San Gabriel Mountains-Palmdale-Vasquez Rocks
Field Trip Guide
© Pedro C. Ramirez
California State University, Los Angeles
Department of Geological Sciences
By
Pedro C. Ramirez, Ph.D.
Angelique C. Hamane, M.S.
Important Notes!!
Your attendance and participation on this field trip is required. You will not get a passing grade in the
laboratory portion of Geol 150 if you miss the trip. Remember, this laboratory is worth 25% of your overall
grade for the course.
When: Saturday, April 29, 2006. Meet at 6:45 a.m. We will depart at 7:00 a.m. sharp!! We will return at
approximately 3:00 p.m.
Where to meet: Meet in front of the Administration Building near the flag pole (just opposite the kiosk –
Information Booth).
What to expect: We will take a bus to the San Gabriel Mountains, Palmdale area and Vasquez Rocks County
Park. Most of our stops are just a few steps from the bus, but expect a few short hikes. BE AWARE OF
TRAFFIC!
 We will look at geologic features along the San Gabriel Mountain road cuts
 We will make drawings of outcrops and determine their relative ages
 We will see the San Andreas fault and some of the associated geologic features
 We will see and describe some beautiful sedimentary rocks at Vasquez County Park
What to wear:
 Hiking boots or good tennis shoes (no sandals or flip flops)
 Layered clothing
 We will depart from campus, travel up to elevations of approximately 5,000 feet, and
through the high desert – so temperatures will vary.
What to bring:
 This field guide
 Clipboard (or something to write on)
 Sharpened pencil
 Sack lunch and drink (absolutely no alcohol)
 There will be no opportunity to purchase food or drink
 Restrooms are limited
 Camera (optional)
 Your curiosity
SAN GABRIEL MOUNTAINS
North American Plate
Pacific Plate
http://www.geos.ed.ac.uk/homes/sbinnie/SBM
Introduction
The San Gabriel Mountains occupy the central part of the Transverse Ranges at the northern margin of the
Los Angeles Basin and southern margin of Mojave Desert. It is bounded by the San Andreas Fault to the north
and the San Gabriel Fault to the south. The big picture, geologically, is that a block of the western San Gabriel
Mountains moved upward and southwestwardly several feet as the Pacific Plate moved northward relative to
the North American Plate.
The bedrock geology of the San Gabriel Mountains that we will see on this field trip are (1) Precambrian-age
basement rock composed of metamorphic gneiss and amphibolite, (2) Precambrian anorthosite, (3) Triassic
Lowe granodiorite, (4) Mesozoic plutons, and (5) Quaternary alluvium. See Geologic Time Scale at back of
field guide.
San Gabriel Terrane
Rocks that we will see in the San Gabriel Mountains are as follows: Please try and collect a sample of each
throughout our trip.
Precambrian Basement (Mendenhall Gneiss)
The Mendenhall Gneiss is a 1.4 billion-year-old metamorphic rock that has dark layers composed of biotite and
hornblende and white layers of quartz and plagioclase (Oakeshott, 1958). This rock was once granite-type
rock possibly granodiorite that was metamorphosed by changes in pressure and temperature.
Amphibolite is a metamorphic rock that was once basalt or andesite. It is a composed mostly of dark minerals
enriched in iron and magnesium.
Precambrian Anorthosite-Syenite
The anorthosite is a distinctive white to gray rock. It is distinct because it is composed almost entirely of
plagioclase. This rock is approximately 1220 million years old. The anorthosite probably formed in a magma
chamber where plagioclase crystals precipitated and settled to the bottom of the chamber. The plagioclase
accumulated on the chamber floor to form the anorthosite body. These rocks are most interesting because
they have the same composition as moon rocks.
Triassic Lowe Granodiorite
The 220 million year old Lowe Granodiorite is a pluton that intruded into the Precambrian anorthosite-syenite
body. Feldspar in the granodiorite ranges from 65-90% and quartz may comprise as little as10% of the rock.
This pluton has a distinctive differentiation sequence with darker rocks near the base dominated by dark gray,
medium-grained hornblende diorite that grades upward into an orthoclase hornblende quartz diorite that
passes into an even lighter rock composed of biotite phenocrysts in a nearly white groundmass of plagioclase
(Ehlig, 1981).
Meosozoic Plutons
Two phases of plutons occur in the San Gabriel Mountains following emplacement of the Lowe Granodiorite.
The first phase consists of Mesozoic dikes; the second of late Cretaceous plutons.
Mesozoic dikes: The Mesozoic dikes consist of amphibolite – a dark gray to black metamorphic rock.
These dikes cut the Lowe Granodiorite but are truncated by the Cretaceous plutons. The dikes range
from a few cm to a few meters thick.
Cretaceous pluton (Mount Josephine Granodiorite): This 80-million-year-old pluton ranges from diorite
to granite and is medium-grained that is typically composed of quartz, plagioclase, hornblende and
biotite. These rocks have intruded the Precambrian basement rock (Mendenhall gneiss) and pieces of
the gneiss occur inside the intrusion, which indicates that the Mount Josephine Granodiorite is younger
than the Mendenhall gneiss according to the Principle of Inclusions.
Quaternary alluvium
Quaternary alluvium is unconsolidated sediments, which include gravel, sand, and silt, that have been
deposited by streams at canyon mouths. These recent deposits come from high or elevated source areas
undergoing erosion. The sediment is washed down the valleys during heavy rains. Clasts in the alluvium
present in the San Gabriel Mountains are composed of pieces of the Precambrian basement rock and of
anorthosite-syenite blocks, Triassic Lowe Granodiorite, and Mesozoic plutons.
San Gabriel Mountains-Palmdale-Vasquez Rocks
From campus, we will take the I-10 west, to the I-5 north toward Sacramento, to CA-2 north toward Glendale.
We will merge onto the I-210 west toward CA-2/Pasadena. EXIT CA-2/Angeles Crest Hwy and turn LEFT onto
Angeles Crest/CA-2. Set your odometer to 0.0 miles
Total miles (miles from last stop)
2.6 (2.6)
STOP #1 – Gould Mesa Overlook
This stop displays two geologic processes 1) how rivers cut through bedrock, and 2) how the San Gabriel
Mountains experienced multiple intermittent uplifts to its present-day state.
Rivers are usually confined between banks of the main stream channel. Rivers have incised or cut into the
rocks of the San Gabriel Mountains forming V-shaped valleys. When major floods occur, the river overtops the
banks and inundates the alluvial plain. These floods redistribute sediments onto the flood plain producing a
broad, gently-sloped platform or floodplain.
The “step-like” terraces evident at this stop support intermittent uplift of the San Gabriel Mountains. Movement
along the San Gabriel fault displaces the rocks upward from a few centimeters to a few meters. After each
uplift, the river continues to cut into the bedrock toward its new base level creating a new flood plain. This
gives the appearance of steps along the banks of the river.
As we drive toward our next stop, look out the window. You will see light pink
Mesozoic granite dikes cutting through basement rocks composed of dark
amphibolite, and light and dark layered gneisses.
As we continue, you should notice the lighter uniformly colored rocks. These
rocks represent the younger Triassic Lowe Granodiorite (220 Ma)
The next stop is to the left of the road. Be careful of opposing traffic. Park in the parking lot and
walk toward the trail and look north across Clear Creek valley.
8.1 (5.5)
STOP #2 – Clear Creek Overlook
This is a good stop for identify a fault by looking at such features as the color of rocks and the abundance of
vegetation. Sometimes, when rocks change abruptly from one rock color to another, it is possible that a fault
has brought these two rocks bodies against each other. Furthermore, vegetation may vary from one rock to
another. Certain rocks are too rigid and/or lack pore spaces to permit roots to anchor or water to penetrate;
while others are more suitable for plant growth. Look for areas where the abundance or type of vegetation
differs. Where these differences occur may mark the trace of the fault.
1. Make a sketch of the area across the valley. Put a scale on your drawing and indicate north with an
arrow and the letter “N”.
2. Do you see a difference in the color of the rocks exposed along the road and those near the far
mountain tops?
a. Road cut rock color ________________
b. Mountaintop rock color ________________
3. Do you see a difference in the amount of vegetation between the hillside closest compared to the
higher mountaintop? _______ Which mountain has more (circle one)? farthest
closest
4. What geologic feature could explain these differences? ____________________________________.
a. In which direction does this feature run? (use compasses) ____________________________
The lighter colored rock at the top is Mount Josephine Granodiorite (80 Million years old).
The darker colored rock near the road is Mendenhall Gneiss (1.4 billion years old).
Our next stop is where the CA-2 and CA-3 meet. Pull off to the right in the parking lot of the Angeles National
Forest Information Booth.
9.1 (1.0)
STOP #3 – Angeles National Forest Information Booth
This is the junction of CA-2/Angeles Crest and CA-3/Angeles Forest Hwy. After this stop, we will turn north on
CA-3/Angeles Forest Hwy toward Palmdale.
We will examine at the road cut across the highway and discuss the relative age dates of features exposed
through cross-cutting relationships.
Alluvium
Nonconformity
Granite
dikes
Granodiorite
© Pedro C. Ramirez
The sequence of events of this outcrop can be interpreted the following way . . .
1. The darker Triassic Lowe Granodiorite rock formed in a magma chamber deep beneath the Earth’s
surface. Coarse-grained crystals (phaneritic texture) indicate that this rock cooled slowly and
crystallized below the surface.
2. The light-colored dikes (Mesozoic granite) were formed as magma cut through the dark-colored
(Mendenhall Gneiss) rocks and cooled beneath the surface.
3. The igneous rocks were then uplifted, exposed, and eroded. Thick layers of igneous rock have been
eroded and removed. An erosional surface developed on the rocks.
4. A debris flow or landslide from a higher source deposited the alluvium resting on top of the igneous
rocks forming an unconformity, known as a nonconformity (igneous or metamorphic rocks overlain by
sedimentary rocks). The erosional surface marks the nonconformity (blue line). The Mesozoic granite
is approximately 80 million years old and the alluvium is recent. This means that an 80-million-year gap
exist in the rocks exposed along this road cut.
Our next stop will be a large turnout to the right of the road.
10.3 (1.2)
STOP #4 – Excellent Nonconformity Example
This road cut presents an outstanding example of a nonconformity and provides an excellent opportunity for
you to apply the Law of Cross-Cutting Relationships to puzzle out the relative ages of events.
Dark igneous rocks = Lowe Granodiorite (220 million years old)
Light igneous rocks = Mesozoic Plutons/granite dike (80 million years old)
Poorly sorted (boulder, cobble, pebble, sand, silt) sedimentary layer = Recent alluvium
1. Draw a cross-section of the outcrop exposed along the road cut. Put a scale on your drawing and an
arrow with the symbol “N” to indicate magnetic north.
2. Determine the relative ages of these geologic features. (Hint: remember our previous stop)
a. ___________________________________________________________________________
___________________________________________________________________________
b. ___________________________________________________________________________
___________________________________________________________________________
c. ___________________________________________________________________________
___________________________________________________________________________
d. ___________________________________________________________________________
___________________________________________________________________________
3. What type of unconformity exists in this location? ________________ Why? ____________________
_________________________________________________________________________________
When we begin to travel to our next stop we will pass the Big Tujunga dam and
reservoir. The dam is an approximately 250-foot concrete wall constructed to
prevent flooding. The reservoir fills with sediment transported by Big Tujunga
Creek. Consequently, the holding capacity (the amount of water present behind
the dam) decreases. The Los Angeles County Department of Public Works must
continuously remove the sediment to prevent flooding in the lowlands.
If you look to the right of the road, you will see evidence of ancient landslides.
Huge angular rock debris lines the mountainside. This would not be a good
place to be if an earthquake were to occur – anywhere along this road for that
matter!!
Our next stop will be to the right just after crossing the bridge over the Big Tujunga Creek.
15.4 (2.6)
STOP #5 – Big Tujunga Creek
Please use extreme caution when crossing the highway. We will walk a short distance down a paved path to
look down the Big Tujunga Creek.
This stop is for a quick look-see. This is an excellent stop to see how present-day rivers deeply cut through
bedrock. Notice the V-shape of the valley. This type of erosion is typically formed by rivers.
Our next stop is immediately at the end of the tunnel on the left side of the road.
15.9 (0.5)
STOP #6 – Hidden Springs Picnic area (Rest Stop)
This is a good place to stop for lunch. There are picnic tables. There is a short, somewhat steep path toward
Mill Creek with huge boulders and lots of shade.
As we drive toward out next stop, you should notice that the bedrock will change
color from tan to light gray. You will have passed through another unconformity
(disconformity) between the younger Mount Josephine Granodiorite (80 million
years old) from older Precambrian Anorthosite (1.2 billion years old).
We will pass many good exposures of Anorthosite but we will not stop at these
sites. It would be too hazardous to cross the busy highway to get up close and
personal
Our next stop is to the left of the road. Please use your blinker early to notify others of your intent to cross the
road.
19.5 (3.6)
STOP #7 – Anorthosite “Moon Rocks”
Anorthosite is a very interesting rock in that it is composed almost entirely of calcium-rich plagioclase. In the
San Gabriel Mountains, anorthosite is a mostly white rock (plagioclase) with some dark layers. These dark
layers are mostly ilmenite – a titanium-rich iron oxide. Ilmenite is easily identified by its dark color, high
density, and magnetism.
Anorthosite rocks are very rare and very important. This is the same rock that covers 83% of the Moon.
Anorthosite on the moon is also with bands of dark ilmenite. Be sure to collect a sample.
There is a long stretch between stop #7 and stop #8. At about the 20-mile mark,
you will pass the contact between light-colored anorthosite and brownish Lowe
Granodiorite, which will be exposed throughout the rest of the hillside.
At the 33.2 mile mark, we will come to where Vincent Road and CA-3 meet. Turn right and then left onto
Highway 14. Take HWY-14/Antelope Freeway north to Palmdale.
36.7 (3.5)
STOP #8 – Vista Point Outlook
Look at the satellite image below. Can you locate the San Andreas Fault? (Hint: It runs diagonally from NW to
SE and the rocks are slightly lighter in color toward the north)
San Andreas Fault
You are here
Look toward the Mojave Desert. You will see many features of the San Andreas fault.
Palmdale Reservoir is a sag pond. Sag ponds form along faults zones. When faults move, they crush and
grind rocks. Depressions of soft, weakened rocks are then filled with water to form ponds.
Linear Ridges just to the northwest are also associated with strike-slip faults and help outline the location of
faults. When we will continue to drive north on Hwy 14, we will pass between the two ridges. Here we will see
intense faulting and folding, which is a result of deformation along the San Andreas fault.
California Aqueduct is concrete-lined aqueduct that transports water from Northern California to Southern
California. It begins at the Sacramento River Delta and crosses the Central Valley. It often runs parallel to
Interstate 5 and continues into Southern California. It is the main water transport structure of the California
State Water Project and at nearly 450 miles (720 km) in length, it is the longest water channel in California
(Wikipedia).
Road Cut
San Andreas Fault
We will continue north on Highway 14. Immediately after Avenue S you will pass
a major plate boundary in California. You will be passing the San Andreas Fault
moving from the Pacific Plate onto the North America Plate.
On both sides of the highway, you will see severely deformed strata along the
road cuts. Here, folded shale and siltstone of Pliocene age are well exposed.
You will see faulting throughout the beds. These features are the result of the
grinding and squeezing along the San Andreas Fault as the Pacific Plate
continues to move northward against the North American Plate.
If you missed it, don’t worry. You will pass it once again as we make our way to
Stop #9.
At 38.2 miles you will EXIT Palmdale Blvd (Rte 138) and make a left. Get onto the right lane. Make your way
back onto HWY-14 South/Antelope Freeway toward Los Angeles
57.2 (20.5)
Introduction
STOP #9 – Vasquez County Park (Vasquez Rocks) (http://aeve.com/digitaldesert/)
© Pedro C. Ramirez
Vasquez Rocks is a 745 acre Los Angeles County Park, located in Agua Dulce midway between Santa Clarita
and Palmdale. The Vasquez Rocks are formations sculpted by 8 to 15 million years of earthquake activity
along the Elkhorn fault, an offshoot of the San Andreas Fault. Compressed, folded and tilted as much as 50
degrees, and up to nearly 150 feet high, erosion gradually stripped away soil from around the rocks leaving
these ancient and spectacular creations. Course-grained yellowish-gray sandstone beds several feet thick
make up these strange jutting and sweeping formations. Shale, basalt, and reddish-brown conglomerate can
be found layered between the sandstone slabs.
History
From as early as 2000 B.C. to the late 1700's, the Tataviam, a tribe of Serrano Indians lived in the vicinity.
Water was available on a year around basis from the springs at Agua Dulce. The Tataviam took advantage of
the outcroppings by living beneath the overhangs and using features of the sandstone for certain utensils such
as grinding bowls. Spanish contact in the 1700's changed everything for the indians. They were taken to
missions where they consequently dwindled in numbers as they intermarried with other cultures. The little that
is known of the Tataviam comes from artifacts found in archeological sites near the Vasquez Rocks.
Origin of Name
In the mid 1800's a Robin Hood bandit named Tiburcio Vasquez and his gang used the caves and rocks as a
hideaway from sheriff's posses and vigilantes who were always on his trail. The desperadoes would rustle
cattle, rob stagecoaches, steal cattle and the like, then lose the posses in the intricate maze. The rocks served
him well. Eventually, rather than being discovered in hiding, his weakness for women would do him in.
Vasquez had an affair with the wife of one of his lieutenants. The angry husband had shot Vasquez and fled
the area to inform the sheriff of the hideout. Returning with two posses there was a shootout at the tallest rock.
Searching the captured and killed it was discovered that Vasquez was not among them. But the wounded
Vasquez without his hideout couldn't escape for long. He was found in a shack in Cahenga Pass, taken to San
Jose where in March of 1875 he stood trial and was hanged.
The park has been used for location shooting in many movies and television series. Do you recognize any of
these movies?
http://www.81x.com
http://music.calarts.edu
Standing at the SE end of the parking lot looking NW:
1. Make a sketch of the tilted rocks. Put a scale on your drawing and indicate magnetic north with an
arrow.
2. Describe the rock layers selected by your instructor. (grain size, angularity, sorting, color, etc.)
3. What type of rocks are these (igneous, sedimentary, or metamorphic)?________________________
List 2 pieces of evidence. (a) _______________________
(b) ________________________
4. What was being eroded to produce the sediment (HINT: What was the source? Another hint: It was a
highland and your were just there.) ____________________________________________________
Principle of Original Horizontality
5. Are the beds tilted or flat? __________ How do you suppose they got this way?________________
________________________________________________________________________________
6. If they were originally deposited horizontally, which way are they tilted (N, E, W, NE, SW, etc.)
________________________.
Law of Superposition
7. Looking NW. If younger rocks are deposited on older rocks, in which direction do the younger rocks
lie? _______________. Older rocks? __________________
8. Dark brown volcanic rocks are exposed along the road cut toward the north. Are they older or younger
that the sedimentary rocks found at this stop? ____________________.
Putting It All Together
9. What are some of the similarities of these rocks and the rocks you just observed in the San Gabriel
Mountains?
Similarities
1.
2.
3.
Differences
1.
2.
3.
10. Spend some time (approximately 45 minutes to one hour – depending on how much progress we’ve
made) to explore this area. There are trails you can walk if you choose. Are the rocks at the top of the
section similar or different to the rocks at the bottom of the section? Describe the type of changes you
see in the rocks.
Geologic Time Scale
Modified from McRoberts (1998)
http://www.indiana.edu/~geol105/1425chap5.htm
Sedimentary Textures:
References:
Ehlig, P. (1981).
McRoberts, C. (1998). Laboratory Notes. Invertebrate Paleontology. State University of New York, College at
Cortland.
Oakeshott, G. B. (1958). Geology and mineral deposits of San Fernando quadrangle, Los Angeles County,
California: California Division of Mines Bulletin 172, 147 pp.