Afault line, the San Joaquin Hills blind thrust, lies eight miles below the hills.[3] Scientists have suggested that the San Joaquin Hills have been formed by uplift from this fault.[4]
The main ridge of the San Joaquin Hills runs southeast from the Upper Newport Bay area, attaining its maximum height of roughly 1,000 feet (300 m) near Laguna Beach. Many of the high ridges exceed 600 to 800 feet (180 to 240 m) in height. The hills stretch over a distance of about 16 miles (26 km) and can be up to 3 to 4 miles (4.8 to 6.4 km) broad. In many places, the San Joaquin Hills drop directly into the Pacific Ocean, creating the steep sea cliffs that characterize the region. Streams that originate on the west slope of the range include Buck Gully, Los Trancos Creek, Muddy Creek, El Moro Creek, Emerald Creek, Prima Deshecha Cañada, Segunda Deshecha Cañada, and Christianitos Canyon. The east side is drained by Bonita Creek, Sand Canyon Wash, San Joaquin Wash, and La Cañada Wash. Cities bordering the range include Newport Beach, Irvine, Laguna Hills, Aliso Viejo, Laguna Beach, Laguna Niguel, Dana Point, San Juan Capistrano, and San Clemente.
Because of erosion phenomena during the previous Ice Age, the San Joaquin Hills are not one continuous mountain range, but instead are bisected by numerous water gaps cut by rivers flowing southwest from the Santa Ana Mountains. In the north, Newport Bay was formed by the Santa Ana River switching its course to the south, cutting across the northern part of the range. Laguna Canyon was formed by San Diego Creek, but eventually the larger creek changed course and left a wind gap through the hills. The largest canyon by far is Aliso Canyon, cut by Aliso Creek. Further south, the hills dissipate into smaller ridges and are bisected by the canyon of Salt Creek. The southern extent of the hills are cut by San Juan Creek.
In past ages the land that is now the San Joaquin Hills lay under the ocean: construction for the San Joaquin Hills Toll Road unearthed a number of artifacts and more than 40,000 fossils, up to 35 million years old, including 5-million-year-old fossilized whales.[6]
Prior to the arrival of Europeans in the area, residents of the nearby Tongva and Acjachemen village of Genga frequented the hills.[7]
The hills take their name from Rancho San Joaquin, owned by Jose Andres Sepulveda. Formed in 1842 by the merger of two smaller ranches, this ranch included the northern part of the hills and likely had Laguna Canyon as its southeastern boundary. Rancho San Joaquin later became part of the Irvine Ranch.[8] Mexican Rancho Niguel was located in the southeastern section of the hills.
The San Joaquin Hills blind thrust may be the source of the earliest recorded earthquake in California, a large earthquake felt in what is now northern Orange County on July 28, 1769, by Gaspar de Portolá.[9]
State Route 73, also called the San Joaquin Hills Toll Road, extends through the length of the hills from Newport Beach to San Juan Capistrano. State Route 133 crosses the hills through Laguna Canyon in Laguna Beach, and they are also crossed in several other places by less major roads.
^Grant, Lisa B.; Mueller, Karl J.; Gath, Eldon M.; Cheng, Hai; Edwards, R. Lawrence; Munro, Rosalind; Kennedy, George L. (1999), "Late Quaternary uplift and earthquake potential of the San Joaquin Hills, southern Los Angeles Basin, California", Geology, 27 (11): 1031–1034, doi:10.1130/0091-7613(1999)027<1031:LQUAEP>2.3.CO;2.
^Roan, Shari (November 19, 1989), San Joaquin tollway work breaks historical ground, Orange County Register; Downey, Cheryl (April 28, 1993), Fine whale skeletons unearthed: Paleontologists discover nearly intact skeletons along route of San Joaquin Hills toll road, Orange County Register; Downey, Cheryl (May 11, 1993), A window to our past: Scientists who are checking the San Joaquin Hills tollway route remove and preserve fossils millions of years old before construction starts, Orange County Register; Brennan, Pat (June 18, 1993), Fossil whale makes a delicate journey to county museum: Five million-year-old `Bones' is carefully loaded onto a truck for its trip to Buena Park, Orange County Register; Brennan, Pat (April 4, 1998), Highway to history: Bones reveal a scenic road through the county's past, Orange County Register; Chang, Daniel (May 1, 1999), Dig this!, Orange County Register; Peralta, Jessica (May 17, 2002), Bones about it: Toll road construction unearths fossil treasure trove for school lectures, Orange County Register.
