Field Trip 5: San Gabriel Mountains and the San Andreas Fault
Overview
San Gabriel Mountains with downtown Los Angeles in the foreground. - photo by David McNew.
This trip will require traversing winding mountain roads. Still, it will offer some beautiful vistas, close-up views of some of the striking bedrock making up the San Gabriel Mountains, and some of the most direct topographic evidence for the San Andreas fault in southern California. The itinerary also includes a visit to the Wrightwood debris flow and Pallet Creek, where the first paleoseismology study was done, yielding invaluable information about the recurrence rate of earthquakes on the San Andreas fault.
The route is accessible by car, van, or charter bus. However, it’s possible that some drivers may find the windy mountain roads challenging to drive and that charter buses may have difficulty finding enough room to park in some of the suggested turnouts. Ideally, your group would travel in a few vans to keep them consolidated but nimble. If a charter bus service is used, you have a discussion with a bus company representative ahead of time to ensure that their driver is aware of the challenging driving conditions.
Plan a full day for this trip, but avoid days with inclement weather or following snowfall. Check road conditions ahead of time. The itinerary starts at the SR-2 and I-210 junction in La Canada Flintridge.
Geology
The San Gabriel Mountains are part of the east-west trending Transverse Range, a collection of mountains that go "against the grain" of most of North America's roughly north-south trending mountain ranges. They separate the San Fernando and San Gabriel valleys to the south from the Soledad Basin and the Mojave Desert to the north and are positioned between the Santa Monica Mountains and San Bernardino Mountains to the west and east, respectively. The San Andreas fault is rapidly uplifting the San Gabriel Mountains along the northern margin of the range and the Sierra Madre-Cucamonga fault system along its southern margin (Carter, B.A., 2011).
The rocks of the San Gabriel Mountains are intriguing, as they “contain some of the oldest and most unusual… rocks in California” (Sylvester and Gans, 2016). The lithology is mostly a complex mix of primarily Proterozoic to Mesozoic age igneous and metamorphic rocks that are tectonically separated into an upper and lower plate by the Vincent Thrust, a.k.a. Vincent-Orocopia-Chocolate Mountains thrust fault. The oldest upper plate rocks are nearly 1.7 billion years old high-grade metamorphic rocks, including gneiss, migmatite, and amphibolite. From approximately 1.4 to 1.2 billion years ago, these rocks were intruded by magma that solidified to form a complex anorthosite-syenite-gabbro body. The anorthosite is noteworthy because it is a rare rock, consisting almost entirely of plagioclase feldspar, making it white. During the subduction of the Farallon Plate, magma was generated, intruding Earth’s crust along a line parallel to the trench, over the full length of California. Some of the main plutonic rock bodies found in the San Gabriels from this subduction include the 220 million-year-old (myo) Lowe Granodiorite, the 122 myo Wilson Diorite, and the 80 myo Josephine granite. The Vincent Thrust separates these upper plate rocks from the lower plate rocks. The lower plate consists of the Pelona Schist, a rock body that formed when the Farallon Plate subducted beneath the North American Plate about 75 million years ago, metamorphosing and seafloor sediments covering the top of the plate. Consequently, the Vincent thrust fault represents the ancient plate boundary between the Farallon Plate and the North American Plate.
At 750 miles long, the San Andreas fault is California's most important tectonic structure. This right-lateral strike-slip fault represents the plate boundary between the Pacific and North American Plates, accommodating about one inch of plate motion per year. Movement of the plates happens as intermittent slips or ruptures along the San Andreas fault, which can produce major earthquakes, such as the magnitude 7.9 San Francisco earthquake in 1906 and the 1857 Fort Tejon in southern California. Careful analysis of strata offset by past earthquakes shows that major earthquakes happen along the San Andreas fault at a rate of one major earthquake per 150 years. Major earthquakes typically disturb the ground surface, making ground cracks or fault scarps when one block of the crust has been uplifted relative to the crust on the other side of the fault.
Since 1857, disruptions to the ground surface, such as scarps and ground cracks, have been erased by erosion and urban development in most places, making the trace of the fault difficult to locate on the Earth’s surface. However, this field trip will take us to several locations where the fault scarp from the 1857 earthquake is clearly evident, providing topographic evidence for the location of the fault.
Geologic map of the San Gabriel Mountains (from Sylvester and Gans, 2016).
Learning Objectives
Through participation in this field trip, students should be able to:
- Use texture to identify igneous rocks
- Use texture to identify metamorphic rocks
- Use topographic features, like scarps or vegetation patterns, to locate the trace of a fault
- Identify landslide scars
- Identify ductile structures in strata, such as folds
- Recognize landforms associated with rapidly growing mountains
Key Vocabulary
- Fault Scarp – a low, steep hill, caused by the vertical offset of the ground surface by movement along a fault
- Metamorphic rock – a rock that has been altered from a pre existing rock by heat, pressure, and/or hot fluids
- Plutonic/Intrusive igneous rock – a rock that forms from the crystallization of magma
- Thrust Fault (thurst) – low angle faults (closer to horizontal than vertical) that are caused by compressional stress
Pre-Field Trip Questions
- Using the following link and/or your textbook, make a sketch of an igneous rock with coarse-grained (phaneritic) texture: igneous rocks Links to an external site.
- Using the following link and/or your textbook, (a) define foliation and describe how foliation texture is created, (b) make a sketch of a metamorphic rock with foliated, gneissic banding: metamorphic rocks Links to an external site.
- Draw a fault scarp in a profile/cross-section perspective that shows the fault extending down into the earth from the surface. Label your drawing.
- Watch this short video, The San Andreas Fault
Links to an external site.
, then answer this question, “How likely is it that we could experience the Big One, a major earthquake on the San Andreas fault, while we are on our field trip?” Explain.
Directions
From Cerritos College, I-605 north, to I-210 west, exit Exit at “2” north, Angeles Crest Hwy, La Cañada Flintridge.
Field Trip Stops
Starting in La Cañada Flintridge and traveling north/east on SR-2, Angeles Crest Highway. Mileages associated with stops 1-4 below are approximated from the I-210/SR-2 interchange.
Stop 1 – Arroyo Seco Overlook; 2.4 miles
Addresses learning objective:
6. Recognize landforms associated with rapidly growing mountains
Looking south, down the Arrow Seco.
Arroyo Seco is one of the principal drainages of the south-facing watersheds of the San Gabriel Mountains. This deep, steep-sided, v-shaped valley is characteristic of rapidly growing mountains, where uplift outpaces erosion.
Activity 1: ask students to sketch the cross-section profile of Arroyo Seco Canyon. How might this profile look different than the profile across a meandering river? – The profile for a meandering river would more or less be flat, with the only slopes being those of the semi-circular stream channel itself.
Traveling north, parking for stop 2 is on the left side of the road.
Stop 2 – George’s Gap trailhead to view the San Gabriel Fault; 8.3 miles (PM 32.8)
Addresses learning objectives:
3. Use topographic features, like scarps or vegetation patterns to locate the trace of a fault
Overview of the Transverse Mountain Ranges
- From west to east, the Transverse Ranges include the Santa Monica, San Gabriel, and San Bernardino Mountains, terminating with the Little San Bernardino Mountains that contain part of JTree NP
- Unique 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
- The western Transverse Ranges were rotated 110 degrees clockwise over the past 16 million years because the northward movement of the Pacific plate captured a massive block of crust from the ancestral Peninsular Range
- Geologically young, having grown within the past 5 million years and perhaps even less
- Rapid growth continues with an uplift rate of approximately 1-2 mm/yr or nearly 8 inches per 100 years!
The San Gabriel Fault was the main branch of the San Andreas Fault from 11-4 million years ago. Looking north from the northeast end of the parking lot one can estimate the trace of the San Gabriel Fault, which is delineated by the alignment of the darkest green vegetation about a quarter of the way up the side of Josephine Peak, relative to Big Tujunga Canyon Road (marked by brown roadcuts). The light-colored rock making up Josephine Peak is the Josephine granite, one of the youngest plutonic rocks of the San Gabriel Mountains. (Sylvester and Gans, 2016).
Josephine Peak and the approximate trace of the San Gabriel fault.
Stop 3 – Clear Creek ranger station/information center; 9.2 miles
The San Gabriel Fault runs through the intersection of SR-2 and Angeles Forest Highway, continuing eastward to the Red Box ranger station.
Continuing along SR-2, roadcuts expose San Gabriel Gneiss and the Josephine Mountain granite that has been sheared up, up, and oxidized along the San Gabriel Fault.
Stop 4 – Red Box picnic area (at turn off for Mount Wilson); 13.8 miles
Addresses learning objectives:
1. Use texture to identify igneous rocks
2. Use texture to identify metamorphic rocks
Red Box offers a nice spot to get out and stretch your legs and use the potty (located on the east side of Mount Wilson Red Box road) or take a lunch break. It also provides an opportunity to attempt to identify some of the rock types that make up the San Gabriel Mountains. Embedded in concrete fence supports, and walls, and placed as boulders along the perimeter of the parking lot you’ll find examples to study, including Lowe Granodiorite, Wilson Diorite, and Gneiss, perfectly exposed and a safe distance from the potentially noisy and dangerous SR-2.
Boulder of gneiss at Red Box.
Red Box picnic area parking lot.
Activity 2: Choose some of a few rock specimens and ask students to, (1) identify the texture; (2) identify the type of rock (plutonic igneous or metamorphic); and, (3) the specific name of the rock, i.e. Lowe Granodiorite.
Optional: proceed up to Mount Wilson, which offers a variety of interesting and educational options Mount Wilson website Links to an external site. .
Stop 5 – Lowe Granodiorite; exposed in roadcuts approximately 7 miles east of Red Box at PM 45.4.
Addresses learning objective:
1. Use texture to identify igneous rocks
The Lowe Granodiorite is one of the most significant and distinctive rock bodies in the San Gabriel Mountains. Its white matrix is spotted with large crystals of hornblende, making it easily recognizable. It’s also the oldest of the Mesozoic plutonic rocks at 220 million years old. Its age has significant implications for changing our understanding of the timing of the break-up of Pangaea. Generally, most earth scientists believe that Pangaea began to break up around 190-200 million years ago, at which time subduction was initiated along the western margin of North America. However, the age of the Lowe granodiorite means that the supercontinent may have started breaking up 20-30 million years earlier than previously thought.
Lowe granodiorite texture with car keys for scale.
Stop 6 – Proterozoic Gneiss and granitic intrusions; 4.3 miles east of Red Box/~0.8 mile east of Upper Big Tujunga Canyon Road
Addresses learning objectives:
1. Use texture to identify igneous rocks
2. Use texture to identify metamorphic rocks
To observe a roadcut showing the interaction of granitic and metamorphic rock bodies, use these turnouts: (1) trailhead for Silver Moccasin Trail, approximately just over 1/2 mile east of Upper Big Tujunga Road of roadcut or, (2) 0.8 mile after Upper Big Tujunga Road. The outcrops are exposed in the roadcut between these two turnouts. The second offers better and safer access. To get the best views requires walking along the road, but this should only be done if extreme caution is exercised due to traffic.
A complex mix of granite and gneiss in roadcut.
Other excellent exposures of gneiss can be found along both the north and south side of Big Tujunga Canyon Road, with turnouts, just 0.2 mile north of N2.
Stop 7 – Punchbowl fault and Pelona Schist at Vincent Gap; 36.4 miles east of Red Box at PM 74.8
Addresses learning objective:
3. Use topographic features, like scarps or vegetation patterns, to locate the trace of a fault
In Vincent Gap the Punchbowl fault separates reddish brown Miocene age sanstone and siltstone from gray Pelona Schist (see Stop 8 discussion). Better exposures of the Pelona Schist can be found by utilizing the two large road pullouts, the first about 0.3 mile and the second about 0.5 mile from Vincent Gap.
Pelona Schist exposed in a roadcut just east of Vincent Gap.
Stop 8 – Inspiration Point trailhead; 39.5 miles east of Red Box at PM 78.0 (park on right/south side of highway)
Addresses learning objective:
3. Use topographic features, like scarps or vegetation patterns, to locate the trace of a fault
6. Recognize landforms associated with rapidly growing mountains
Geography and Geology
- San Antonio, a.k.a. Mount Baldy is to the southeast
- Baden-Powell to the southwest
- The East Fork of the San Gabriel River is at the bottom of the steep, deep canyon to the south
- The Vincent-Orocopia-Chocolate Mountain thrust (Vincent Thrust) is one of the most important structures in southern California
- Visible at about eye-level along the northeast flank of Mt. Baden-Powell above light-colored sills of felsic composition
- Separates gneiss of over a billion years old in the upper plate from the Pelona Schist of about 75 million years old in the lower plate
- The Pelona Schist is composed of seafloor sediments that were metamorphosed as the Farallon Plate subducted beneath North American, thus, the Vincent Thrust represents the ancient plate boundary between the Farallon and North American plates
- The Punchbowl Fault is an inactive strand of the San Andreas fault
Cartoon of Vincent Thurst Fault as viewed from Inspiration Point. – D.D. Trent, 1975.
Photograph from Inspiration Point showing the approximate location of Vincent Thrust across Mt. Baden-Powel.
Stop 9 – San Andreas Fault; Big Pine Visitor Center Interpretive Site (Stone building) immediately east of the junction of SR-2 and N4.
Addresses learning objectives:
3. Use topographic features, like scarps or vegetation patterns, to locate the trace of a fault
San Andreas Fault and Wrightwood fun facts
- The Scarp from the magnitude 7.9 1857 Fort Tejon earthquake Links to an external site. is on the north side of the junction, creating the slope behind the building
- The San Andreas fault runs roughly parallel to SR-2 through the town of Wrightwood
- Erosion along the fault has created Swartout Valley, in which the town of Wrightwood was developed in the years since the 1857 quake
- Lone Pine Valley further east was also made by erosion along the fault
- Mountain High Ski Resort, on the south side of the highway, was the first commercial ski lift in California, opening in 1932 as the Blue Ridge Ski area
Fault scarp from the 1857 San Andreas fault earthquake, expressed as the hill behind the building.
Fault scarp from 1857 San Andreas Fault earthquake, expressed as brown hill above parking lot.
Activity 3: ask students to point out scarp and estimate its height.
Drive approximately 4.6 miles east of the Big Pine Visitor Center Interpretive Site on SR-2, turn right on Sheep Creek Drive. Left when Sheep Creek Drive ends at Lone Pine Canyon Road, then proceed less than a quarter mile to Sheep Creek and park on the turnouts south or north of the road.
Stop 10 – Sheep Creek and the 1941 debris flow scar
Addresses learning objective:
4. Identify landslide scars
Assuming you were able to safely park, exit vehicles, assemble on the south side of the road for good views of the scar from the 1941 debris flow. Evident in the photo is the nearly vegetation-free, snow-covered part of the mountainside.
1941 debris flow scar (treeless patch).
Debris flows were triggered by rapid snow melt in May of 1941, which saturated older landslide deposits, creating a disordered mass of rock, mud, and tree parts that moved with a viscosity comparable to wet cement down the Sheep Creek channel, eventually flowing out of the mountains into the Mojave Desert.
Sheep Creek with debris flow deposits.
Activity 4: Identify three different types of rock found in the debris flow deposits. – In particular look for actinolite schist.
Map of debris flow area (Google Maps).
Activity 5: Using the Google “terrain view” map above Links to an external site., trace out the boundary of any landslide scars depicted by the contour lines. Note: Sheep Creek is represented by the dotted and dashed blue line; the vehicles are parked where the creek intersects Lone Pine Canyon Rd. North is to the top of the page.
For stop 11, return to Wrightwood and the SR-2/N4 (Big Pines Highway) junction, then take CA 138 (N4) towards Palmdale in order to follow the westward trend of the San Andreas fault.
Alternatively, one could continue eastward on Lone Pine Canyon Road, following the trace of the San Andreas fault down Lone Pine Canyon, eventually reaching the junction with CA-138 at Mormon Rocks. At about 0.8 mile from Sheep Creek, there is a gated road on the left (north) side of the road with just enough room for a few vehicles, but not enough for a bus. Walk around the gate and down the road about 100 yards to the top of the small hill for a good view down Lone Pine Valley and along the San Andreas fault zone.
Looking down Lone Pine Canyon and the San Andreas fault zone.
Mileages for the following stops are given from the SR-2/N4 junction to Pallet Creek Road.
Stop 11 – Apple Tree Campground and 1857 San Andreas fault scarp; 2 miles
Addresses learning objective:
3. Use topographic features, like scarps or vegetation patterns to locate the trace of a fault
The 1857 earthquake fault scarp can be found by taking the dirt road off the east end of the parking lot. Continue to the end of the road and have a look around. You’ll find fault gouge zones and trees with dramatically curved tree trunks. These trees were toppled from the violent shaking and since have compensated by growing skyward – resulting in the curved tree trunks.
Curved tree trunk.
Stop 12 – Jackson Lake; approximately 3 miles
Addresses learning objective:
3. Use topographic features, like scarps or vegetation patterns, to locate the trace of a fault
Water has accumulated on one side of a shutter ridge, which is the hill on the far side of the lake. This ridge was formed by lateral offset along the San Andreas fault so that it now impedes the flow of water in such a way that a lake has formed behind the shutter ridge (Hough, 2004).
Jackson Lake with a shutter ridge at the far end of the lake.
Stop 13 – San Andreas fault scarp; approximately 12 miles, at the intersection with Bob’s Gap Road. Note: N4 becomes Valyermo Road at this point.
Addresses learning objective:
3. Use topographic features, like scarps or vegetation patterns, to locate the trace of a fault
The scarp is most obvious on the northeast corner of the intersection and includes a house atop it for scale.
House atop fault scarp from 1857 earthquake.
Stop 14 – Pallet Creek; approximately 14.3 miles
Turn left on Pallet Creek Road, just past Saint Andrew’s Abbey, and continue about 1.4 miles on Pallett Creek Road to a pull-out on the south side of the road. This is a good spot to discuss one of the more important thesis studies undertaken in the field of geology during the past 50 years or so.
Pallett Creek is where then-graduate student Kerry Sieh performed his legendary (at least within earth science circles) study of the San Andreas fault. Kerry utilized “heavy equipment” in the form of a backhoe tractor to dig trenches along the fault. This gave him a cross-sectional perspective of the fault, where he could see strata offset by fault movement. He used carbon-14 radiometric dating to determine when a specific stratum had been offset by earthquakes and through doing so, he was able to determine a recurrence interval for the San Andreas: on average, a major earthquake once per 150 years.
Stop 15 – various spots to stop and observe the San Andreas fault scarp
Addresses learning objectives:
3. Use topographic features, like scarps or vegetation patterns to locate the trace of a fault
From Pallett Creek, continue along Pallett Creek Road, then turn right onto Longview Road. Proceed about 0.8 mile then turn left on Fort Tejon Road. Continue for about 2.9 miles to the intersection with 106th Street. The San Andreas fault is off to the left (south), trending along the base of the low hills. Question, what mountain range is in the background?
Looking north along 47th street. Vegetation surrounds a home perched atop the San Andreas fault scarp.
After another 1.9 miles or so, make a left onto Mount Emma Road, and continue 2.3 miles to Cheseboro Road. After making a right on Cheseboro Road, proceed about a third of a mile and make a left on Barrel Springs Roa. This runs parallel to the fault, which is marked by the hill immediately to the north. At 47th Street, the fault scarp is visible as the low hill, upon which vegetation surrounds the yard of a home.
Cottonwood trees mark the trace of the San Andreas fault.
As you approach 42nd street, notice the line of Cottonwood Trees taking advantage of groundwater trapped along the fault plane and therefore reflecting the trace of the San Andreas fault. At 40th, the fault scarp is immediately north of the road. After 37th Street, Barrel Springs curves towards the north and crosses over the California aqueduct and the San Andreas Fault.
Barrel Springs becomes 25th Street north of Pearblossom Highway. Make a left onto Avenue S and proceed west until you pass under the 14 freeway, parking in the dirt parking area just after passing the foot of the offramp for the northbound traffic.
Stop 16 – Palmdale Roadcut
Addresses learning objectives:
3. Use topographic features, like scarps or vegetation patterns, to locate the trace of a fault
4. Identify ductile structures in strata, such as folds
Folded strata exposed in the Palmdale roadcut. – CC
In the parking area, one is standing on the San Andreas fault. Walk up the trail about ¼ mile towards the top of the ridge for excellent views of the Palmdale roadcut. This ridge formed through intense compression of the relatively young, Pliocene Anaverde Formation. This sliver of crust is caught between two branches of the San Andreas fault. The main branch is along the southern side and a secondary branch, the Littlerock fault, is found along the northern margin of the ridge, making it an example of a squeeze block (see Chapter 3) or a pressure ridge (Sylveter and Gans, 2016). Excavations for geologic studies show that rock has been offset 50 feet in a right-lateral sense on the south side of the ridge.
Activity 6: Ask students to answer the following questions: What type of rock is the Anaverde Formation and how can you tell? – Sedimentary. You can see the strata. What type of structures has the strata been deformed into? – Anticlines and Synclines. What type of stress caused these structures? – Compression.
Activity 7: Ask students to make a map-view perspective drawing of Highway 14 before and immediately after a major earthquake along San Andreas fault at this location.
Post Fieldtrip Questions
- What type of rock makes up the San Gabriel Mountains? Give some specific names of the rock units.
- On the DEM map (digital elevation map) of southern California, mark the trace of the San Andreas fault.
- Make a cross-sectional drawing showing a fault breaking through Earth’s surface to make a fault scarp. Label the fault, fault scarp, and hanging wall and footwall. Name the type of fault.
DEM map of southern California. - CC.