Most of the rock visible in Catoctin dates from the Precambrian or Cambrian times (500 million years ago or more) and was formed from a lava flow of molten rock. This cooled and was then covered by sea-bottom sediment. Next, the heat and pressure of mountain building changed the original lava into a metabasalt.Younger rocks were once present, lying on top of the rock we see today, but over millions of years they have been eroded away by wind, rain, running water and ice. A few thousand feet below Catoctin remains an intrusive granite basement rock that is over one billion years old.

Rough outcrops of weathered metamorphic rocks, primarily Weverton quartzite and Catoctin Greenstone, typify the geology of Catoctin. These tough materials provide a protective cap for the mountains, and can be easily viewed at Chimney Rock, Wolf Rock, and Hog Rock.

In the eastern half of the park at Chimney Rock and Wolf Rock, the rocks have been metamorphosed into a hard, weather-resistant rock called quartzite. This area is part of the Weverton Formation. The Weverton Formation, named for its formation near the town of Weverton, is the main ridge-making formation in the eastern mountains in Maryland. The formation is composed of quartz cemented together by a secondary, less developed quartz. At Chimney Rock there are also numerous joints (fractures) in the rock. Melting water that filled spaces then froze and expanded breaking away pieces of rock, a process known as frost wedging, formed these joints.

At Hog Rock, located in the center of the park, the Catoctin metabasalt formation is the bedrock. This metabasalt is dark greenish-gray metamorphosed igneous rock, which is highly resistant to weathering. It is known as the Catoctin Greenstone.

Thurmont Vista is a good place to observe a geologic occurrence in the landscape. More than 180 million years ago a great border fault occurred when the area now occupied by the valley slid down about one mile, probably over millions of years, from the area now occupied by the mountain top.

The Loudoun formation lies between the Weaverton and Catoctin formations on the slope to the east of park Central Road and Catoctin Hollow Road. This formation is composed of conglomerate (a sedimentary rock of irregularly sized gravel) and phyllite (metamorphosed shale). The Loudoun formation is less resistant to weathering and has worn away to create a valley between the two hills of more resistant rock. Park Central Road north of the Visitor Center, follows part of this eroded valley.

Catoctin's geology was of great importance to Native Americans and early European settlers. Running through the western portion of the park is a zone of metavolcanic rock called metarhyolite. This dark blue-gray rock was used by Native Americans for creating projectile points and tools. Tools made of Catoctin metarhyolite have been found as far away as the eastern shore of the Chesapeake Bay.

Iron ore was found at the base of the mountains by early settlers. Between 1776 to 1903, the Catoctin Furnace heated the iron ore along with charcoal and limestone to produce nearly pure iron. This was the first important industry in the Mechanicstown, or Thurmont area.