Old Rag Mountain - Geology

Geology

Old Rag Mountain is underlain by Old Rag Granite, named for its ubiquitous exposure on the mountain, formed during the Grenville Orogeny about a billion years ago. About 400 million years after the Grenville orogeny during the Catoctin Formation, deposition of basaltic magma occurred during the formation of the Iapetus Ocean forming a layer of greenstone over the granite. This was followed by the Weverton Formation, Harpers Formation, and Antietam formation in which sand and rock sedimented on the ocean floor forming quartzite and sandstone deposits. Finally a period of sedimentation of shells and skeletons of foraminifera resulted in deposition of a layer of limestone. Approximately 700 million years after the Greenville Orogeny, the Iapetus Ocean began to close resulting in the Alleghenian Orogeny when the Old Rag Granite and layers of rock deposited upon it where transported westward and eventually thrust up over the limestone bed around it, forming Old Rag and the Blue Ridge Mountains.