Jacks Mountain

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Jacks Mountain
Elevation	2,321 feet
Location	Pennsylvania, USA
Range	Appalachian Mountains
Coordinates	40°37′43″N 77°37′53″W / 40.62861, -77.63139
Topo map	USGS Barrville (PA) Quadrangle

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

It is named for Jack Armstrong, an 18th-century Indian trader. In the spring of 1744, Armstrong and two servants encountered three Delaware men from Shamokin, Pennsylvania in the Juniata River narrows. For unknown reasons, a disagreement between the parties resulted in the murders of Armstrong and his men.

U.S. Route 22 (US 22), the William Penn Highway, and the former Pennsylvania Railroad Main Line, now the Norfolk Southern Railway's Pittsburgh Line, follow the Juniata River through the Jacks Narrows water gap between Mapleton and Mount Union. U.S. Route 322 and the former Milroy Branch of the Pennsylvania Railroad pass through the Mann Narrows Water Gap along the Kishacoquillas Creek near Reedsville.

[edit] Geology

Just below the ridge crest lies the contact between the older Ordovician Juniata Formation on the northwest slope, and the younger more erosion resistant Silurian Tuscarora Formation 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 contain 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.