Masses of rock strongly resembling pebbles and boulders of granite occur in the pre-Cambrian sediments of the Black Hills about four miles west of Custer, South Dakota. These pseudo pebbles and boulders range in size from 1/8 inch up to nearly one foot in diameter, have smooth surfaces, and are well rounded. Evidence is presented that they were formed from continuous thin layers of calcareous sediment within siltstone beds. These were pinched out into small lenses during folding and rounded by rolling along shear planes. Later they were partially replaced by feldspars, quartz and micas by contact action of an intrusive granite
