The upper Miocene Castaic Formation was deposited in the Ridge basin along the east side of the San Gabriel fault. Paleoecologic analysis of the Castaic Formation furnished information about the paleotopography along the fault, and provides information about activity on the fault during the late Miocene. The northern part of the Castaic Formation was deposited in an embayment bounded on the west by land extending southward immediately west of the fault to the vicinity of Castaic. Southeastward from Castaic, this positive feature extended as a submarine sill, behind which the maximum water depth in the Castaic basin was not less than 50 fathoms. The microfauna in the mudstones deposited in this silled basin consists of diatoms, radiolaria, normal planktonic forams, and dwarfed and deformed benthic forams. The southern part of the Castaic Formation was deposited on a broad shelf open westward to the Pacific Ocean. Sediments along the southeastern margin of the basin suggest little relief and slight tectonic activity in the areas of the southern Soledad basin and the western San Gabriel Mountains, in contrast to the high relief west of the San Gabriel fault along the northwestern boundary of the Castaic basin. The Castaic Formation was deposited in a normal-marine environment. Lateral faunal variations, interpreted in the past to indicate brackish conditions, are attributed instead to variations in substrate and topography along the basin margin.