The Piru Gorge Sandstone member of the Ridge Route Formation (Pliocene) was deposited as a turbidite-deltaic complex within the tectonically active Ridge basin. In cross section, the Piru Gorge Sandstone is 190 m thick and over 5 km long. The sandstone consists of (1) a lower turbidite sequence, overlain by (2) multistacked channel-levee cycles, and (3) interchannel deposits. These sequences are arranged into several megasequences, each up to 60 m thick. The lower turbidite interval is up to 10 m thick and consists of facies B, C, and D of Mutti and Ricci Lucchi. The interval is interbedded with, and grades laterally into, lacustrine shale. The channel-levee sequence is up to 30 m thick and consists of crossbedded channel sandstone, slump folded strata, and levee deposits arranged in fining- and thinning-upward cycles. The sequence makes up the middle and upper parts of each megasequence. Interchannel deposits are interbedded with, and laterally transitional with, the channel-levee sequences. The deposits consist of thick intervals of organic-rich, bioturbated, mudcracked mudstone and sandstone that locally have rootlets and animal tracks.

Paleocurrent, thickness, and petrographic studies indicate the Piru Gorge Sandstone was derived primarily from the north-northwest with minor contributions from the east and west. Petrographic data indicate the arkosic sandstones were derived from granitic source terranes.

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