Blog: US 101 – Buellton to Santa Maria

Posted 21 April 2016 by A.G. Sylvester ©

This part of US 101 lies in the Santa Maria basin, which, technically, is part of the Coast Ranges. The boundary between the north edge of the Transverse Ranges and the south edge of the Coast Ranges is approximately marked by the Santa Ynez River, which US 101 crosses at Buellton (exit 139).

The rocks you’ll see along the highway are very different in kind and age south and north of the Santa Ynez River (exit 139): Cretaceous marine shale and sandstone are south of the river; Pliocene and Pleistocene terrestrial sedimentary rocks predominate north of the river. Some geologists have mapped a major strand of the Santa Ynez fault along the south edge of the river to explain this great difference, but one geologist maintained that the contact is an unconformity now buried beneath the river sediments.

The Santa Ynez River once transported much sand to the sea.  Longshore currents then conducted the sand around Points Arguello and Conception to Santa Barbara-Ventura beaches until the large water storage Bradbury dam was completed across the river in 1956. Two lesser dams upstream also trap some of the river water and sand, so that the river at Buellton is usually dry during the summer. As a consequence, Santa Barbara beaches are increasingly sand-starved.

Between Buellton and Palmer Road, US 101 follows the course of Zaca Creek through a broad anticline in the Purisima Hills consisting mostly Monterey Shale exposed here and there in stream cuts. Beyond the turnoff to CA 154 (exit 146), the highway makes a broad left bend to proceed WNW along the trough of the Los Alamos syncline all the way through Los Alamos (exit 154) to Palmer Road. The north limb of the syncline dips gently southward. Monterey Shale is overlain here and there by almost flat-lying, weakly consolidated, alluvial sediments deposited from streams draining the San Rafael Mountains on the eastern skyline. You may see some shale outcrops in abandoned railroad cuts a few miles north of the CA 154 turnoff, but otherwise, the rocks, such as they are, are covered with vineyards. The south limb of the syncline dips more steeply, but the rocks are obscured by vegetation.

From Clark Avenue (exit 164) to Betteravia Road (exit 169), US 101, surrounding houses, and premium agricultural fields lie on Holocene sand dune deposits. The sand has been transported to the sea by the Cuyama/Santa Maria River and then blown back ashore by strong westerly winds.

You cross from Santa Barbara County into San Luis Obispo County before the bridge over the Santa Maria River at PM 100. The cliff on the north side of the river exposes fluvial sandstone deposited by the Santa Maria River.

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Errata: Revised 19 May 2016

 Roadside Geology of Southern California, 1st Printing, January 2016

By Arthur G. Sylvester and Elizabeth O’Black Gans

Page 7: Arrows in normal fault diagram should be reversed in slip direction.

Page 105, line 2: Salton Trough extends into the Gulf of California, not the Gulf of Mexico.

Page 158, line 1: 1892 earthquake, not 1890.

Page 205, figure caption: longitude = 119° 45.12N

Page 269, figure caption: Attribution should read: “Modified from Yeats and Grigsby, 1987, and Hopps and others, 1992.”

Page 269, line 5 should read: “south-dipping Padre Juan fault,” not Pitas Point fault.

Page 272, figure caption: La Conchita mudslide on Jan. 10, not 14.

Page 334, figure caption should read: “Modified from Hopps and others, 1992, and Nicholson and others, 2007.”

Page 370, Add to Transverse Ranges references, Yeats, RS, and FB Grigsby, 1987. Ventura anticline: Amphitheater locality, California, pp. 219-223, in Hill, ML, editor, Decade of North American Geology Centennial Field Guide, Volume I, Geological Society of America, Boulder, Colorado, 490 p.

Page 390: Photo credit: Molly Gans.

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Blog: CA 60 – East Los Angeles to Beaumont

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.


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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Blog: CA 118 – Moorpark to Granada Hills

Posted 19 April 2016 by A.G. Sylvester ©

CA 118 from Moorpark through the Simi Valley passes through the Sespe Formation (late Oligocene, early Miocene) exposed in several canyons on the south flank of Big Mountain north of the highway, especially in the vicinity of Madera Canyon Road (exit 22).

At the east end of Simi Valley and the west end of San Fernando Valley, between Kuhner Drive (exit 30) and De Soto Avenue (exit 35), are splendid bold outcrops and road cuts of hard, light brown sandstone. These thick-bedded sedimentary rocks are assigned to the Chatsworth Formation of Upper Cretaceous age (about 70 million years). Total thickness of the formation, whose base is not exposed, is about 6,000 feet, comprising thousands of individual turbidite flows. It is broadly folded into a gently west-plunging synform. Hill slope exposures are tan because of surface weathering of iron-bearing mica and clay minerals over time, whereas road cuts are gray because they have been exposed to weathering only a relatively short period of time – too short for oxidation to occur. You may study these rocks more closely along old Santa Susanna Pass Road by exiting south on Kuhner Drive (exit 30), proceeding eastward, and then returning to CA 118 via Topanga Canyon Boulevard (exit 34, CA 27). Good views of the formation may also be had at Stoney Point Park near the end of Topanga Canyon Boulevard.

DSC00211CalabasasCOPY Each sandstone bed in the Chatsworth Formation formed as a massive, deepwater marine flow or turbidite deposit, which flowed tens of miles down submarine canyons and was deposited on the ocean floor at depths of 4,000 to 5,000 feet. Fossils are sparse in the formation, because they are usually battered and ground up during turbulent transport over such distances. The line across the center of the image is CA 118. (34° 16.5N, 118° 39.0W)

The upscale community of Porter Ranch lies north of CA 118 between De Soto and Zelzah Avenues. A utility company stored natural gas in a subsurface reservoir of a depleted oilfield near the community. One of the aging wells into the reservoir developed a massive leak of methane and ethane in 2015 that wasn’t controlled and capped until early 2016. Residents were obliged to leave their homes for several months during the remedial work. An estimated 100,000 tons of gas was released, making it the largest such leakage in US history in terms of environmental impact.

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Blog: CA 76 – Oceanside to Lake Henshaw

Posted 19 April 2016 by A.G. Sylvester ©

CA 76 goes up the flood plain of the San Luis Rey River from Oceanside past the airport to Mission San Luis Rey de Francia (PM 4.9). The mission is situated on river terrace overlooking the flood plain.

The small rounded hills a short distance beyond and north of the mission are Miocene volcanic plugs, locally called the “Sleeping Indian Hills.” Morro Hill (elev. 922 feet), the prominent hill on the skyline, is a volcanic neck.

Bonsall (SD PM 11.8)

CA 76 penetrates well into the Peninsular Ranges batholith. One of the main components of the western series of plutonic rocks is Bonsall tonalite, which is a specific form of granitic rock that consists mainly of plagioclase feldspar. The remaining minerals comprise quartz, biotite, and hornblende. The dark gray blobs in the granite, called mafic enclaves, are typical in the Bonsall tonalite as is its “boulder pile” mode of outcrop. You will see good examples of both in the steeper hills around Pala Mesa (SD PM 14.9) and where CA 76 crosses I-15 (SD CB 16.5). Many of the stream courses you cross between here and Lake Henshaw are choked with such big boulders, suggesting an immature landscape evolution. A nick point in San Luis Rey River between Harrah’s Rincon Casino and the La Jolla Indian Reservation is yet another line of evidence of immature landscape development.

Pala (SD PM 23)

The hills north of the community of Pala are laced with haul roads leading to gem-rich pegmatite dikes. Since the early 1900s these quarries have yielded incredible specimens of both pink and green tourmaline, kunzite, topaz, beryl, and other semi-precious gems. Most of the mines are inaccessible because they are on private property.

White pegmatite dikes in brown gabbroic rocks near Jamul, CA. (32° 43.7N, 116° 53.6W)

Look for good views of Palomar Mountain off to the east as you drive from Pala to Pauma Valley where the highway rises steeply out of the San Luis Rey River flood plain. The Elsinore fault strikes along the steep, southwest face of the mountain, which is famous for its observatory with the 200-inch Hale Telescope, once the largest in the world. With its clear air and position 6,100 feet above sea level, Palomar Mountain was a splendid site for the telescope, but it is now hampered by light pollution from San Diego.

CA 76 practically traces the Elsinore fault as does the course of the San Luis Rey River as far as Lake Henshaw in Warner basin. The lake is an enlarged sag pond along the Elsinore fault zone which lies east of the Henshaw Ranger Station, the store, and the restaurant. In fact, the San Luis Rey picnic area lies squarely upon the fault. The roadcut on the north side of the highway is strongly fractured and contorted as a result of deformation in the fault zone.

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Blog: Burnt Shale and Saugus Formation CA 23

Posted 13 April 2016 by A.G. Sylvester ©

The Monterey Shale has several pockets of “burnt rock” resulting from the combustion of natural gas in very rich organic layers. The pockets are in a belt about 14 miles long along Oak Ridge south of Fillmore. The largest pocket is in Grimes Canyon where vesicular black, reddish-brown, yellow, and white rock is sold as “lava rock” and used for decorative stone. Low temperature oxidation of pyrite appears to play an important role as a catalyst for the spontaneous combustion of the hydrocarbon in the rock. Rock temperatures exceeded 3,000° F, melting the rocks and producing small intrusive masses.

To reach the Grimes Canyon quarries, proceed south from Fillmore on CA 23 past the Elkins Ranch oil field (PM 21.5) at the base of the grade. Shallow wells produce from the Sespe Formation at the crest of the Oak Ridge anticline.  Outcrops on both sides of the canyon at PM 20.5 expose the Monterey burnt shale. The quarries are on privately owned and require trespass permission to visit them.

Continue uphill and south on CA 23. All the south-dipping beds are on the south flank of the Oak Ridge anticline. Quarries west and east of the large roadcuts in the switch backs at crest of the grade (PM 19.6) expose yellow sand and gravel of the lower Saugus Formation, which consists of interbedded shallow-marine to brackish water sandstone, siltstone, pebble-to-cobble conglomerate, and coquina beds that grade laterally and vertically into non-marine sandstone, siltstone, and conglomerate.  The quarries produce bulk sand and gravel for road base and plaster. The lower Saugus Formation was deposited in a delta by west-flowing streams in Pleistocene time (about 2.5 to 0.7 million years ago).


Roadcut at the crest of CA 23 in yellow sand and gravel of the Saugus Formation (34° 20.57N, 118° 54.46W).

CA 23 continues downhill about three miles to Moorpark through light gray to brown pebbly soil derived from weathering of upper Saugus Formation gravel, sand and clay, which also dip south in the south flank of the Oak Ridge anticline.

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Blog: Malibu Canyon – County Road N1

Malibu Canyon
Colorful northwest-dipping sandstone beds of Sespe Formation (Oligocene) overlain by lower Topanga Formation sandstone (Miocene) in steep roadcut on County Road N1. (34° 03.8N, 118° 42.5

Posted 19 January 2016 by A.G. Sylvester ©

Malibu Creek is the only stream that transects the Santa Monica Mountains. Its headwaters are in the mountains north of the range divide. It must have existed here before the mountains did and cut its canyon as the mountains rose.

Entry into the canyon is via County Road N1 from PCH alongside the Pepperdine University. The campus is situated on northwest-dipping, variegated sandstone beds of the Sespe Formation (Oligocene) overlain by lower Topanga Formation sandstone (Miocene). About a mile or so up the canyon, the highway crosses a buried thrust fault and then goes into a deep gorge where the stratigraphic section is repeated.

About 4 miles up the canyon near its intersection with Piuma Road, County Road N1 passes from the sedimentary section into a 2-mile-long stretch of interbedded lava flows and breccia of the 16- to 13-million-year-old Conejo Volcanics. These extrusive rocks erupted both on land and in the sea in middle Miocene time. Outcrops of the volcanic rocks in the western Santa Monica Mountains are typically unstratified dark brown breccias and highly fractured lava flows.

Near its intersection with Mulholland Drive, County Road N1 goes down the north side of the Santa Monica Mountains via an alluvial valley lying in a syncline of upper Topanga Formation. You won’t see outcrops along this stretch of road until you are within 1 mile of US 101.

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Roadside Geology of Southern California

Roadside Geology of Southern California

 by Arthur Gibbs Sylvester and Elizabeth O’Black Gans

Roadside Geology of Southern California is the motorist’s guide to the geologic features of southern California, including  Mojave Desert, Salton Trough,  Peninsular Ranges,  Los Angeles basin, and the Transverse Ranges as seen along  main highways. It does not include offroad byways or hiking trails.  The book will be about 400 pages with over 300 colored maps, photographs, and interpretative diagrams. Publication is being scheduled for spring 2016.

The publisher will be Mountain Press Publishing Company of Missoula, Montana, who has published many Roadside Geology titles covering about 30 states.