Roadside Geology of Southern California

Posted 21 April 2016 by A.G. Sylvester ©

AKA: The Great Southern Overland Stage Route

Geology along County Road S2 between Ocotillo and Warner Springs

Map prepared 2015 by Libby Gans©

County Road S2 begins in the arid Yuha Desert at Ocotillo in the southwest corner of the Salton Trough and gradually ascends the east edge of the Peninsular Ranges into oak and pine meadowland near Warner Springs. Much of the route faithfully follows the historic southern Overland Trail that was later followed by the equally historic Butterfield Overland Mail stagecoaches (1858–1861). Some of the route skirts the west edge of Anza-Borrego Desert State Park, a designated and protected state wilderness area and one of the largest state parks in North America with an area of 916 square miles, almost the size of the Rhode Island. S2 offers several vistas across the park and access to a few unimproved dirt roads that penetrate it. You would be well advised to have a proper off-road vehicle before attempting any of those roads, and note that all natural and cultural features in the park are fully protected. Excellent guidebooks by Paul Remeika and Lowell Lindsay (1992) and Lowell Lindsay and Diana Lindsay (2006) provide directions to the park and its roads and sites. Distances along S2 are indicated by green mile markers (MM) and increase from north to south.

The south half of S2 follows the trace of the Elsinore fault along the straight southwest edge of the Coyote Mountains from Ocotillo to Agua Caliente Springs and then along the northeast edge of the Tierra Blanca Mountains escarpment. Subparallel fault strands in a 200-foot-wide zone mark the fault trace along the bases of straight, uplifted mountain fronts where crystalline basement and sedimentary rocks are juxtaposed. Aligned vegetation lineaments, springs and seeps, benches, scarps, and aligned saddles mark some of the individual fault traces.

The Coyote Mountains have a core of crystalline basement rocks overlain by early Miocene Alverson Formation volcanic flows, mid-Miocene to Pliocene Imperial Formation, and Pliocene to Pleistocene fanglomerate. The Imperial Formation, deposited on the floor of an early version of the Gulf of California, contains a rich fauna of fossil molluscs having an affinity to Gulf Coast taxa, suggesting the Gulf of California was once connected to the Caribbean Sea. The Pliocene beach sand deposits contain seashore-loving sand dollars (Dendraster sp.), sea biscuits (Clypeaster sp.), and oysters shells (Ostrea sp.) that look very much like what you would collect along any San Diego region beach or backwater bay today. Fossil collecting is not permissible within Anza-Borrego Desert State Park.

 

Volcanic Hills

Enter Anza-Borrego Desert State Park at MM 56 and pass through a water gap eroded through a basaltic lava flow at MM 55. These volcanic rocks, part of the Alverson Formation, form the hummocky terrane west of the highway and are similar in type and age to other dark-colored olivine basalt and lighter-colored andesite units that crop out in several places in the Anza-Borrego region. One of these units is the Jacumba basalt near Table Mountain, about 15 miles to the south and 3,000 feet higher. The elevation difference of these rocks indicates the amount of vertical movement along this section of the Elsinore fault zone. These and similar volcanic rocks extend southward along the fault for 30 miles across the border into Mexico. Their extrusion is thought to have coincided with the initial rifting of Baja California from the Mexican mainland about 23 to 15 million years ago. The Alverson andesite has been dated at about 17 million years.

 

Carrizo Badlands

A short, dirt side road along the highway at MM 51.5 leads to a sweeping view of the Carrizo Badlands and the entire Vallecito–Fish Creek drainage basin. These badlands formed in interbedded lake and alluvial sediments, such as marl, claystone, sandstone, and conglomerate, which were deposited in the southwest part of the Salton Trough in Pliocene and Pleistocene time.

The region was an extensive depositional basin, now surrounded by mountains on all sides, where considerable subsidence and sedimentation occurred in recent geologic time. The 9,000-foot-thick sedimentary section is famous for its rich assortment of mega- and microvertebrate fossils, especially in the Pliocene and Pleistocene Imperial and Palm Spring Formations. Two to four million years ago the Carrizo Badlands were covered with grasslands, lakes, streams, and scattered forests that supported large herds of mastodons, llamas, camels, horses, and tapirs. Since that time, the remains of those animals became fossilized, and the basin has been folded, faulted, and uplifted. Hiking or driving through this huge area is the only way to gain a proper appreciation for its vastness and unparalleled solitude.

View northeast across Carrizo Badlands. Canis Latrans Creek is in the foreground. (32°50N, 116°10W)

At the crest of Sweeney Pass (MM 51.3) you stand on a mesa covered with a thin veneer of desert pavement on light-colored sedimentary rocks deposited in lake and savanna environments in Pliocene and Pleistocene time. If you were to walk about 10 miles cross-country in a northeast direction, you would go from these richly fossiliferous sediments at your feet down section into late Miocene alluvial fan deposits in Split Mountain Gorge.

North of Sweeney Pass, County Road S2 goes down through bouldery alluvial fan deposits in Sweeney Canyon to Carrizo Creek (MM 49). The creek drains 1,200 square miles of watershed on Jacumba and In-Ko-Pah Mountains and flows to San Sebastian Marsh below sea level near the Salton Sea.

 

{B} Faults at Mountain Palm Springs and Canebrake

About four hundred native California fan palms (Washingtonia filifera) abound in six main palm groves easily reached on a good, half-mile gravel road (turnoff at MM 47) that traverses a cholla forest west of County Road S2. Typically these palm trees flourish in areas of near-surface water, commonly where groundwater is dammed behind a subsurface zone of impermeable rocks, such as gouge along a fault zone. Here the palms are at the juncture of the Elsinore and Tierra Blanca faults.

Native palm oasis at Mountain Palm Springs Camp located at the juncture of the Elsinore and Tierra Blanca faults. The bedrock here is a pretty, white, brecciated, biotite-hornblende granite of the great La Posta pluton. (32°51.7N, 116°13.3W)

Several Elsinore fault strands exhibit evidence of recent earthquake activity along County Road S2 between Sweeney Pass and Canebrake Ranger Station. The highway lies along the northeast edge of the Tierra Blanca Mountains, whose mountain front is a youthful scarp on the west side of County Road S2 opposite the Well of Eight Echoes. Right-slip along this fault strand is at least 4 miles. Prominent alluvial fans extend from the mountain front toward the highway from two narrow canyons. Such canyons, with a narrow stem at the mountain front that widens into a goblet shape farther into the mountains, are called wineglass canyons and are evidence of recent uplift. Small, aligned scarps, about 3 feet high, can be traced across the modern fans between Canebrake (MM 45) and Agua Caliente Springs (MM 40). The scarps look youthful enough to have formed during the 1892 Laguna Salada earthquake. The rough, corrugated surface of the Canebrake fan consists of granitic rock brought down from the Tierra Blanca Mountains.

Vallecito Creek (MM 43) leads to an extensive area of serrated brown hills and winding washes in the Vallecito Badlands east of County Road S2. See the earlier discussion about the Carrizo Badlands.

 

{B} Agua Caliente Springs to Rancho Vallecito

Historically, hot (167° to 185°F) groundwater percolated up from depth at Agua Caliente Springs (MM 38) along the Elsinore fault zone. The hot water not only altered strongly fractured granitic rocks to light-colored, easily weathered clay and claystone, but it also provided warm water for mineral baths and bathing pools. In the mid-twentieth century, an earthquake reorganized the subterranean plumbing, shifting the hot water discharge away from the old rock Indian pools to a nearby site now developed as a state park. Since the earthquake, the old pool has filled with cold spring water.

Northeast of Aqua Caliente Springs, County Road S2 proceeds a couple of miles through a broad area of arroyos, roadcuts, and intervening hillsides, all carved in grayish tan, poorly bedded, pebbly to bouldery conglomerate, and mantled with cactus. These sediments, deposited on the margin of the basin, are assigned to the Pliocene and Pleistocene Canebrake Conglomerate, and are mainly granitic boulders and cobbles derived from the Pinyon-Vallecito highlands to the north and northeast. This conglomerate grades laterally into lake and river deposits of the Palm Spring Formation of late Miocene and Pliocene age.

Rancho Vallecito is on the north edge of the Sawtooth Mountains Wilderness. From there you have a fine view of the impressive escarpment of the Laguna Mountains, 5 miles to the southwest, surmounted by Garnet Mountain (elevation 5,679 feet) to the southwest and Oriflamme Mountain (elevation 4,813 feet). About 5 miles northeast of Rancho Vallecito is massive, dark gray Whale Peak (elevation 5,349 feet) in the Vallecito Mountains.

 

{B} Campbell Grade

Campbell Grade (MM 30) crosses a bedrock spur that connects the Vallecito Mountains north of County Road S2 with the Sawtooth Mountains to the south. Vallecito Creek has cut a gorge across the spur. The main trace of the Elsinore fault cuts through here, too, but the fault is obscured in this area by large rock slides that came off the east side of the Peninsular Ranges. Minor faults are clearly exposed in roadcuts along County Road S2 where it lies in granite and older rocks intruded by the granite. From a viewpoint at the top of the grade, look southeast and see the trace of the 1858–1861 Butterfield Overland Mail stagecoach route.

 

{B} Box Canyon

County Road S2 follows the Elsinore fault from Campbell Grade through Mason Valley and then turns abruptly eastward into Box Canyon at the north end of Mason Valley along the south side of Granite Mountain (elevation 5,633 feet). Miners worked several prospects in pegmatite intrusions in Julian Schist in the Box Canyon area, presumably for tourmaline and related gem minerals. Dikes and layered metasedimentary rocks give a striped appearance to hillsides on both sides of County Road S2 around Granite Mountain. The east side of Granite Mountain is a large Quaternary landslide.

Gray and brown Julian Schist and quartzite in roadcut exposures in Box Canyon are intruded by light-colored pegmatite dikes associated with intrusion of the 94-million-year-old La Posta pluton of the Peninsular Ranges Batholith. (33°00.8N, 116°27.0W)

The road in Box Canyon was cut by the Mormon Battalion with hand tools in 1847 and was the first road into southern California. It was widened in 1858 to permit passage of the famed Butterfield Overland Mail stagecoaches. Here you will also see a fine display of vegetation that characterizes this part of the Colorado Desert, including barrel cactus, ocotillo, agave, and the tallest cholla specimens in the Anza-Borrego region.

 

Earthquake Valley

County Road S2, CA 78, and the Pacific Crest Trail intersect at Scissors Crossing (MM 17) below the south face of Grapevine Mountain (elevation 3,955 feet). County Road S2 jogs briefly onto CA 78, and then continues north of CA 78 for 12 miles along the Earthquake Valley fault in Earthquake Valley. Local lore says that Earthquake Valley once held a natural lake and that an earthquake in the early 1900s caused it to drain into San Felipe Creek, thence through Sentenac Canyon to the Salton Sea.

Geologic map of Elsinore fault strands between Earthquake Valley and Temecula.            Map prepared 2015 by Libby Gans©

The Elsinore fault is more or less continuous from I-8 as far as Lake Elsinore. In Earthquake Valley, however, several major subsidiary fault strands leave the Pinyon and Vallecito Mountains and head northwestward, parallel to the main fault. County Road S2 follows the Earthquake Valley fault to Warner Valley. The Agua Tibia fault strikes from the northeast side of Lake Henshaw to the southwest side of the Palomar Mountain Range and the Palomar astronomical observatory. CA 79 mostly follows the Agua Caliente fault zone from Warner Springs to Aguanga. Numerous subparallel faults slice the 10-mile-wide zone of granitic crystalline rocks and the metasedimentary rocks they intruded between the main Elsinore fault and the Agua Caliente fault zone. All of these faults are considered to be active, although none has generated an earthquake much greater than magnitude 4.5 in historic time.

The Elsinore fault strikes along the west edge of Warner Valley, giving rise to such features as the linear Mesa Grande ridge and Lake Henshaw, a sag pond where water accumulated along the fault zone because drainage was impounded. Warners Ranch (PM 0.7) was a stop on the southern overland route into California. The first Butterfield stagecoach stopped here in 1858 on its 2,600 mile, twenty-four-day trip from Tipton, Missouri, to San Francisco.

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