Marine Eocene strata underlie about 26 square miles in the northeastern Orocopia Mountains, Riverside County. The newly discovered section, which totals about 4,800 feet in thickness, lies in a structural trough within basement rocks and is overlain unconformably by about 5,000 feet of undated non-marine clastic and volcanic rocks.

The Eocene beds consist of interbedded siltstone, sandstone, and breccia with some sandy limestone and conglomerate. On the east, at the base of the section, large granitic boulders up to 30 feet in diameter lie along the unconformity with granite. These give way upward to thick lenses of coarse granitic breccia with interbeds of buff siltstone and arkosic sandstone. The upper part of the section on the east consists of massive buff siltstone with sandstone and boulder beds. On the west the section consists largely of interbedded siltstone and sandstone with conspicuous isolated boulders of granite.

Mollusks and Foraminifera, including orbitoids, occur at many localities throughout the section. Some of the characteristic forms are: Turritella andersoni cf. lawsoni Dickerson, Turritella uvasana cf. applini Hanna, Clavilithes sp., Marginulina mexicana (Cushman) var., Pseudophragmina (Proporocyclina) psila (Woodring) and Pseudophragmina (Proporocyclina) clarki (Cushman).

This fauna indicates middle Eocene age, and the strata are possibly correlative with similar rocks of the Coast Ranges.

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