Discover Pennsylvania and Explore the Geodiversity of the Allegheny Portage Railroad
- Wayne Munday
- Apr 4
- 3 min read
Sip back and discover Pennsylvania and explore the geodiversity and fossils of The Allegheny Portage Railroad National Historic Site. Roughly a 90 minute drive on the US-22 eastwards from Pittsburgh or 3 hour drive westwards from Washington DC on the I-70. The Allegheny Portage Railroad National Historic Site spans 59Km through the Allegheny Mountains in Blair and Cambria Counties of Pennsylvania. This railroad represents innovative engineering and a much needed solution to transporting people and goods over the mountains where slopes exceeded five degrees at elevations ranged from 346 - 734 meters above sea level.
Opened in early 1834 the Allegheny Portage Railroad was the final component of the Pennsylvania Main Line Canal and the first railroad to traverse the Allegheny Mountains. It established a direct connection between Philadelphia and Pittsburgh to facilitate commerce, passenger travel and military movements. In 2011 the Allegheny Portage Railroad was recognised as a National Underground Railroad Network to Freedom where the railroad was used as an underground transportation route by enslaved African Americans to gain freedom in the years before the Civil War.
The region's geology is part of the Allegheny Front and the Ridge and Valley physiographic provinces. The Ridge and Valley province is bordered by the Blue Ridge and Piedmont provinces to the east and the Appalachian Plateau to the west and is subdivided into three distinct sections: the Hudson, Central, and Tennessee sections.
The fossil record of the area includes marine stromatolite algae and terrestrial land plants dating back to the mass extinction at the end of the Permian Period. The Ridge and Valley physiographic province is a segment of the Appalachian Highlands and extends from upstate New York to central Alabama forming a distinct landscape of alternating ridges and valleys.
This region was shaped by the tectonic compression associated with the formation of the supercontinent of Pangea that led to the folding of Paleozoic sedimentary rocks. The valleys consist of limestone and shale while the ridges are composed of more resistant sandstones and conglomerates and form the province’s trellis drainage pattern shaped by the erosion of the folded and tilted rock layers.
The railroad connected Hollidaysburg and Johnstown surmounting the Allegheny Mountains and linking the Pennsylvania Main Line Canal’s Eastern and Western Divisions. Historically, the Allegheny Front a 280Km long escarpment with an elevation change of about 610 meter posed a significant barrier to transportation.
The Allegheny Front consists of linear and rounded hills dissected by narrow valleys that gradually ascend to the escarpment’s summit. In contrast, the western side has undulating hills composed of horizontally layered sedimentary rocks that remain largely undeformed. The Allegheny Front separates the Appalachian Plateaus from the Ridge and Valley Provinces and extends from northern Pennsylvania to south eastern West Virginia. The eastern side of the Allegheny Front features highly folded, faulted, and deformed rock formations shaped by the Alleghenian orogeny one of the mountain building events that formed the Appalachian Mountains occurring almost entirely within the Permian Period between 299 - 251 million years ago.
The Allegheny Portage Railroad National Historic Site contains 300 million year old Mississippian and Pennsylvanian limestones with karst features that runs through a large forested watershed area known as Blair Gap Run that flows east.
The dominant rock types in the Allegheny Front include sandstone, siltstone, and shale and the notable formations and features in the region include the Blue Knob Mountain (950 meters), Hunter Rocks, Turtle Rocks, Wolf Rocks and the Wopsononock Lookout. The Allegheny Portage Railroad National Historic Site tells a story of engineering ingenuity of the natural world to overcome a challenge.
