He National Park Service - Center for Urban Ecology
Chimney Rock, a scenic overlook along Catoctin Mountain Park’s Wolf Rock/Chimney Rock Loop trail, is composed of the Weverton Formation’s metamorphosed sedimentary units.

Chimney Rock, Catoctin Mountain Park

Geology of the Blue Ridge

The rocks of the Blue Ridge physiographic province bear witness to the power of large scale volcanism, metamorphism, and erosion. They are the oldest rocks in the National Capital Region and form the “basement” for all the other rocks.

The Blue Ridge province was once part of an ancient supercontinent that existed 1 billion years ago in the Proterozoic Eon. Approximately 600 million years ago, rifting began to pull this supercontinent apart.

The rift created a basin for a new ocean which existed long before the Atlantic, the Iapetus Ocean. It also left behind igneous rocks, which were formed when magma rising from deep within the Earth erupted on the surface. This is now the most common type of rock of the Blue Ridge province.

As the Iapetus Ocean grew larger and the land masses spread farther apart, river and marine sediments were deposited on top of the igneous rocks. Eventually the Iapetus Ocean began to close, and three different mountain-building episodes occurred when land masses collided into the ancient North American continent.

These collisions metamorphosed, folded, and uplifted the previously deposited igneous and sedimentary rocks. The result was the Appalachian Mountains, which at that time soared to heights resembling the modern Himalayan range, which is also forming by collisions between two large land masses.

Erosion during the following 265 million years cut down the once towering Appalachian Mountains to modern elevations, and exposed the metamorphosed igneous and sedimentary rocks that were once buried deep within the core of the mountain belt.

You can stand on the ancient rocks associated with Iapetus Ocean rifting on the tall ridges of Catoctin Mountain Park. Metamorphosed sedimentary rocks (originally formed on the margin of the continent as the rift widened) now form the heights surrounding Harpers Ferry National Historical Park.

The Harpers Formation

The Harpers Formation, a metamorphosed siltstone deposited along the edge of an expanding Iapetus Ocean basin, is exposed in this dramatic and scenic precipice at Harpers Ferry. This heavily folded unit made construction of transportation routes difficult, particularly along waterways, and resulted in a need to construct this tunnel.

View from Jefferson Rock

This view from Jefferson Rock (Harpers Formation) was particularly admired by Thomas Jefferson, who called it “perhaps one of the most stupendous scenes in Nature”. Metamorphosed sedimentary rocks form these ‘stupendous’ heights. Click the link below learn more about Jefferson’s description of this picturesque landscape.

Thomas Jefferson at Harpers Ferry
Learn How Native American Indians Used the Rock at Catoctin Mountain Park

Geologic Time Scale (USGS)
Geologic Glossary (USGS/NPS)
Geologic Map of the National Parks in the National Capital Region
Geologic Map of Harpers Ferry National Historical Park
Map of C & O Canal National Historical Park
NPS Resource Reports for Parks




National Park Service
Center for Urban Ecology
4598 MacArthur Blvd NW
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