The late Miocene marine Castaic Formation in Ridge basin is over 2,000 m (6,600 ft) thick and consists mostly of slope and deep-sea fan facies. The Castaic Formation is vertically transitional into the overlying Marple Canyon Sandstone Member of the Ridge Route Formation and laterally interfingers with the Violin Breccia to the southwest. The slope facies consists of poorly bedded mudstone interbedded with sandstone, conglomerate, and coquina deposits interpreted as turbidite-filled slope channels. The slope facies follows the northwest trend of the basin and occurs on the northeast, north, and southwest sides of the basin. The slope channels have laterally adjacent levee deposits. Paleocurrents in the channels are to the west-northwest and southwest-southeast, whereas pa eocurrents in the levee deposits are to the northwest-southeast. Large slide blocks, slump-folded strata, and breccia beds are common in the slope facies.

Deep-sea fan deposits consist of inner and middle fan channel and interchannel facies and outer fan depositional-lobe facies. They are confined to the center of the basin, interfinger into slope facies to the southwest and northeast, and are overlain by nonmarine fan-delta complexes to the north. The channels contain thick sandstone deposits that thin and fine upward and are laterally discontinuous, whereas the adjacent interchannel deposits consist of thin-bedded sandstone and mudstone sequences which form inclined wedges of highly slump-folded strata. Depositional-lobe facies thicken and coarsen upward and consist of interbedded sandstone and mudstone which form laterally continuous deposits with minor channeling and slump folding. Paleocurrents in these deposits are to the south-so theast and suggest sediment transport down the axis of the basin from the north-northeast.

Ridge basin was a relatively shallow-marine trough about 6 km (4 mi) wide and 10 to 20 km (6 to 12 mi) long. The deep end of the basin was to the southwest and connected to the Ventura basin across the San Gabriel fault. Typical deep-sea fans did not develop in Ridge basin during Castaic Formation time; instead extensive slope deposits formed along the margins of the basin and thick but narrowly confined turbidite-filled channels and depositional lobes filled the valley or trough of this basin.

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