[3][4] The interior of the Perris Block has various low bedrock mountains, hills and bedrock plains with intervening sediment-filled valleys, that make up six erosional surfaces sculpted by the effects of the vertical oscillation of the block during the Plio – Pleistocene era.
These sediments under the Pomona and San Bernardino Valleys form the Inland Santa Ana Basin aquifer.
East of the Santa Ana River lie the La Loma Hills, Box Springs Mountains and northeastward of them, across the canyon of Spring Brook and of the Pigeon Pass Valley, the range of mountains formed by Blue Mountain, Reche Summit, Olive Hill and the Kalmia Hills that border the northeast edge of the Perris Block along the San Jacinto Fault Zone to the Perris Plain.
The western side of the central Perris Block and across its width eastward south of the Santa Ana River, is bordered by the range of the Temescal Mountains.
Above the San Jacinto Basin, the northwest corner of the Perris Plain is drained by Sycamore Canyon and Tequesquito Arroyo into the Santa Ana River.