Jacks Mountain is a stratigraphic

Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....

 ridge

Ridge

A ridge is a geological feature consisting of a chain of mountains or hills that form a continuous elevated crest for some distance. Ridges are usually termed hills or mountains as well, depending on size. There are several main types of ridges:...

 in central Pennsylvania

Pennsylvania

The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...

, United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, trending southeast of the Stone Mountain

Stone Mountain (Pennsylvania)

Stone Mountain is a stratigraphic ridge in central Pennsylvania, United States, trending northwest of the Jacks Mountain ridge and Jacks Mountain Anticline. The ridge line separates Kishacoquillas Valley from the Seven Mountains area...

 ridge and Jacks Mountain Anticline

Anticline

In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...

. The ridge line separates Kishacoquillas Valley

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, lying between Stone Mountain ridge to the north and Jacks Mountain ridge to the south. The valley is located in Mifflin County...

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

Mifflin County, Pennsylvania

Mifflin County is a county located in the U.S. state of Pennsylvania. As of 2010, the population was 46,682. Its county seat is Lewistown. It is named after Thomas Mifflin, the first Governor of Pennsylvania.-Geography:...

, Huntingdon

Huntingdon County, Pennsylvania

Huntingdon County is a county located in the U.S. state of Pennsylvania. In 2010, its population was 45,913.Huntingdon County was created on September 20, 1787, from part of Bedford County. Its county seat is Huntingdon.-Geography:According to the U.S...

, Snyder

Snyder County, Pennsylvania

Snyder County is a county located in the U.S. state of Pennsylvania. As of 2010, the population was 39,702. Snyder County was formed in 1855 from parts of Union County...

, and Union

Union County, Pennsylvania

-Demographics:As of the census of 2000, there were 41,624 people, 13,178 households, and 9,211 families residing in the county. The population density was 131 people per square mile . There were 14,684 housing units at an average density of 46 per square mile...

 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

Fur trade

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

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

Lenape

The Lenape are an Algonquian group of Native Americans of the Northeastern Woodlands. They are also called Delaware Indians. As a result of the American Revolutionary War and later Indian removals from the eastern United States, today the main groups live in Canada, where they are enrolled in the...

 from Shamokin

Shamokin (village)

Shamokin was a multi-ethnic Native American trading village on the Susquehanna River, located near the site of the modern Sunbury, Pennsylvania. Early in the eighteenth century, the village consisted of Iroquois migrants from the north, as well as Shawnee and Lenape settlers moving away from the...

 who was in debt to Armstrong. Mushemeelin and two Delaware companions tracked Armstrong to the Juniata River

Juniata River

The Juniata River is a tributary of the Susquehanna River, approximately long, in central Pennsylvania in the United States. The river is considered scenic along much of its route, having a broad and shallow course passing through several mountain ridges and steeply-lined water gaps...

 narrows and, in 1744, killed the trader and his two servants. The site was thereafter known as "Jack's Narrows".

U.S. Route 22

U.S. Route 22

U.S. Route 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 near the Newark Liberty International Airport.US 22 also carries the names of the William...

 (US 22), the William Penn Highway, and the former Pennsylvania Railroad Main Line, now the Norfolk Southern Railway

Norfolk Southern Railway

The Norfolk Southern Railway is a Class I railroad in the United States, owned by the Norfolk Southern Corporation. With headquarters in Norfolk, Virginia, the company operates 21,500 route miles in 22 eastern states, the District of Columbia and the province of Ontario, Canada...

's Pittsburgh Line, follow the Juniata River through the Jacks Narrows water gap

Water gap

A water gap is an opening or notch which flowing water has carved through a mountain range. Water gaps often offer a practical route for road and rail transport to cross mountain ridges.- Geology :...

 between Mapleton

Mapleton, Pennsylvania

Mapleton is a borough in Huntingdon County, Pennsylvania, United States. The population was 473 at the 2000 census.-History:The H.O. Andrews Feed Mill was listed on the National Register of Historic Places in 1990.-Geography:...

 and Mount Union

Mount Union, Pennsylvania

Mount Union is a borough in Huntingdon County, Pennsylvania, 45 miles southeast of Altoona, on the Juniata River. In the vicinity are found bituminous coal, ganister rock, fire clay, and some timber. A major Easter grass factory is located in the northern quadrant of the borough limits; until May...

. U.S. Route 322

U.S. Route 322

U.S. Route 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...

 and the former Milroy Branch of the Pennsylvania Railroad

Pennsylvania Railroad

The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....

 pass through the Mann Narrows Water Gap along the Kishacoquillas Creek

Kishacoquillas Creek

Kishacoquillas Creek is a tributary of the Juniata River in Mifflin County, Pennsylvania, in the United States.Kishacoquillas Creek drains the Kishacoquillas Valley, running along the foot of the Jacks Mountain ridge where it intersects with Honey Creek before passing through the Mann Narrows...

 near Reedsville

Reedsville, Pennsylvania

Reedsville is a census-designated place in the Kishacoquillas Valley of Mifflin County, Pennsylvania, United States. The population was 858 at the 2000 census...

.

Geology

Just below the ridge crest lies the contact between the older Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 Juniata Formation

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 Appalachian Mountains.-Description:The Juniata is...

 on the northwest slope, and the younger more erosion resistant Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 Tuscarora Formation

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.-Description:...

 geologic layer forming the steeper southeast slope and the crest. The Bald Eagle Formation

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:...

 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

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 the prominent ridge-forming Bald Eagle sandstone unit in the Appalachian...

 contains brachiopods and other marine fossils. 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.