Geology

The rock found in Catoctin Mountain Park began around 500 million years ago as layers of igneous rock - basalt and rhyolite - and sedimentary rock - shale, sandstone, graywacke, and conglomerate. Around 250 million years ago, the North American continent collided with Africa, folding and uplifting the rock into a massive mountain range as great as the modern Himalayas. The intense heat and pressure metamorphosed the existing rock. Shale, sandstone, grawywacke, and conglomerate changed into phyllite, quartzite, metagraywacke, and metaconglomerate. Basalt and rhyolite became metabasalt and metarhyolite. Since then, the rock has slowly eroded, exposing layers of geologic history you can find in Catoctin Mountain Park.

 

Metabasalt (left) and metarhyolite began as igneous rocks that metamorphosed under intense heat and pressure.

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Clockwise from top left: phyllite, quartzite, metaconglomerate, metagraywacke. Each began as deposits of sedimentary rock that were then metamorphosed.

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Where to See Rock Formations

There are four rock formations in Catoctin Mountain Park: Harpers, Weverton, Loudoun and Catoctin.

The Harpers Formation, on the far southeastern corner of the park around the Lewis property, contains phyllite, quartzite and metagraywacke.

You'll find the Weverton Formation in the area that includes Thurmont Vista, Wolf Rock, Chimney Rock, and Headquarters and it contains quartzite, metaconglomerate, phyllite and metagraywacke. Chimney Rock and Wolf Rock have the hardest rock, quartzite, which has eroded more slowly than surrounding rock.

The Loudoun Formation, in the area around the Catoctin Mountain Park Visitor Center, contains phyllite, metaconglomerate and metagraywacke.

The Catoctin Formation, in the area around Blue Ridge Summit Overlook, Hog Rock Overlook and Cunningham Falls, contains metabasalt. The Catoctin Formation, on the far western part of the park that includes Owens Creek Campground, contains metarhyolite.

 

Deposition, uplift, and erosion have exposed several geologic layers to be found in Catoctin Mountain Park.

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