The Perris Block, 30 to 90 mi southeast of Los Angeles, is an eroded mass of Cretaceous and older crystalline rock, sculptured by six erosion surfaces. Two are narrow valley systems and the other four nearly horizontal planes or remnants thereof. The oldest surface is a bowl and narrow valley system with its base at an elevation of 1,100 to 1,400 ft; it is partially filled with alluvium which has yielded a lower Pliocene (Clarendonian) mammalian fauna. It was probably warped slightly before any of the younger surfaces were cut. The nearly horizontal erosion surfaces, listed in the proposed order of their formation, are now at elevations of approximately 1,700, 2,500, 2,100, and 1,500 ft and are thought to be in quasi equilibrium with the present major drainage systems. The second narrow valley system has a base near sea level; it was probably formed just after the 1,700 ft horizontal surface.

The Perris Block lies between the Los Angeles Basin and the lofty San Jacinto Mountains. The latter are marked by an elevated, low-gradient valley system. During Pliocene and Pleistocene time the Los Angeles Basin sank many thousands of feet, the San Jacinto Mountains rose considerably, and the Perris Block oscillated vertically. The vertical tectonics must have been governed by deep horizontal flow, isostatic in nature, and related to the somewhat greater flow-couple indicated by right-lateral strike-slip faulting on the San Jacinto fault system.

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