Rocks of the middle Tertiary (mostly Oligocene) Sespe and Lospe Formations crop out in the Santa Maria basin and vicinity. Lithofacies are typical of alluvial fan/plain deposits: clast- and matrix-supported conglomerates interbedded with planar to crossbedded sandstones are commonly overlain by sandstone-shale sequences.

Although Lospe deposits near San Simeon, in northwestern San Luis Obispo County, and at Point Sal, near Santa Maria in Santa Barbara County, are presently more than 90 km apart on opposite sides of the San Simeon-Hosgri fault zone, they were derived from the same western source consisting partly of silica-carbonate rock and ophiolite terranes. Proximal fan lithofacies near San Simeon and medial to distal facies at Point Sal indicate an easterly draining system in which San Simeon was closer to the source. Point Sal sandstone clasts containing more than 5% detrital potassium-feldspar were probably derived from an Upper Cretaceous-Paleogene sedimentary terrane to the southwest.

The middle Tertiary paleogeology of the Santa Maria basin was dominated by an easterly draining, aggrading alluvial fan complex near Point Sal. Displacement along a proto-Hosgri fault probably initially uplifted the highlands west of the fan complex. Lospe alluvial deposition at Point Sal was separate from Sespe-Lospe deposition to the south, southeast and north, with the possible exception of an area 3 km northeast of Santa Maria.

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