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: 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 ...

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 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 small city of Stone Mountain, Georgia. The park is the most visited tourist site in the state o ...

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 Ferguson and Dry Valleys. Jacks Mountain lies in Mifflin,

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 ...

, Snyder, and

Union Union commonly refers to: * Trade union, an organization of workers * Union (set theory), in mathematics, a fundamental operation on sets Union may also refer to: Arts and entertainment Music * Union (band), an American rock group ** ''Un ...

Counties, and the ridge line forms part of the border between Huntingdon and Mifflin Counties. It is named for Jack Armstrong, an 18th-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, polar and cold temperate mammalian animals have been the mos ...

r. In the autumn of 1743, Armstrong confiscated the horse of Mushemeelin, a

Delaware Indian The Lenape (, , or Lenape , del, Lënapeyok) also called the Leni Lenape, Lenni Lenape and Delaware people, are an indigenous peoples of the Northeastern Woodlands, who live in the United States and Canada. Their historical territory inclu ...

from Shamokin who was in debt to Armstrong. Mushemeelin and two Delaware companions tracked Armstrong to the Juniata River 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 U.S. Route 22 (US 22) is a west–east route and is one of the original United States highways of 1926, running from Cincinnati, Ohio, at US 27, US 42, US 127, and US 52 to Newark, New Jersey, at U.S. Route 1/9 in the Newark Airport Intercha ...

(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 in the United States formed in 1982 with the merger of Norfolk and Western Railway and Southern Railway. With headquarters in Atlanta, the company operates 19,420 route miles (31 ...

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 combin ...

, follow the Juniata River 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 prac ...

between

Mapleton Mapleton may refer to: Places Australia * Mapleton, Queensland, a rural town and locality in the Sunshine Coast Region Canada * Mapleton, New Brunswick, a rural community in Kings County * Mapleton, Moncton, New Brunswick, a neighborhood * Maple ...

and Mount Union.

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

and the former Milroy Branch of the

Pennsylvania Railroad The Pennsylvania Railroad (reporting mark PRR), legal name 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. It was named ...

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, Pennsylvania ...

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. 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 northwest 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 southeast slope and the crest. The

Bald Eagle Formation The Ordovician Bald Eagle Formation is a mapped bedrock unit in central Pennsylvania, USA. It is a ridge-forming unit in the Appalachian Mountains. Description The Bald Eagle is defined as a gray to olive-gray and grayish-red, fine to coarse-gra ...

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 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 be ...

contains

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, whi ...

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 ...

. 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 * 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 {{UnionCountyPA-geo-stub