Roadside Geology of Southern California

Posted 20 April 2016 by A.G. Sylvester ©

From its trisection with I-5, US 101, and I-10 on the east edge of the concrete-channeled Los Angeles River and Civic Center, CA 60 proceeds eastward through Boyle Heights, crosses I-710 in East Los Angeles, and proceeds on older alluvium along the south edge of the Repetto Hills to Montebello. The red soil is typical of that developed on older alluvium (Pleistocene). Between Montebello and its intersection with the Rio Hondo River in south El Monte, CA 60 passes through vaguely bedded, shallow marine “Pico” silty sandstone in the north limb of the Montebello anticline. The Montebello oil field stretches along the south flank of the anticline.

CA 60 crosses the Rio Hondo, CA 19, the San Gabriel River, and I-605 and goes into the northwest end of the Puente Hills in the City of Industry. The gap between the Repetto Hills and the Puente Hills about one-half mile south of CA 60 is known as the Whittier Narrows. The San Gabriel River is unique here because it divides into two main distributaries after leaving its mountain canyon. One of these distributaries forms the headwaters of the Rio Hondo that flows through the west side of Whittier Narrows, thence southwestward to the Los Angeles River, and then enters the Pacific Ocean at Long Beach. The other branch, the San Gabriel River, flows through the east side of Whittier Narrows and thence across the Los Angeles basin, emptying into the ocean in Anaheim Bay at Seal Beach. The earth-fill dam was constructed across the narrows in 1954 as a flood-control structure.

Structurally the San Jose Hills are an east-west trending anticline that extends into the northwest part of the larger Puente Hills. The Puente Hills are bounded on the east by the Chino fault and on the southwest by the Whittier fault and its subsidiary branch, the Workman Hill fault. Together these faults are northwest extensions of the Elsinore fault. A half-mile wide zone of right-stepping en echelon folds lies between the Chino and Whittier faults, indicating that they comprise a right-lateral shear couple. Most of the displacement has occurred on the Whittier fault, which dips north about 70 degrees. Its north side has been thrust upward about 7,000 feet relative to the south side with about 4,000 to 7,000 feet of right-slip judging from stream displacements. The main rocks in the San Jose and Puente Hills are sedimentary strata of the Monterey Formation that dip gently north and east. The red soil is common in this area

East of Industry, a northeast-trending stretch of CA 60 passes through the community of Diamond Bar, located on a light gray, thin-bedded shaly siltstone member of the Monterey Formation. Where CA 60 bends eastward, it proceeds through nearly flat-lying sandstone member of the Monterey Formation. The Chino fault lies along the north flank of the hills where the hills meet the flat valley.

 

San Bernardino Valley

The San Jose Hills and Puente Hills form the west and south margins of the San Bernardino Valley, more popularly and culturally known as “The Inland Empire”. Most of the valley soil developed on alluvium shed from the San Gabriel Mountains to the north. The southwest part of the valley is underlain by at least 1,300 feet of non-marine Quaternary deposits and several thousand feet of marine sedimentary rocks, probably Pliocene and upper Miocene in age. The straight northwest-trending boundary between the Puente Hills and the valley marks the locus of the Chino fault, a high-angle reverse fault that dips steeply southwest, placing upper Miocene sandstone, siltstone, and conglomerate on the southwest side of the fault upon Pliocene conglomerate on the northeast.

 

Jurupa Mountains and Valley

You enter the Peninsular Ranges batholith at the Country Village exit from CA 60. In vicinity of the Pedley Avenue exit, look on the north side of the freeway and observe the white streaks, which are pegmatite dikes, in the hillsides which are reddish brown because of weathering gray granite that contains a high content of iron-rich minerals, mainly biotite and hornblende.

Other geologic features of the Jurupa area and City of Riverside are described on p. 182 of Roadside Geology of Southern California and are not repeated here.

 

Box Springs Mountains

CA 60 and I-215 coincide for five miles through the city of Riverside. They continue past the campus of the University of California at Riverside (UCR) and split at the south end of the Box Springs Mountains. The mountains are an ecological reserve between Riverside and Moreno Valley comprising refuge habitat for fauna that have been driven out of surrounding areas now covered with urban developments.

The Box Springs Mountains are made of granite, part of the Peninsular Ranges batholith, as is evident from the “boulder pile” character of much of the mountains. Horizontal joints impart a pseudo-layering that led one writer to conclude incorrectly that the mountains are made of metamorphosed sedimentary rocks. The best place to observe the nature of the granite bedrock and its jointing is in the vicinity of the Central Avenue/Watkins Drive exit from CA 60.

A big “C” was constructed in 1957 on the west face of the mountains about 1,500 feet above the UCR campus by its students. Moreno Valley High School students embedded a similar “M” on the south side of the mountains in 1966. Because of its isolated prominence in an extensive urban area, the mountain crest is adorned by several TV and telecommunications antennae.

Moreno Valley

Moreno Valley is part of the Perris plain. The relative flatness of its surface is the result of weathering and erosion of homogeneous granitic bedrock whose plagioclase and micas break down by chemical weathering, leaving a grus of uniform grains of quartz and K-feldspar. Running water then erodes and transports these grains across relatively gentle slopes, ultimately to nourish the beaches along the southern California coastline. The hills and ridges rising from the Perris plain are erosional remnants that have yet to be consumed.

 

San Timoteo Badlands

California 60 proceeds eastward from the flat valley floor, crosses the north strand of the San Jacinto fault, and then heads abruptly into a deep, winding canyon cut into foothill badlands of the San Jacinto Mountains. The rocks are a terrestrial sedimentary succession, the San Timoteo Formation, consisting of weakly indurated, light-gray to buff, arkosic sandstone, pebble conglomerate, and interbedded light-reddish to light-gray silty claystone. Basal beds of the formation have granitic clasts as large as 2-3 feet in diameter transported from granitic outcrops to the north and east. The formation was deposited between 5.4 and 1.3 million years ago as alluvial fans derived from the rising San Jacinto Mountains. The finer-grained strata were deposited onto an expanding plain in the San Jacinto Valley during Pliocene and Pleistocene time. The San Timoteo beds are folded into a gentle anticline against the San Jacinto fault, but most of the sequence along the highway dips gently northward. Vertebrate fossils diagnostic of Pliocene and Pleistocene time have been recovered in the San Timoteo Formation, which overlies the Mt. Eden Formation. The latter formation contains a wealth of fossils of Pliocene age, including mastodons, dogs, cats, bears, pigs, ground sloths, camels, deer, antelope, raccoons, wolverines, horses, and rhinos.

A rich and scientifically important cache of over 1,500 bone fragments representing 35 vertebrate species was discovered here in 2010, consisting of giant ground sloths, camels, llamas, deer, horses, and small rodents. They represent a period of geologic time about 1.4 million years ago, a time when the area was considerably more green and lush than it is today. The fossils are on display at the Western Science Center in the town of Hemet.

Beaumont

CA 60 terminates at its junction with I-10 in the town of Beaumont, which lies at the west end of San Gorgonio Pass.

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