En Route Talking Points

Here, are some talking points that may enrich your travel time to the field trip destinations covered in this manuscript. Please edit as needed... Southern California has a rich human and natural history and there's lots to talk about! The talking points are organized by freeway.

Freeway map of southern California.

Freeway map of southern California (Google Maps).

The San Gabriel Freeway, Interstate 605 (I-605)

The 605 follows the San Gabriel River across the Los Angeles Basin
San Gabriel River
- River transports sediment from the San Gabriel mountains to its base level, the Pacific Ocean at Seal Beach
- River erosion creates natural pathways
- Pathways become footpaths
- Footpaths become horse trails
- Horse trails become thoroughfares
- Thoroughfares become highways
- Whittier Narrows "water gap", between Beverly Blvd and State Route 60 (The 60)
  - The San Gabriel River and Rio Hondo River, just to the west, created the low-lying Whittier Narrows by eroding faster than the hills have been uplifted.
  - The narrows are about 2 miles wide and 800 feet deep (relative to Whitter Hills), bisecting the Puente Hills into 2 parts: the Montebello Hills to the west and Whittier Hills to the east (From, Geology Underfoot in Southern California, “Vignette 9 – A Boon to Communication, The Whittier Narrows”).
The Los Angeles Basin
- Started forming 16 million ago (Sylvester and Gans, 2016) when the northward movement of the Pacific Plate tore off a chunk of the North American Plate (western Transverse Ranges). As this block of crust rotated, tensional stresses rifted open a series of basins.

An animation showing the formation of the San Andreas Fault and the rotation of the western Transverse Mountain Range.

- The basins were first filled with lava flows, evidence for this includes the 16 million-years-old Glendora Volcanics, later, sediment eroded from the continental highlands filled the depressions with up to 30,000 feet of silt, sand, and gravel (Sylvester and Gans, 2016).
Whittier and Puente Hills (east of the 605) are being actively uplifted along the Whittier fault, the northern branch of the Elsinore fault, which runs along the base of these hills; this fault is active as evidenced by the recent Whittier (1987, M 5.9), Chino Hills (2008, M 5.5), and La Habra (2014, M 5.1) earthquakes.
- Comprised of steeply tilted, south-dipping sedimentary rock that is typical of sediment in the LA Basin: mudstone, siltstone, sandstone, and conglomerate
Gravel Pits
- Between the I-10 and I-210
- Mining of the San Gabriel River Alluvial fan for sand and gravel
San Gabriel Mountains

The Foothill Freeway, Interstate 210 (I -210) west of the 605 Freeway

The 210 spans the foothills of the San Gabriel Mountains, which are largely vegetated and developed bajadas, coalesced alluvial fans
- The alluvial fans formed as the San Gabriel Mountains were uplifted and eroded by streams; where streams exit, steep and narrow mountain canyons, they slow and deposit their sediment load, forming alluvial fans
- Many of these stream channels are dammed at the mouth of the canyons to create “catch flow basins” that impede dangerous boulder-sized clasts that come with the inevitable debris flows
Sierra Madre fault zone
- Runs along the base of the foot of the mountains
- Has facilitated as much as 10,000 feet of vertical uplift of the crust, which is today expressed as the San Gabriel Mountains
- Associated with the San Fernando fault that produced the 1971 Sylmar earthquake
I-210 west at the CA-134 interchange, look for “benched” road cuts, a common practice used to increase slope stability
Driving through Sylmar takes one past the epicenter of the 1971 magnitude 6.5 Sylmar earthquake that caused over a billion dollars in damage and 65 deaths, mostly from the collapse of the Olive View Hospital
- Movement along the Sierra Madre Thrust, caused the earthquake and the mountains to grow by 8 feet
Los Angeles Aqueduct
- As the 210 mergers with I-5, look ahead, and you should notice a long pipe-like feature coming down the hillside just to the right of the freeway

The Los Angeles Aqueduct.

Los Angeles Aqueduct.

- This is a chute for carrying water, baffled for aerating
- One of three aqueduct systems that brings water to the greater Los Angeles area, the other two being the California Aqueduct which delivers water from the western Sierra Nevadas, and the Colorado River Aqueduct brings water from the Colorado River
- Designed by self-taught engineer William Mulholland to divert water from the Owens River and its tributaries to the burgeoning city of Los Angeles

William Mulholland

William Mulholland.

- - Water would be used to boost land values and profits for developers
- Remarkable engineering feat considering it was designed and built over 100 years ago, so water flows downhill over its entire length, requiring no pumps and only gravity to transport water from its source 235 miles to L.A.
- Building the aqueduct necessitated some shady business dealings, violence, and even deaths
  - Residents of Owens Valley had plans to use the water from the Owens River to develop agriculture and livestock
  - Fred Eaton, a former mayor of Los Angeles and politically well-connected, used a contact in Owens Valley to, through deception, buy up land in Owens Valley and with it water rights to the Owens River
  - Mulholland and Eaton were also working behind the scenes with a collection of friends and business partners to buy up cheap land in the San Fernando Valley, which would be made drastically more valuable once it was provided with a reliable water source
    - This story serves as part of the plot for the critically acclaimed 1974 movie Chinatown, starring Jack Nickolsoninks to an external site.
  - While completed ahead of schedule and under budget, the completion of the aqueduct consumed machines, mules, and men, with several construction-related deaths
  - Owens Valley residents rebelled upon learning that all the water of “their river” was being diverted to Los Angeles
    - In 1924 seventy armed Owens Valley men took control of an aqueduct gate and shut off the flow of water
    - In 1927 a 45-foot section was blown up
    - The uprisings were permanently quelched when Mulholland sent out machine gun-armed horseback patrols with orders to shoot to kill anyone disturbing the aqueduct
I-5/I-14 interchange
- Some of the overpasses here collapsed during both the 1994 and 1971 earthquakes

State Route 14 (CA-14)

Placerita Canyon Road
- Placerita Canyon earned its name and fame for the placer (gold deposited as sediment in a stream channel) deposits that were discovered here in 1842, six years before the Gold Rush began in the western Sierra Nevada Mountains.
Santa Clara River
- Longest undammed river in southern California (Prothero, 2011) and significant in that its channel hasn’t been modified by human construction
- Braided stream, common in the foothills of mountains
- Multiple intertwining channels, weaving around channel bars
- Ephemeral stream channel in that it is dry unless it has just rained
- The dry channel is deceiving, because water is flowing in the subsurface as groundwater, which will eventually be utilized by communities downstream (Sylvester and Gans, 2016)
Vasquez Rocks
- After passing Agua Dulce Road, look to the left (north) for spectacular stacks of tilted, red-brown sediment, jutting out of the ground
- See Chapter 4 for a more thorough description
Lamont Odet Vista Point
- We are at the very edge of the Pacific Plate. Looking northward, our line of sight crosses the California Aqueduct, the San Andreas fault, and the plate boundary with the North American Plate, and Antelope Valley beyond.
- Disappointingly, the San Andreas fault is not an obvious gash, since it has been over 160 years since its last major break in southern California and erosion has “smoothed out” disturbances, like scarps and ground cracks. Instead, the trace of the fault are the low hills, running northwest-southeast, through which Highway 14 cuts through and lie on the opposite side of Lake Palmdale, continuing east into the mountains.
- Lake Palmdale was originally a sag pond, a depression that collects water where there has been subsidence of the crust due to faulting. Here, the San Andreas fault is segmented, “stepping over” to create a pull-apart basin and sag pond – note how the arrows on either side of the pull-apart basin to pointing away from each other, creating tensional stress and subsidence. Recently, the sag pond was damned to store more drinking water for the cities of Paldale and Lancaster that spread out from the edge of the North American Plate before us.

A map view illustration showing how a pull-apart basin can form.

Forming a pull-apart basin.

- The California Aqueduct provides the greater Los Angeles area with a significant portion of its water. Constructed in the 1960s, it brings water from the western Sierra Nevada Mountains and transports it 700 miles, making it one of the longest aqueducts in the world. It was built above ground so repairs can be made more quickly after damaging San Andreas fault earthquakes; a matter of days vs. weeks or even months if the water were contained in underground pipes (Sylvester and Gans, 2016)
Palmdale Roadcut
- The ridge through which CA-14 transects formed through intense compression of the relatively young, Pliocene Anaverde Formation. This sliver of the crust is caught between two branches of the San Andreas fault: the San Andreas proper along the southern side and a secondary branch, the Littlerock fault, trending along the northern margin of the ridge, making the ridge an example of a squeeze block (see pages 18-19) or a pressure ridge (Sylvester and Gans, 2016). Excavations for the geologic study show that there is about 50 feet of right-lateral offset along the south side of the ridge.
Antelope Valley
- Once through the Palmdale roadcut, you have crossed onto the North American Plate. “Antelope Valley”, containing the bustling communities of Palmdale and Lancaster, is a more appealing name invented by developers to describe what is the southwestern corner of the Mojave Desert.
- As recently as a few tens of thousands of years ago, lakes covered large parts of Palmdale and Lancaster. As our climate has warmed and dried, the lakes evaporated.
  - Many archaeological sites occupy what would have been ancient lakeside beaches (Sylvester and Gans, 2016)
- Rosamond Hills and Soledad Mountain (just north of the community of Rosamond, west of the highway)
  - The colorful rocks that make up these hills are Miocene-age rhyolitic volcanics, which have been hydrothermally altered, adding to the coloring
  - Mined extensively for gold, silver, and radioactive minerals
  - Rock collectors have frequented this area for geodes, chalcedony, and opal
- Garlock Fault
  - Trace is marked by the straight eastern mountain front of the Tehachapi mountains
  - One of the longest faults in California at 160 miles
  - An exception to the norm, in that it is a left-lateral strike-slip fault – most horizontal faults in southern California are right-slip faults
  - Offset drainages abruptly jump to the right when crossing the fault
- El Paso Fault and El Paso Mountains
  - El Paso Fault
    - Runs along the foot of the El Paso Mountains
    - Branch of the Garlock Fault
    - Facilitating the uplift of the El Paso Mountains

A Google Earth image of the Garlock Fault.

Google Earth image with approximated location of the Garlock fault. - Google Earth.

El Paso Mountains
- Contain Red Rock Canyon State Park
- Many cuestas of colorful sedimentary rock separated by ephemeral stream channels
El Paso Mountains “Gorge”
- Pass used by CA-14 through the El Paso Mountains
- Formed by stream erosion, making it a water gap – note stream channel and stream terraces immediately to the right of the highway

Interstate 10 (I-10), between I-605 and I-15

This “The 10” takes advantage of the San Gorgonio Pass, 1 of 3 topographic passes through the mountains surrounding the greater Los Angeles area, the other two being the Tejon and Cajon passes, utilized by I-5 and I-15, respectively.
Just east of Azuza Ave. the hills (San Jose Hills) buttressing the north side of I-10 are partly composed of lava flows known as the Glendora Volcanics (see above)
- ~16 million years old
- Volcanic eruptions were triggered by the rifting that formed the Los Angeles Basin
- Exposed as olive brown rock, as opposed to buff-orange rock exposed in preceding hills
Slover Mountain, approximately 2 miles west of the 91 freeway, was originally a 700 feet high mountain of marble that's been quarried for cement

I-10 east of I-15

- Santa Ana River at I-10/215 interchange
  - The largest river in the Los Angeles region
  - Tributaries bring water from the San Gabriel, San Bernardino, San Jacinto, and Santa Ana mountains
  - The mouth of the river divides Huntington Beach from Newport Beach
- I-10/CA-111 junction – look south for alluvial fans along the base of Mt. San Jacinto
- San Gorgonio Pass
  - Whitewater rest stop (just after I-10/CA-111)
    - Bathroom break
    - Overview of the Transverse Mountain Ranges
      - Whitewater is situated at the base of the San Bernardino Mountains, near the eastern end of the Transverse Mountains
      - A unique mountain range in that they are one of the only east-west oriented mountain ranges in the western hemisphere, going against the grain of the other mountain ranges in California, which all trend roughly north-south
      - Joshua Tree National Park contains the easternmost extent of this prominent mountain range, the Little San Bernardino Mountains
      - The San Bernardino and Little San Bernardino Mountains have been growing intermittently for about 9 million years, with recent earthquakes indicating that they continue to be uplifted along local faults
      - The San Andreas fault delineates the southern margin of the mountains
    - San Gorgonio Pass (from Cabazon-Morongo Casino to North Palm Springs)
      - This valley between the two tallest mountains in southern California, San Gorgonio (11,503 feet) to the north and San Jacinto (10,834 feet) to the south is one of three passes through the mountains surrounding the greater L.A. area, the other two being the Cajon and Tejon, for the 15 and 5 freeways, respectively.
      - This topographic passageway also creates a wind tunnel for air rushing in from the Pacific to fill in an area of atmospheric low pressure that develops over the desert. As this air squeezes through the pass, it speeds up, resulting in one of the windiest places in California. Consequently, this area was chosen as the San Gorgonio Pass Wind Farm site. Note wind turbines as you continue your journey to CA-62 north.

State Route 62 (CA-62)

CA-62, Twentynine Palms Highway
- Leaving I-10, you’ll proceed nearly 7 miles across the alluvial deposits from the San Bernardino Mountains before reaching the base of the Little San Bernardino Mountains. Over this stretch of the 62, you cross the plate boundary between the Pacific and North American plates, as defined by two branches of the San Andreas fault, the Banning fault and Mission Creek fault, crossed at mile markers PM 1.6 and PM 6.6, respectively. CA-62 ascends steeply through a deep gash in the mountain front where foliated, dark to light-colored rock frames the highway on either side. This rock is Proterozoic migmatite (Trent, personal communication), a high-grade metamorphic rock.
- The Morongo Canyon Preserve, about 11 miles after leaving the I-10, offers a nice opportunity to mix some biology into your field trip. Desert plants, birds, and mammal tracks can be studied via a well-designed and thoughtfully shady boardwalk trail system. There are also instructional displays, maps, and handouts to enhance the learning experience.
- Descending into the community of Yucca Valley, consider that the highway runs roughly parallel to the Pinto Mountain Fault. This left-lateral fault facilitates some of the uplift of the Little San Bernardino Mountains and Joshua Tree NP.

Interstate 15 (I-15)

Northbound on the I-15 (from the 91)
The 15 freeway takes advantage of the Cajon Pass
- One of three topographic passes through the steep mountains surrounding the greater Los Angeles area, connecting it rest of the U.S.
- Stream erosion along the San Andreas fault zone has created the Cajon Pass, dividing the Transverse Range into the San Gabriel and San Bernardino mountains to the east
Cucamonga Fault
- Trends along the base of the San Gabriel Mountains, just west of the freeway
- The eastern extension of the Sierra Madre Fault (thrust) along which there have been as much as 10,000 feet of vertical uplift of the crust we see today as the San Gabriel Mountains
- Fault intersects I-15 just before the I-215 interchange

Block diagram showing faults under Los Angeles and the San Gabriel mountains.

From Fuis et al. (2001)

The Transverse Mountain Range
- Eastern Block
  - San Gabriel Mountains on the Pacific Plate and the San Bernardino Mountains, Joshua Tree National Park, and the Little San Bernardino Mountain on the North American Plate
  - Contain old and odd metamorphic and intrusive igneous rocks
- Western Block
  - Santa Susana, Santa Monica, and Santa Ynez Mountains, the Northern Channel Islands
  - Relatively younger (~150 million years old) sedimentary rocks
  - Rotated up to 110 clockwise after being peeled away from the Penisula Ranges 16 million years ago
- Stream erosion along the San Andreas fault zone has created the Cajon Pass, dividing the Transverse Range into two geographically distinct mountain ranges, the “San Gabriels” and the “San Bernardinos”
- San Gabriels
  - Compressional tectonic forces have uplifted the mountains over the past 5 million years at a rate as fast as 70 feet per 1000 years (Sylvester and Gans, 2016)
    - San Gabriels could be growing as fast as the Himalayan Mountains (Prothero, 2011)
      - Rapid uplift evidenced steep slopes, deep, steep-sided canyons, and triangulated ridges
    - This block of crust is being squeezed upward from movement along the Sierra Madre Fault-Cucamonga and San Andreas fault zones
  - Comprised of a complicated mix of Mesozoic and Proterozoic metamorphic and igneous rocks, some as old as 1.7 billion years, including:
    - Anorthosite: a rare igneous rock made up almost entirely of plagioclase
    - Proterozoic dikes
    - Granitic plutons emplaced during the subduction of the Farallon Plate during the Mesozoic time
    - Pelona Schist: seafloor sediments and basalt subducted and metamorphosed as the top part of the Farallon Plate
- San Bernardinos
  - Uplift started about 9.5 million years ago in response to compressive stresses, lifting this block of crust upward along reverse faults
  - Proterozoic metaplutonic and metasedimentary (metamorphosed intrusive igneous and sedimentary rocks)
  - Paleozoic metasedimentary rocks
  - Diorite to granitic plutonic rocks, ranging from 255 to 75 million years old (Sylvester and Gans, 2016)
The I-15 crosses over Lytle and Cajon creeks at 5 and 7.5 miles north of the I-210 interchange. These two major tributaries of the Santa Ana River contain abundant cobble and boulder-sized clasts of bedrock from the adjacent mountains.
The San Andreas fault and Pacific Plate/North American plate boundary
- About 2.5 miles after passing Kenwood Ave, just after I-15 curves due north, you cross over the San Andreas fault, the plate boundary between the Pacific and North American plates
I-15 to the I-40 interchange
- The 15 climbs up the Cajon Pass then descends onto the southern margin of the Mojave Desert
  - The Cajon Pass is one of three, the other two being the Tejon Pass, utilized by the I-5, and the San Gorgonio Pass, containing the I-10, that allows land travel out of the greater Los Angeles area

Interstate 40 (I-40)

I-40 Desert Oasis rest stop 28 miles east of I-15/I-40 interchange
- A restroom stop for students
- Instruct students to begin looking for lava flows and volcanoes after leaving the rest stop