ThePerris Block is the central block of three majorfault-bounded blocks of the northern part of thePeninsular Ranges. The Perris Block lies between theSanta Ana Block to the west and theSan Jacinto Block to the east.[1] The Perris Block was named byWalter A. English in 1925 for the city ofPerris, located near the center of the block.[2]
Structurally stable for millions of years, the Perris Block is an internally unfaulted, eroded mass ofCretaceous and oldergranitic rocks of theSouthern California Batholith andmetasedimentarybasement rocks. These rocks compose various ranges of mountains and hills andmonadnocks and underlie the valleys within it. It is bounded on the west by theChino Fault andElsinore Trough, on the east and northeast by theSan Jacinto Fault Zone including theSan Jacinto Valleygraben. It is bounded on the north by theCucamonga Fault Zone, in theSan Bernardino Valley andSan Jose Hills Fault in thePomona Valley. To the south, the Perris Block is bounded by theSan Felipe Fault Zone between it and theTemecula,Aguanga, andAnza sedimentary basins that lie betweenTemecula andAnza.[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 sixerosional surfaces sculpted by the effects of the vertical oscillation of the block during thePlio –Pleistocene era.[5][3]
The northern part of the Perris Block lies north of theSanta Ana River. Here in the Pomona Valley and San Bernardino Valley it has been mostly buried by the sediments from theTransverse Ranges as they rose over the last 2-3 million years. Exceptions are theJurupa Mountains andPedley Hills that still rise above that deposition. The now obliteratedSlover Mountain also did so before it was mined out of existence. These sediments under the Pomona and San Bernardino Valleys form theInland Santa Ana Basinaquifer.
East of the Santa Ana River lie theLa Loma Hills,Box Springs Mountains and northeastward of them, across the canyon ofSpring Brook and of thePigeon Pass Valley, the range of mountains formed byBlue Mountain,Reche Summit,Olive Hill and theKalmia 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 theTemescal Mountains. They run southeastward along the Elsinore Trough to theTemecula Basin. Eastward, these mountains run to the south of the Santa Ana River toSycamore Canyon, bordering on theBox Springs Mountains. They enclose theGavilan Plateau and partially enclose theRiverside Valley on the west, south and east. They extend their ancient eroded surfaces eastward to thePerris Plain and thePlains of Leon, which they border on the west, southward to the Temecula Basin.
ThePerris Plain, anuplifted peneplain, is drained primarily in theSan Jacinto Basin, theSan Jacinto River watershed flowing intoLake Elsinore, that is dotted bymonadnocks and by several other ranges of mountains and hills. Above the San Jacinto Basin, the northwest corner of the Perris Plain is drained by Sycamore Canyon andTequesquito Arroyo into the Santa Ana River. Below the San Jacinto Basin, the Perris Plain is bounded on the south by thePlains of Leon, which continues the plains of the Perris Block and by the mountains of the southern Perris Block.
Plains and Valleys of the Perris Block within the San Jacinto Basin:
Mountains and hills of the Perris Block within the San Jacinto Basin include:
The Perris Block south of the Perris Plain and San Jacinto Basin is characterized by the extension of the Perris Plain, thePlains of Leon that includes within it theDomenigoni Valley and westernDiamond Valley, the upper part of the valley ofWarm Springs Creek and connectingtablelands,French Valley,Auld Valley andBuck Mesa.[6] The southern Perris Block also includes the hills and mountains to the east and south of the Perris Plain, that with the Plains of Leon are all drained by the creeks tributary to theSanta Margarita River into the Temecula Basin.
Tucalota Creek, drains theRawson Mountains (south of Diamond and Domenigoni Valleys),[7]Black Mountain, the western slopes ofRed Mountain, theMagee Hills, theTucalota Hills andBachelor Mountain intoLake Skinner then toSanta Gertrudis Creek, a tributary ofMurrieta Creek.[8] The west slope of the southern Magee Hills, and the north slope of theBlack Hills are drained bySanta Gertrudis Creek or its tributaries, another Murrieta Creek tributary.Billy Goat Mountain,Oak Mountain,Round Top the west and south slopes of theBlack Hills are drained byTemecula Creek or its tributaries, as are the south slopes of Red Mountain,Little Cahuilla Mountain andCahuilla Mountain.
33°51′9″N117°15′19″W / 33.85250°N 117.25528°W /33.85250; -117.25528