Rockfish Gap is underlain by greenstones of the Catoctin Formation, a late Neoproterozoic geologic unit exposed throughout the Blue Ridge Mountains in Virginia, Maryland, and southern Pennsylvania. Catoctin greenstones were originally erupted as basaltic lava flows 550 to 565 million years ago and later metamorphosed and deformed during the late Paleozoic Alleghenian orogeny (300 to 330 million years ago). Rockfish Gap lies astride a north-northwest trending fracture zone; some fractures are intruded by diabase dikes of Jurassic age. The broken bedrock along this fracture zone is more easily eroded than unfractured bedrock - likely the reason for the formation of the gap at this location.