The bedrock beneath Swift Run Gap is 1.05 billion year old granitic rocks of the Blue Ridge basement complex. The type location for the Swift Run Formation, a Neoproterozoic metasedimentary unit, is approximately east of the gap. A steeply dipping, northwest-striking transverse fault cuts through Swift Run Gap, and differential erosion of the fractured bedrock along this fault may be responsible for the development of a gap at this location.