Catoctin Rocks!

Geology is a science that studies rocks and soils to learn about the Earth's history. For this activity, read the text below and answer the questions about Catoctin Mountain Park's geology.

The geologic formations in Catoctin Mountain Park were deposited about 500 million years ago as volcanic lava flows and layers of sediment. Then, South America and Africa collided with North America producing high mountain ranges like the ancestral Appalachian Mountains and the Blue Ridge Mountains, which are a province of the Appalachian Mountains range.

Catoctin Mountain originally had two different types of rocks: sedimentary and igneous.

Sedimentary rocks formed when sediments that were deposited by wind, water or ice, hardened and solidified over time, as they were buried by more sediment. Millions of years ago, there used to be siltstone, sandstone, shale, graywacke (type of sandstone), and conglomerate in the Catoctin area.

Igneous rocks formed when molten rock (magma) cooled. Millions of years ago, there used to be basalt and rhyolite in the Catoctin area.

Catoctin's rocks are now metamorphic rocks that formed when they were altered (not melted) by heat and pressure. This metamorphism happened in the Catoctin area when the continents collided. Rocks in Catoctin were metamorphosed turning shale into phyllite, quartz sandstone into quartzite, graywacke into metagraywacke, conglomerate into metaconglomerate, basalt into metabasalt (or greenstone), rhyolite into metarhyolite.

Metamorphic rocks are harder than the surrounding sedimentary rocks of the valley, so that's one of the main reasons Catoctin Mountain is higher in elevation.

There was also a major fault in this area, called a Triassic Border Fault. Around 200 million years ago, during the Triassic period, beds of rock broke along what is now U.S. Route 15 and created a fault. West of the fault, the land moved up and was eroded to become Catoctin Mountain. East of the fault, the land slid down and is now the Frederick Valley.

The frigid temperatures of the Ice Age arrived in the Catoctin area some 2 million to 10,000 years ago. No glaciers reached the area, but prolonged freeze-and-thaw cycles helped fracture the rocks.

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 rocks, being quartzite, and that's why they are some of the highest points and vistas because they withstand weathering better.

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.


Question 1. A collision of what continents produced the high mountain ranges like the ancestral Appalachian Mountains and the Blue Ridge Mountains?

A. South America and Africa collided with North America

B. Europe collided with North America

C. Asia collided with North America

Question 2. What types of rocks are now in the Catoctin Mountain area?

A. Sedimentary

B. Metamorphic

C. Igneous

Question 3. What is the name of a major fault in this area?

A. Permian Border Fault

B. Jurassic Fault

C.Triassic Border Fault

Last updated: April 10, 2015

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