Allegheny Front - Geology

Geology

Most of the Allegheny Front is capped by a nearly horizontal, erosion-resistant stratum (rock layer) of white Pottsville conglomerate, sometimes where flat with younger Carboniferous strata on top. The silica-cemented gravel-containing Pottsville rock formed as part of a vast delta in the Pennsylvanian geologic period.

The region was then uplifted and folded during the Alleghenian Orogeny about 320-250 million years ago, during the Carboniferous and Permian geological periods, forming the modern Appalachian Mountains as the ancient continents of North America and Africa collided due to plate tectonics processes.

Subsequent erosion (for more than 200,000,000 years) has prefentially removed various softer surrounding rocks, especially the easily dissolved Greenbrier limestone, leaving areas of the hard Pottsville conglomerate as a caprock protecting softer rock strata immediately beneath. The Pottsville bedrock outcrops conspicuously in various more exposed areas along the Front's eastern edge. Many of these clifftops offer broad scenic views, unlike most mountaintops within the generally forested Appalachians, and the more readily accessible are popular tourist destinations, including Dan's Rock on Dans Mountain in Maryland and Dolly Sods and an overlook along U.S. 50 in West Virginia.

Since the eastern side of the Front is drained by Chesapeake Bay rivers that drop fairly rapidly through the Ridge and Valley Province to the low-elevation waters of the Great Appalachian Valley, erosion on the eastern slope of this caprock layer has been much more intense than on the western slope, where drainage to low elevations is spread over a greater distance (particularly considering stream meanders) through the Appalachian Plateau to the Ohio River. These contrasting patterns of erosion produce a steep escarpment along much of the Front's eastern edge, with the mountains west of the Front generally grading more gently into the Appalachian Plateau.