Perris Block
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The Perris Block is the central block of three major fault-bounded blocks of the northern part of the Peninsular Ranges. The Perris Block lies between the Santa Ana Block to the west and the San Jacinto Block to the east.[1] The Perris Block was named by Walter A. English in 1925 for the city of Perris, located near the center of the block.[2]
Structurally stable for millions of years, the Perris Block is an internally unfaulted, eroded mass of Cretaceous and older granitic rocks of the Southern California Batholith and metasedimentary basement rocks. These rocks compose various ranges of mountains and hills and monadnocks and underlie the valleys within it. It is bounded on the west by the Chino Fault and Elsinore Trough, on the east and northeast by the San Jacinto Fault Zone including the San Jacinto Valley graben. It is bounded on the north by the Cucamonga Fault Zone, in the San Bernardino Valley and San Jose Hills Fault in the Pomona Valley. To the south, the Perris Block is bounded by the San Felipe Fault Zone between it and the Temecula, Aguanga, and Anza sedimentary basins that lie between Temecula and Anza.[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.[5][3]
The northern part of the Perris Block lies north of the Santa Ana River. Here in the Pomona Valley and San Bernardino Valley it has been mostly buried by the sediments from the Transverse Ranges as they rose over the last 2-3 million years. Exceptions are the Jurupa Mountains and Pedley Hills that still rise above that deposition. The now obliterated Slover Mountain also did so before it was mined out of existence. 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.