Ridge Basin is a graben dropped between the San Gabriel and the San Andreas faults as a result of local tension or differential pressure where the Garlock and the San Andreas rift faults meet and westward movement causes the latter to be bent into a great arc. Following the inception of the graben in or about Middle Miocene time its point sank with unprecedented rapidity, forming an inland lake in which 21,000 feet of Pliocene and Lower Pleistocene(?) sediments were deposited to comprise the thickest post-Miocene section in California.

Resting with marked unconformity on Eocene strata older than the basin is the basin sequence; reading upward, fragments of non-marine Mint Canyon (early Upper Miocene) yielding Hipparion, Merychippus, Protohippus, and Parahippus(?) near-by, then 2,000 feet of upper Monterey (late Upper Miocene) yielding a Neroly marine macrofauna at the base and grading upward to brackish strata, then 18,000 feet of Pliocene(?) lacustrine deposits more or less brackish at the bottom, and finally 3,000± feet of Lower Pleistocene(?) lacustrine beds.

The 23,000-foot post-Mint Canyon succession is continuously exposed as a northwest-plunging syncline between Castaic and Gorman, the axis of which is essentially the course of the new Ridge Route highway. The southwest edge of the succession is everywhere angular conglomerate deposited along the San Gabriel fault scarp. The conglomerate grades northeasterly to shale, which, in turn, grades back to pebbly sandstone farther northeast.

The vertical throw of the San Gabriel fault which bounds the basin on the southwest scissors near Castaic, and then increases from zero at the pivot to 23,000± feet 20 miles northwest. The particular throw occurred between Middle Miocene and Middle Pleistocene times concomitant with deposition. Following this extreme activity the deep-seated fault plane froze, and has now been essentially dead for a million or so years between Castaic and Beartrap Canyon.

After deposition ceased the basin was deformed and degraded approximately 15,000 feet in the Coast Range revolution of Middle and Upper Pleistocene time. During this interval crystalline Frazier Mountain was dragged or thrust across the extreme northwest point of the basin from an original site computed to have been east of the junction of the Garlock and San Andreas rift faults, thus causing piracy of drainage systems to occur on a grand scale, and giving to Piru Creek its present anomalous upper course.