer to: People *Andrea Milroy, Miss World Venezuela 2004 * Bobby Milroy (born 1978), Canadian badminton player and former president of the World Badminton Player's Federation * Duncan Milroy (born 1983), Canadian NHL hockey player ...

of the

Pennsylvania Railroad The Pennsylvania Railroad ( reporting mark PRR), legal name as the Pennsylvania Railroad Company, also known as the "Pennsy," was an American Class I railroad that was established in 1846 and headquartered in Philadelphia, Pennsylvania. At its ...

pass through the Mann Narrows Water Gap along the

Kishacoquillas Creek Kishacoquillas Creek (pronounced Kish-e-kō-kwil´-lis) is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map , accessed August 8, 2011 tributary of the Juniata River in Mifflin County, Pennsylvani ...

near Reedsville.

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, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

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

on the northwest slope, and the younger more erosion resistant

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 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 third and shortest period of t ...

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, US. Description The Tuscarora is a thin- to thick-bedded fine-gra ...

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

brachiopods Brachiopods (), phylum Brachiopoda, are a phylum of 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, while the fron ...

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 ...

. The Reedsville forms the less steep lower slope that becomes shallower toward the base of the ridge. Across the Kishacoquillas Valley, on the opposite side of the Jacks Mountain anticline, the same rock layers are repeated in reverse order on the Stone Mountain ridge.

References

Geographic Names Information System
Feature ID: 1177904. * USGS Allensville 7.5-Minute Quadrangle * {{Mountains of Pennsylvania Landforms of Mifflin County, Pennsylvania Landforms of Snyder County, Pennsylvania Ridges of Huntingdon County, Pennsylvania Landforms of Union County, Pennsylvania Ridges of Pennsylvania Water gaps of Pennsylvania