The geologic formations in Catoctin Mountain Park were deposited about 500 million years ago as volcanic lava flows and layers of sediment.
Catoctin Mountain originally had two different types of rocks: sedimentary and igneous.
Catoctin's rocks are now metamorphic rocks which formed when they were altered (not melted) by heat and pressure. This metamorphism happened in the Catoctin area when the continent of North America collided with South America and Africa. 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.