The dacite volcanic plugs, lava domes, intrusive sheets, and felsitic rhyolite-dacite between Morro Rock and Islay Hill are part of the Mono Rock-Islay Hill Complex of the Oligocene epoch (from 27 to 23 million years ago). The complex lies east of the San Gregorio-San Simeon-Hosgri fault (the SG-SS-H fault). This complex is one of three probable sources of the volcanic clasts within the sandstone and conglomerate of the Miguelito and Edna members of the Pismo Formation in the Point Sur area 145-160 km (90-100 mi) to the north and west of the SG-SS-H fault. Based on paleomagnetic signatures, the Morro Rock-Islay Hill Complex was rotated 40 to 50 degrees, perhaps during late Miocene or early Pliocene time.