Geology and Hydrology in the Eastern San Gabriel Mountains

Annual Field Trip Number 36, Stops and Discussions
June 19 and 20, 2010
Geology and Hydrology in the Eastern San
Gabriel Mountains
A Journey through the River of Time
Saturday, June 19, 2010 (Kenney, Reynolds, Goetz, Harris, and El-Zaynab field
presentations)
#A1: Lost Lake: A Sag Pond and Terraces along the San Andreas Fault
#A2: Cosy Dell Formation (K-T Boundary)
#A3: Vaqueros-Cosy Dell Fm. Contacts
#A4: Crowder Formation Outcrops
#A5: Cajon Valley Formation, Victor Valley Fan & Cajon Fault
#A6: Vincent Gap and Grassy Hollow (Lunch Stop)
#A7: Slope Stability Project at the Angeles Hwy 2 Bridge
Sunday, June 20, 2010 (Nourse, Saint , and Summers field presentations)
#B1: Cow Canyon Saddle Overlook
#B2: USFS Visitor’s Center
#B3: Gneiss-Dikes Intrusions and 1938 Floods
#B4: Manker Flats and San Antonio Falls (Lunch Stop)
#B5: 1969 Flood Terraces and SAF Outcrop
#B6: San Antonio Dam Overlook
Introduction to Geology and Hydrology in the Eastern San Gabriel Mountains
The eastern San Gabriel Mountains, as illustrated by the oblique aerial view on the cover page and the
geological cross-section (Figure 1) constitute a spectacular field laboratory for geological and
hydrological processes and their interaction with ecology and human activities. The San Gabriel
Mountain block is bounded by the Frontal thrust faults (Sierra Madre-Cucamonga Systems), in the south
and the San Andreas Fault System, a right-lateral transform fault system, that now forms the plate
boundary between the Pacific and the North American plates, The mountains are dissected by rightlateral and left-lateral strike slip faults, many of which are accentuated by deep canyons, forming the
headwaters of our river systems. During this trip we will encounter the San Gabriel River, the San
Antonio Creek, the Lytle Creek and the Cajon Creek and examine their response to tectonic shifts,
floods, landslides and fires.
On the first day we will visit the San Andreas Fault System in the Cajon Pass and in the Wrightwood
area, under the leadership of Miles Kenney, Bob Reynolds, Chris Goetz and Chris Harris. On the second
day we will visit the San Antonio Creek, from its source to the mouth, examining its response to the San
Antonio left-lateral strike slip fault, the landslides, the floods, the fires and the dam, under the leadership
of Jon Nourse of CalPoly Pomona, who has done field work and supervised student theses since 1991.
We will be camping at the Mt. Baldy Ranch Road R.V. Park in the Cow Canyon Saddle, overlooking the
San Gabriel Fault and at the tow of the Cow Canyon Landslide.
Figure 1. Geological cross-section across the eastern San Gabriel Mountains, with several of the formations and
the faults that we will encounter during the field trip.
(from NAGT Field Guide, 2001)
ROAD LOG
(compiled by Prem Saint, Boris Zaprianoff and Vik Mathur)
Figure2. East San Gabriel Mountains with stops for Day 1 and Day2
Day 1, Saturday, June 19, 2010
Highlights: San Andreas Fault Zone in the Cajon Pass, Sedimentation Story of the K-T
Boundary, Crowder Formation, Cajon Valley Formation; Cajon Fault, Victor Valley Fan, and
Slope Stability Project on the Angeles Highway 2 Bridge
0.0 (0.0)
Start at Cow Canyon Saddle at the entrance of Mt. Baldy Ranch R.V. Park, 30601 East
Glendora Ridge Road, Mt. Baldy, CA 91759, driving down towards Mt. Baldy Village.
0.8 (0.8)
Turn right onto Mt. Baldy Road, passing through the village and the school, with
outcrops of Pre Cambrian gneisses and landslide deposits and chaparral vegetation.
5.4 (4.6)
Turn left on Shinn Road. Cross the bridge on the San Antonio Creek and San Antonio
Fire Station, approaching San Antonio Dam on your right.
7.6 (2.2)
Curve right onto Mountain Road and San Antonio Heights Village.
9.6 (2.0)
Turn left onto the I-210, Foothill Freeway, East ramp, towards San Bernardino.
18.7 (9.1)
Exit on I-15 North towards Barstow.
27.4 (8.7)
I-215 joins I-15.
31.0 (2.6)
At the top of the rise on I-15, we are crossing the trace of the San Andreas Fault, the site
of potential disruption following a major earthquake on the fault. To our right (east), two
branches of the fault are very close together. To the left (northwest), the fault zone
appears as the large, 0.25 mile-wide shear zone of Lone Pine Canyon with fault-related
features like Lost Lake, a sag pond.
Figure 3. San Andreas Fault in the Cajon Pass, along with Los Angeles Basin utility lines, railroads, gas pipes,
electrical lines which could be severed during future earthquakes, causing severe economic disruptions.
32.8 (1.8)
Prepare to exit at Cleghorn Road, 0.5 mile ahead.
33.3 (0.5)
Exit at Cleghorn Road.
33.5 (0.2)
Stop, TURN LEFT and proceed under the freeway to the junction of Cleghorn Road and
Cajon Boulevard. Proceed south toward Blue Cut.
35.3 (1.8)
TURN RIGHT onto Swarthout Canyon Road and cross Cajon Creek.
35.8 (0.5)
Caution; cross BNSF railroad tracks.
36.3 (0.5)
Caution; cross BNSF railroad tracks. Proceed north toward Lost Lak
36.6 (0.3)
TURN RIGHT into the parking lot at Lost Lake. Stop A1
STOP A1: LOST LAKE: A SAG POND ALONG THE SAN ANDREAS FAULT, TERRACE
DEPOSITS
Field Presentations by Miles Kenney and Robert Reynolds
The sag pond at the Lost Lake is a product of groundwater seepage in an area of tectonic depression in
the San Andreas Rift Zone, a linear valley in the Lone Pine Canyon. Dr. Ray Weldon conducted a
detailed study of a series of stream terraces associated with Cajon Creek in the region of Lost Lake
(Weldon and Seih, 1985). The work identified progressive offset of radiocarbon dated, late Pleistocene
and Holocene deposits and landforms by the San Andreas fault that yielded a set of slip rates spanning
the past 14,400 years. Near Lost Lake they determined a slip rate for the San Andreas fault of ~24
mm/yr for the past 14,400 years and inferred that this slip rate had remained relatively constant during
that period of time. The primary data utilized was the identification of a series of stream terraces
associated with Cajon Creek that had been progressively offset by the San Andreas fault.
Their findings provided strong constraints on the local kinematics of the San Andreas Fault System in
the area. For example, other work by Weldon and others determined a slip rate on the San Andreas fault
approximately 10 miles northwest near Wrightwood of ~34 mm/yr which is north of the juncture
between the San Jacinto and San Andreas faults. They postulated that the total slip of 34 mm/year near
Wrightwood was portioned between the San Jacinto and San Andreas faults in the area of Cajon Valley.
The estimated slip rate for the local San Jacinto fault zone is 10 mm/yr, which once added to the
estimated 24 mm/yr totaled 34 mm/yr.
Figure 4. Lost Lake Sag Pond: Groundwater seepage in the tectonic depression in the San Andreas fault zone has
created the pond, here covered with cattails along the shore.
36.9 (0.3)
View south of Blue Cut, its color from glaucophane and actinolite schist. From Blue Cut,
it is a quick walk from the Pacific Plate across the San Andreas Fault to the North
American Plate, to our next stop at the Cretaceous Cosy Dell Formation.
37.9 (1.0)
STOP at Cajon Boulevard. Watch for cross-traffic. Proceed diagonally southeast across
Cajon Boulevard to the unused north-bound lane of Rte 66.
38.0 (0.1)
Stop A2.
STOP A2: COSY DELL FORMATION (K-T BOUNDARY?)
Field Presentation by Bob Reynolds
Cajon pass contains 70 million years of history in the rock record. Sediments of Cretaceous age are
present as is the Earliest Miocene Vaqueros Fm. Continental deposition starts with the Middle Miocene
Cajon Valley Beds. The Inface Bluffs on the northern horizon contain the Brunhes-Matuyama reversal
(765,000 yr) suggesting a record of deposition thru the middle-late Pleistocene.
At this stop, between I-15 and old Route 66, we are examining the Cretaceous Cosy Dell Formation
(Morton and Miller, 2003), formerly called the Paleocene–Eocene? San Franscisquito Formation
(Dibblee, 1967; Woodburne and Golz, 1972). The basal conglomerate is overlain by limey sandstones
and dark, silty limestone. Its Cretaceous age is determined by the presence of elasmosaur vertebrae in
the outcrop to the west (Kooser, 1985). Articulated vertebrae in an outcrop to the northwest (Whale
Mountain) suggest that this large marine reptile died in place and was not reworked from other
sediments (Lucas and Reynolds, 1991). The section may have potential to contain the K/T boundary, but
none of the abundant fossil fish scales, crustaceans, gastropods, pelecypods, or plants from this outcrop
provide additional age control relating to that transition. Paleocene Coccoplithus pelagicus, a nanofossil,
Apectodinum plexus, a dinoflagellate, and Turritella pachecoensis, a gastropod, were reported (Fred
Berry p. c. to Kooser, 1985; Weaver 1951-56) from apparently similar sediments somewhere in Cajon
Pass, but by the 1970s the specimens and locality data could not be found. Return to vehicles.
38.1(0.1)
STOP at Cajon Boulevard. Watch for cross traffic. DRIVE NORTH (right) toward the
Cleghorn onramp.
39.3 (1.2)
Pull right onto the unused northbound lane of Route 66. Stop A3
STOP A3: VAQUEROS –COSY DELL CONTACTS
Field Presentation by Bob Reynolds
To the northwest is Whale Mountain, where the Vaqueros (?) Formation crops out and is in fault contact
with the Cretaceous Cosy Dell Formation. The Vaqueros (?) Formation unconformably overlies
crystalline basement rocks and is unconformably overlain by the middle Miocene (Hemingfordian to
Barstovian North American land mammal age) Cajon Valley Beds (Woodburne and Golz, 1972;
Woodburne, 1991). The Vaqueros Formation contains fossils of an echinoid (Scutella fairbanksi),
elasmobranchs, and the mollusks Ostrea titan subtitan, Pecten sespeensis, Crassatella granti and
Turritella inezana, the latter two species important in establishing the late, early Miocene age
(Woodring, 1942; Woodburne and Golz, 1972).
An unusual dolphin has been described from these Vaqueros (?) marine facies. The fossil is a member of
the archaic superfamily Platanistoidea. The superfamily was widely distributed to all ocean basins in the
Tertiary, but today is represented only by the endangered fresh-water Ganges River dolphin (Barnes and
Reynolds, 2007). The long-snouted skull was a meter in length, and the body was four meters long.
The Cajon Valley Beds (Woodburne 1991; formerly Punchbowl Formation of Woodburne and Golz
1972; Cajon Formation of Meisling and Weldon, 1982, 1989) are exposed to the west. From the south,
visible units are Tcv4 (pink), Tcv5 (green), Tcv3 (red and white), and Tcv 2 (white hogbacks). The
Cajon Valley beds range in age from approximately 18–14 Ma (Reynolds, 1991). The beds contain
fossil horse, camel, deerlets, rodents, and insectivores. Recent BNSF railroad track additions were
monitored by paleontologists who recovered bear, rhino, chalicothere, and the first talpid (mole) from
the Cajon Valley Beds. The early to middle Miocene age of the beds has been established by land
mammal age chronology. Squaw Peak is to the northeast. The Cajon Valley Beds are separated from the
Crowder Formation to the east (not visible) by the Squaw Peak fault.
Pliocene and Pleistocene sediments of the Phelan Peak Formation, Shoemaker Gravels, and Noble’s Old
Alluvium are present in the in-face bluffs to the north-northwest. Return to vehicles. Proceed north
toward Cleghorn onramp.
39.9 (0.6)
Pass the junction of Cajon Boulevard and Cleghorn Road. Bear right (east) under the
overpass.
40.0 (0.1)
Enter I-15 northbound at Cleghorn Road. Stay in the right lane and prepare to exit right
onto Highway 138.
41.0(1.0)
EXIT at Highway 138 and move to the right lane.
41.3 (0.3)
STOP at Highway 138. TURN RIGHT (east) toward Silverwood Lake; pass through
Crowder Formation Units 1 and 2.
41.5(0.2)
The thin brown paleosols in Crowder Unit 1 have produced significant numbers of fossil
taxa (Reynolds, 1991; Reynolds and others, 2008).
41.8 (0.8)
TURNOUT (right) and PARK. Stop A4.
STOP A4: CROWDER FORMATION OUTCROPS
Field Presentation by Bob Reynolds
The Crowder Formation was named by Dibblee (1967) and described as Pliocene in age. Foster (1980)
divided the Crowder Formation into five units and provided detailed information on source areas and
paleo-current direction from the northeast. Subsequent workers (Weldon et al., 1984; Weldon, 1985;
Meisling and Weldon, 1989) defined the age of the lowest Crowder units based on a local magnetic
polarity zonation tied to small mammal assemblages from the paleosols (Reynolds, 1984, 1985, 1991).
The first half of the Crowder Formation was estimated to span a period of time that began prior to 17 Ma
and ended at than 7.1 Ma (Reynolds and others, 2008). Consequently, the age of the latter part of the
Crowder Formation would shed light on structural and erosional events in latest Miocene time, prior to
the deposition of the Phelan Peak Formation.
Figure 5. Cajon Pass Geology and Hydrology
The view north is in the direction of the Crowder Fm. upper Unit 3 (7.3 Ma). The northern skyline
exposes, from the north bound lane of I-15, the Phelan Peak Formation (4.2-1-5 Ma), the Shoemaker
Gravels (1.3-0.9 Ma), and Noble’s Old Alluvium (0.8-0.5, contains the Brunhes-Matuyama reversal
This depositional sequence suggests that there may have been a very short time to develop the major
unconformity between Crowder Unit 5 (6 Ma) and the Phelan Peak Formation (4.2[765,000 yr]). Ma).
Figure 6. Outcrop of the Crowder Formation, Miocene sediments, with rich mammal, reptile, bird and mollusk
fossils, across the road from Stop A4.
Figure 7. Geologic Map showing the Crowder Formation in the Cajon Pass,
from USGS, Morton and Miller, 2003
The Crowder Formation is now considered to be a lithologically distinct formation in fault contact with
the Cajon Valley beds (Morton and Miller, 2003) with a portion of that deposition contemporaneous,
between 18 and 14 Ma (Weldon, 1985; Woodburne, 1991; Reynolds, 1991). The Crowder Formation is
nearly 1000 meters thick and is located northeast of the Squaw Peak Fault, while the 3,000 meter thick
Cajon Valley beds are located southwest of the fault. The latter contain land mammal fossils
representing a period of time between 18 and 14 Ma (Woodburne, 1991; Reynolds, 1991).
The Crowder units have produced forty mammalian species, including two new species (Lindsay and
Reynolds, 2008). There are reptiles, birds and mollusks from the deposits, as well as the remains of
burrowing insects and millipedes. The recovery of more than 12,639 diagnostic fossils from 20 paleosol
horizons within the Crowder Formation illustrates the importance of careful examination and collection
from buried soil horizons.
View east toward vegetation at Sulfur Spring. To the south, bare cliffs of Crowder Unit 3 result from
slope failure. Retrace west to I-15.
42.4 (0.6)
STOP at the junction of Hwy 138 and I-15. Park into the McDonald Area Parking Lot
for discussion on the Cajon Valley Beds, Victorville Fan and the Cajon Fault.
STOP A5: CAJON VALLEY FORMATION, VICTOR VALLEY FAN & CAJON FAULT
Field presentation by Miles Kenney
The Cajon Valley Fault Story:
Total dextral (right lateral) motion between the North American and Pacific Plates during the late
Tertiary has primarily been accommodated by a broad zone of offshore and continental shearing on
strike slip faults, Basin and Range extension, and transrotation of crustal blocks. Total right lateral
motion between the North American and Pacific plates may be as high as 1,100 to 1,500 km during the
past 30 Ma. The San Andreas fault (senso stricto in central California, and including the San Gabriel
fault system and San Francisquito-Fenner-Clemens Well fault system in southern California) has likely
contributed between 300 to 340 km of this total slip. For example, although the 300 km of dextral
motion on the SAF north of the Transverse Ranges took place essentially across a relatively narrow zone
since the early Miocene and possibly Late Eocene, numerous strands and fault systems have
accommodated the motion within and south of the Transverse Ranges in southern California during the
same period of time. Therefore, reconstruction of the Late Cenozoic San Andreas Fault System in
southern California has involved estimating the timing and magnitude of slip across active strands and
inactive strands which were subsequently abandoned and often truncated by younger faults.
The Cajon Valley Fault which is exposed for 13 kilometers along the western margin of Cajon Valley
and can be extrapolated more than 20 km parallel to the SAF toward the northwest beneath Victorville
Fan deposits. Six piercing planes (lithologic contacts) within the Holcomb Ridge – Table Mountain
igneous and metamorphic suite are correlated to exposures of the same suite of rocks in the Western San
Bernardino Mountains of the eastern Cajon Valley region. The Cajon Valley fault is the likely
candidate for the exhibited displacement between these two terrains. Based on geologic mapping,
references to published papers, and thin section analysis, the Cajon Valley fault experienced
approximately 27±5 km of right lateral displacement between 4.1 to 9.5 Ma or possibly to 13.5 Ma.
The magnitude of total displacement across the Cajon Valley Fault of ~27 km places kinematic
constraints on nearby faults within the San Andreas Fault System. The Cajon Valley Fault was likely
the northern strand of the northwestern extension of the SGF, which accumulated 45±15 kilometers of
right-lateral motion between 4.5 to 5 and 12 Ma. Thus, the alignment of the Cajon Valley fault as the
northern extension of the San Gabriel fault as proposed by others is adopted within the present kinematic
model. Since the Cajon Valley Fault probably did not accommodate all the offset attributed to the San
Gabriel fault zone, a second fault strand is required to have accrued the missing ~18 km of dextral slip.
The most obvious candidate would be a fault zone bounding the
southern edge of the Holcomb Ridge – Table Mountain slice in essentially the same location as the
modern San Andreas Fault. This paleo-reconstruction of the Cajon Valley – San Gabriel fault system
requires that the modern San Andreas fault likely existed to southern Cajon Valley since ~10 Ma. In
addition, it indicates the San Bernardino strand of the modern San Andreas fault did not develop until
after the Holcomb Ridge – Table Mountain slice had been removed from the western San Bernardino
Mountains. Thus, the modern San Andreas fault to the region of southern Cajon Valley has likely
existed since ~10 Ma; however, toward the southeast from Cajon Valley, the modern strand of the San
Andreas fault (San Bernardino strand) likely formed around 4 Ma. Other kinematic ramifications of
this model indicate that the Squaw Peak Thrusts did not accrue tens of kilometers of displacement as
previously proposed by Matti and Weldon (1989). Additionally, the reconstruction geometry of the
model indicates that a restraining and releasing bend existed that could account for the progressive
compression and extension deformation documented within the Ridge Basin–Liebre Mountain region as
proposed by Crowell (1982).
The Victorville Fan Story and the evolution of Western San Bernardino and NE San Gabriel Mountains
An interpretation for the timing and magnitude of late Quaternary uplift within the western San
Bernardino Mountains and northeastern San Gabriel Mountains north of the San Andreas Fault is
presented based relative age of structures and stratigraphic relationships primarily identified within
Pliocene to Pleistocene sediments. These formations include coarse grained proximal fan deposits
associated with the Victorville Fan, and finer grained sediments that pre-date deposition of the
Victorville Fan. As originally proposed by Meisling and Weldon (1989), the Victorville fan consisting
of the Harold Formation, Shoemaker Gravels and Nobles Older Alluvium were deposited time
transgressively as sediments were shed off of the San Gabriel Mountains as they moved toward the
northwest due to motion across the San Andreas fault. A series of paleo-reconstructions are provided in
the presented paper that exhibit the source regions in the San Gabriel Mountains and relative
depositional areas in the Victorville fan deposits and tectonic deformation that was occurring during
various times of the late Pleistocene.
Figure 9 Structure contour map of pre-Pliocene erosion surfaces in the western Mojave Desert, San Bernardino
Mountains, and Table Mountain located in the northeastern San Gabriel Mountains. The contours in the San
Gabriel Mountains south of the San Andreas Fault merely follow the existing topography due to lack of age
control and documentation of the preserved erosional surfaces in the range. Except for the San Gabriel
Mountains, no regions south of the San Andreas and Pinto Mountain faults were contoured. Thin red lines
represent Quaternary faults and folds; black regions indicate late Miocene basalt flows
A structure contour map of the regional pre-Pliocene erosion surface in the San Bernardino Mountains
and western Mojave Desert provides a key basis for estimating total vertical deformation. (See Kenney
Article, this volume). These data indicate that a locus of compression, followed by vertical uplift of 1.0
to 1.6 km in a 4 to 15 km wide zone north of the San Andreas fault, migrated from the western San
Bernardino Mountains to the northeastern San Gabriel Mountains during approximately the past 1.5 to
possibly 2.0 Ma (Meisling and Weldon, 1989). The uplift produced a north-dipping monocline with fold
axis parallel to and on the northern side of the San Andreas fault. The upper limb of the monocline is
approximately horizontal between the upper hinge and the San Andreas fault, and narrows in steps
toward the northwest. The uplifted monoclinal region is 12 to 14 km wide near eastern Summit Valley,
6 to 10 km near Cajon Valley, 5 to 7 km at Table Mountain, and 3 to 4 km near Valyermo where initial
uplift and compression are in progress. Between Table Mountain and Valyermo, deformation since 0.78
Ma has involved a series of time transgressive high-angle north-dipping reverse faults which were active
during the local initiation of uplift. These faults became progressively less active as they were uplifted
1.0 to 1.6 km with growth of the northeast-dipping monocline. Thus, the reverse faults provide evidence
for northwestward migration of localized compression adjacent and north of the San Andreas fault from
Table Mountain to Valyermo. In addition, the Llano fault represents a steeply southwest dipping reverse
fault that may also have exhibited northwest migrating time transgressive activity during the late
Quaternary.
The northwest migrating compression and uplift adjacent to the San Andreas fault results from the
interaction of the San Jacinto and Cucamonga fault zones with the San Andreas fault at depth.
Specifically, the northward extension of the Perris Block is pushed below the San Gabriel Mountain
block and collides with the San Andreas fault at a depth of 8 to 9 km beneath Table Mountain. Motion
of the Perris Block is presumably parallel to the San Jacinto fault, which has a slightly convergent slip
vector relative to the San Andreas fault of approximately 20 degrees. Thus, the Perris Block collides
with the San Andreas fault between the northeastern San Gabriel Mountains and deflects it northward
beneath Table Mountain resulting in a subsurface restraining bend. Due to motion across the San
Andreas fault, the north-dipping subsurface restraining bend and lateral ramp migrated toward the
northwest, producing a dynamic locus of compression and uplift in the rocks north of the San Andreas
fault. Once the rocks achieved maximum uplift, they maintained their relatively high elevation without
further deformation, suggesting that a lateral ramp exists behind the leading restraining bend.
Return to the Stop on Highway 138, I-15 crossing.
Proceed west on Hwy 138. Cross over railroad tracks and under the railroad trestle. The spectacular
white arkosic hogbacks called Mormon Rocks are part of the Cajon Valley Beds, Unit 2. Geologists
Figure 10. Mormon Rocks, a hogback of steeply dipping Middle Miocene Cajon Valley beds, consisting of
arkosic sandstones.
initially thought that these arkose units were equivalent to similar-appearing sediments in the Devils
Punchbowl Formation, to the west, at Valyermo. Both arkosic units sit unconformably on a marine
formation that had been referred to the San Franscisquito. Woodburne and Golz (1972) pointed out the
lithologic differences in the underlying marine formation, and the presence of an elasmosaur relegated
the Cajon portion of those beds to a Cretaceous age (Lucas and Reynolds, 1991). Woodburne and Golz
(1972) demonstrated that the age of the Devils Punchbowl arkose was late Miocene (Clarendonian–
Hemphillian NALMA), while vertebrate fossils in the Cajon Valley beds were middle Miocene
(Hemingfordian–Barstovian NALMA). Therefore these look-alike arkosic beds were probably deposited
under similar conditions, but at intervals separated by at least five million years.
43.7 (1.3)
Turn left on Lone Pine Canyon Road. Cross Swartout Canyon Road junction and
continue along the Lone Pine Canyon Road.
Figure 11. Map of the Lone Pine Road and the Lytle Canyons, with landslides and other slope failures
from Hazlett, 2001
Note the burnt chaparral vegetation from the October 2009 fires which postponed our annual field trip.
51.6(7.9)
Entering Wrightwood.
The road towards Wrightwood ascends the wide floor of Sheep Creek Valley, choked with debris and
mudflows derived from the Wright Mountain Ridge, composed of weak Pelona Schist. The steepness of
the Wright Mountain, combined with rapid transpressional uplift, associated with the Cucamonga thrust,
and strike slip faulting at the base of the ridge, has resulted in spectacular debris flows, even visible in
the satellite image, below.
Figure 12. Facing the Lone Pine Canyon, towards Wrightwood in the northwest. The San Andreas Fault zone is
several hundred feet wide and the relatively flat valley floor is covered with low chaparral vegetation. (Photo by
Vik Mathur)
The road towards Wrightwood ascends the wide floor of Sheep Creek Valley, choked with debris and
mudflows derived from the Wright Mountain Ridge, composed of weak Pelona Schist. The steepness of
the Wright Mountain, combined with rapid transpressional uplift, associated with the Cucamonga thrust,
and strike slip faulting at the base of the ridge, has resulted in spectacular debris flows, even visible in
the satellite image, below.
Figure 13. Satellite Image of the San Andreas Fault Zone and the Sheep Creek Debris Flow in the Wrightwood
Area. The destructive debris flows occurred in 1941 and 1969 and involved Pelona Schist. (Satellite image from
www.sanandreas.org)
52.1(0.5)
Crossing the Sheep Creek Landslide Area. Note the greenish debris derived from Pelona
Schist.
The Pelona Schist, in the Lone Pine Canyon, as through most of the Cajon Pass area, is very unstable.
Pieces of actinolite bearing Pelona Schist are common in the debris where Lone Pine Canyon Road
crosses the Sheep Canyon channel. Actinolite occurs as a metamorphic product of mafic rocks, with
green schist and epidote-amphibolite facies. Today we encountered the Pelona Schist at the Blue Cut in
Cajon Pass and tomorrow we will discuss its origin during our stop at Manker Flats and San Antonio
Falls.
Morton and Campbell (1989, p.174, in Hazlett, 2001), describe a three-stage cyclical behavior in the
development of the landslide and the debris flows. “The three stages are independent, occur in sequence,
and are of different duration. Deposits of the first stage, the largest in size, are removed to positions
farther downstream by the activity of the second and third stage landslides.
Figure 14. Wrightwood Area debris flows, landslides and related control works (from Morton and Sadler 1989)
“First-stage landslides are represented by huge slumped masses derived from steep bedrock slopes in the
canyon heads; the material moves down the principal stream drainage, which may be completely filled
with debris. Second stage activity develops as streams cut a network of branching channels into the
massive first stage deposit. The second-stage landslides are chiefly slumps from the other slide mass and
from adjacent bedrock slopes. The movement of these slides is generally down slope toward actively
eroding drainages. Third-stage activity includes mudflows that accompany the spring melting of snow
pack. The debris moves down the stream channels to depositional reaches on major fans. Removal of
sufficient amounts of first-stage events resets the bedrock slope of the main drainage for another first
stage event.
“The first-stage landslides in the Wrightwood area are of prehistoric origin, and their recurrence interval
in any canyon is probably several thousand years. Second-stage landslides last one to years and are
apparently preceded and triggered by a series of high-precipitation sequences, ranging from a few days
to as much as six weeks; peak mudflow activity apparently results when a heavy spring snow melt
occurs during a period of second stage landslide activity.”
52.3 (0.2)
Turn right on Sheep Creek Road, through the residential area of Wrightwood.
52.7 (0.4)
Turn left on SR-2, going west. After leaving Wrightwood, we follow the Swartout Creek,
along the San Andreas fault, past a ski area and the U.S. Forest Service Visitor’s Center.
57.3 (3.6)
Junction of Hwy N4 and SR-2. Continue uphill on SR-2.
59.2 (1.9)
Vista at Inspiration Point
Figure 15. Inspiration Point (Elev. 7386 ft.) Overlook in October 2009, with autumn colors of buckwheat and
rabbit brush flowers. You are looking at the headwaters of the East Fork of the and San Gabriel River and the
north face of the Mt. San Antonio (elev. 10,064 ft.), also known as Mt. Baldy.
59.8 (0.6)
Grassy Hollow U.S. Forest Service Visitor’s Center LUNCH STOP.
STOP A6: VINCENT GAP AND GRASSY HOLLOW
Figure 16. Grassy Hollow Visitor Center Entrance, on a snowy day in April, 2010 (Photo by Vik
Mathur)
62.5 (2.7)
Vincent Gulch Divide Gate (Elev 6565 ft.)
63.6 (1.1)
Stop A7.
STOP A7: SLOPE STABILITY PROJECT AT THE ANGELES HWY 2 BRIDGE
Field Presentation by Chris Goetz and Chris Harris; also see the Goetz and Harris article, this volume
The torrential rains and snowfall that struck Southern California during the winter of 2004-2005,
resulted in extensive slope failures This caused the closure of SR-2, the Angeles Crest Highway,
between Islip Saddle and Vincent Gap for over 4 years from December 2005 and May 2009. The
roadway damage at this site occurred along a steep north facing slope which is underlain by gneissic
bedrock that is partially covered by a discontinuous veneer of talus. The failure at Mile post (MP) 74.05
involved total loss of west bound shoulder and incipient cracking within the westbound lanes.
Subsequent slope failure during the winter of 2006 later removed the eastbound lane as well. East of the
site, there is a perennial creek that drains beneath the roadway in a 3-ft diameter corrugated metal pipe
(CMP). A blockage of the CMP by the rock and wood debris likely caused storm runoff to overflow
onto the roadway, resulting into the roadway failures. Based on the geological/geotechnical
investigations, Caltrans elected to repair the roadway by constructing a bridge at MP 74.08 and by
constructing the embankment, utilizing a geo-grid reinforced slope. Caltrans also improved drainage at
the creek by adding two additional 2-ft. diameter CMPs beneath the roadway.
Figure 17. Slope Failure on SR-2, following the torrential rains of winter 2004-2005. The bedrock is
Precambrian gneiss, exposed in 45-50 degree slopes (Photo from Goetz, URS).
Figure 18. Completed single-span SR-2 bridge, with roadway embankment and geo-grid reinforced slope, and
improved drainage in the perennial stream below.
END OF THE DAY 1 FIELD TRIP.
Day 2, Sunday, June 20, 2010.
Highlights: San Antonio Creek Watershed, Cow Canyon Saddle Landslide, US Forest Service
Visitor’s Center, Igneous Dikes and 1938 Floods, Manker Flats and San Antonio Falls, 1969 Flood
Terraces, and San Antonio Dam.
0.0 (0.0)
Start at Cow Canyon Saddle at the entrance of Mt. Baldy Ranch R.V. Park, 30601 East
Glendora Ridge Road, Mt. Baldy, CA 91759, driving down towards Mt. Baldy Village.
STOP B1: COW CANYON SADDLE OVERLOOK
Field Presentation by Jon Nourse
The valley to the west is controlled by at least three strands composing the north branch of the San
Gabriel Fault zone. The fault recorded about 13 miles of right lateral slip between 12Ma and 5 Ma. To
the east the San Gabriel fault is truncated by the San Antonio Canyon (SAC) fault, with a left lateral
displacement of about 2 miles to Icehouse Canyon (Nourse and others, 1994). The intersection of the
two faults is buried by the Cow Canyon Landslide.
Walk down the road, to the overlook facing east into the head scarps of the Cow Canyon landslide
across the San Antonio Canyon. Along the road are the outcrops of the toe of the slide with distinct
clasts of garnet-biotite quartz feldspar gneiss, quartzite, marble, calcsilicate gneiss and graphitic phyllite
(Figure19, below).
Figure 19. Outcrop of the toe of the Cow Canyon Landslide, with clasts of gneiss, quartzite, marble and graphitic
phyllite.
Return to the cars and drive down the Glendora Ridge Road.
Figure 20. San Antonio Canyon and the vicinity, with Day 2 field stops
0.8(0.8)
Turn left onto Mt. Baldy Road and into Mt. Baldy Village
1.0 (0.2)
Turn left into U.S. Forest Service Visitor’s Center STOP 2
STOP B2: USFS VISITORS’ CENTER
Field presentation by Carolyn Summers, instructor Environmental Education program
Gather around the campfire circle, with displays of the Tongva village and mining camp demonstrations
Figure 21. U.S. Forest Service Mt. Baldy Visitor Center compound, with displays of a Tongva Village and a
Mining Camp, representative of past land use in the area.
Return to Mt. Baldy Road, turn left through the village.
1.2 (0.2)
Buckhorn Lodge and the crossing of San Antonio Creek
1.4 (0.2)
Park in the parking lot on the left, across from the trout ponds
STOP B3: GNEISS-DIKES INTRUSIONS AND 1938 FLOODS
Field presentations by Jon Nourse and Prem Saint
The Buckhorn Restaurant and Lodge, below, and the trout ponds are located at a site where the 1938
Figure 22. The Buckhorn Lodge today. From Osborne, 2005
flood destroyed a flourishing Camp Baldy Resort, operated by a member of the family of the Yosemite
Camp Curry fame.
Figure 22. Historical Pictures of the Curry Mt. Baldy Lodge, destroyed by 1938 floods (from Osborne, 2005, A
Guide to Mt. Baldy & San Antonio Canyon)
Walk down to the flood plain and the riparian zone of the San Antonio Creek. The creek contains
boulders of diverse plutonic and metamorphic rocks, including those from the Telegraph Peak pluton
which is the source of Late Oligocene rhyolite porphyry sills and dikes exposed in the outcrops to the
west of the creek (Nourse and others, 1998). Cross the creek to examine the bedrock of the Precambrian
gneisses with cross-cutting dikes and sills, (Figure 22). The gneiss bedrock represents some of the oldest
rocks in California and were formed in a supercontinent called Rodinia, that existed between 1700 Ma
and 800 Ma (Figure 24). Seventy mafic and intermediate dikes measured in this area cluster into two
orientations: N45W/85NE and N80W/60NE. The younger dike swarm is disrupted by brittle faults with
distinct orientations and styles. Younger, northeast-striking, northwest dipping faults that display
prominent left lateral and oblique-sinistral reverse faults. The perennial flow of the San Antonio Creek
provides a critical watering hole for wildlife, including the bighorn sheep (Figure 25)
Figure 23. Rhyolite Dikes and Sills intruded by younger mafic and intermediate dikes, all intruded into
southwest dipping mylonites and Precambrian gneiss (Photo by Dee Trent).
Figure 24. Reconstruction of the Rodinian Landmass, a supercontinent that existed between 1700 Ma and 800
Ma, during which the Precambrian gneiss of the San Gabriel Mountains and the Joshua Tree Area was formed
(from Trent and Hazlett, 2002)
Figure 25. Riparian Zone of the San Antonio Creek, along with the perennial streamflow provides an
important watering hole for the wildlife, including the Bighorn sheep.
Return to the cars and proceed north on Mt. Road. The road cut across from the parking lot exposes a
part of the dissected Cow Canyon Landslide (Figures 26, 27, and 28), we met at the Cow Canyon
Saddle, stop 1.
Figure 26. Outcrop of the Dissected Cow Canyon Landslide, east of the parking lot across from the trout
2.2 (0.8)
Cross San Antonio Creek bridge with large boulders.
2.6 (0.4)
Go past the Icehouse Canyon trailhead turnoff towards the Mt. Baldy Ski area, crossing
several switchbacks with road cuts in another dissected landslide, the Mt. San Antonio
Slide, with the source area in the between Mt. San Antonio and Mt. Harwood (Figure 27).
Note the change in vegetation as we ascend beyond 500 ft. into various vegetation and
habitat zones, from chaparral to the different Pine and Fir Communities (Figure 29).
Figure 27. Landslides and Debris Flows in the San Antonio Canyon (from Hazlett, 2001)
Figure 28. Geologic Map of the San Antonio Canyon in the vicinity of Mt. Baldy Village. From Dibblee, 2002.
Figure 29. Vegetation and Habitat Zones, as you drive up the Mt. Baldy Road from the foothills to the top of the
Mt. San Antonio (Mt. Baldy), passing from chaparral to the the different pine and fir communities. Where there
are perennial streams, like the Icehouse Creek and San Antonio Creek, locally these zones are interspersed with
riparian zones.
5.0 (2.4)
Manker Flats, with mounds of mine tailings containing Pelona Schist (Figure 30), a
reminder of hydraulic mining for gold in the Mt. Baldy Notch Area that date back to
1880s and 1890s (Trent, 2001).
5.7. (0.7)
Park in the parking lot at the intersection of Manker Flats and San Antonio Falls Road,
display the US Forest Service Day Parking Pass inside your winshield and walk up the
USFS fire road towards the San Antonio Falls.
Figure 30. Manker Flats with mine tailings containing Pelona Schist and other rocks, a remnant of hydraulic gold
mining in Mt. Baldy notch area.
Figure 31. Geological Map of the Manker Flats, San Antonio Falls and the Baldy Notch (from Diblee 2002)
STOP B4. MANKER FLATS AND SAN ANTONIO FALLS
Field Presentation by Jonathan Nourse
Walk up road through a west-dipping section of Pelona schist that composes the footwall of the Vincent
thrust. These rocks were first mapped in detail by Perry Ehlig (1958) as part of his doctoral dissertation.
Greenschist, grayschist, and quartzite derived from mafic igneous rocks, graywacke/mudstone, and
chert, respectively, is strongly sheared and transposed. These oceanic rocks were overthrust by San
Gabriel continental basement during late Mesozoic-Early Cenozoic time (Ehlig, 1981; Nourse, 2002;
Dibblee, 2003). Notice the complex mesoscopic folds, especially well-preserved in metachert layers.
Along this stretch of the road, the metamorphic fabrics are intruded by several rhyolite porphyry dikes
and one mafic dike. From the first switchback (0.6 mi above the gate) is a spectacular view of San
Antonio Falls (Figure 32). These waterfalls in upper San Antonio watershed provide a beautiful
Figure 32. San Antonio Falls with their water source
in a line of springs located above 8200 feet. Note
the exposures of Pelona Schist with rhyodacite sills
and dikes.
Figure 33. Pelona Schist outcrop on the Mt. Baldy
fire Road, with rhyodacite porphyry dikes and sills,
cut by reverse faults, indicating a local
accommodation for transpressional forces (photo by
Trent, 2001)
illustration of local hydrology and geology. The primary water source for San Antonio Creek is a line of
springs located upstream at 8200ft elevation, just above Sierra Hut. At Sierra Springs, water contained in
a porous talus boulder slope (Baldy Bowl) is forced to the surface along a lower boundary with
impermeable rock (Pelona schist). Water passing over San Antonio Falls (elevation 6400 ft) is joined
downstream by flow from Manker springs and Icehouse Creek. Farther downstream San Antonio Creek
supplies water to residents of Mt. Baldy village and Upland. Hydroelectric power is also generated as
the creek drops 1660ft elevation between the Mt. Baldy village and San Antonio Dam. The cliffs before
you reveal rock layers that have been carved and exposed by the erosive action of San Antonio Creek. In
detail, these rocks compose two contrasting packages separated by the Vincent thrust (a fault named for
its exposure near Vincent Gap). The Vincent thrust marks a major tectonic collision zone between an
ancient continent and an adjacent ocean basin. Perry Ehlig (1927-1999) was first to map this fault in
detail during the 1950s as part of his PhD dissertation. Perry walked virtually every inch of the Vincent
thrust, a significant accomplishment in this rugged, dense chaparral country (Figure 34). The gray rocks
Figure 34. Rugged, dense chaparral vegetation, covering the exposures of Vincent Thrust in the Eastern San
Gabriel Mountains.
exposed in the lower part of the falls are metamorphosed sands, silts, and muds, originally deposited in a
deep ocean basin about 80 Ma. The green layers represent metamorphosed basaltic lavas that erupted
within this ocean basin. The Vincent thrust is exposed just above the prominent tree-covered ledge to the
upper left of San Antonio Falls. The steep cliffs above the Vincent thrust contain rocks of continental
origin ranging in age from about 1700 to 80 Ma. Samples of these rocks may be viewed in the talus
slope to the left of the falls. Especially noticeable is a distinct white rock with large black crystals
informally named “dalmationite” by Ehlig. This igneous rock crops out along with a finer grained black
and white quartz diorite near the summit of Mt. San Antonio. Close inspection of the waterfall outcrop
reveals uniformly dipping Pelona schist intruded by two prominent rhyodacite porphyry sills, in turn cut
by thinner mafic-intermediate dikes. Several northeast-striking faults disrupt the continuity of the
rhyodacite sills. Apparent left lateral and/or reverse displacements are compatible with more accessible
exposures observed farther up San Antonio Falls Road. Follow a narrow trail to the base of the falls. A
somewhat treacherous scramble up the talus slope south of the falls will afford up-close examination of
the intrusive relationships. The road climbs another 3 mi to Mt. Baldy Notch, during which it crosses the
San Antonio Canyon Fault four times as it bends from a N30E to N80E strike. Brittle strain near this
fault appears to be partitioned into reverse and sinistral components to accommodate local transpression.
For example, near Manker Creek crossing (1.4 to 1.7 mi above the gate), about 300 m northwest of the
San Antonio fault), rhyolite porphyry sills in Pelona schist are offset by a family of reverse faults.
Differential movements between reverse faults (see also Figure 2-8 of Trent and Nourse, 2001) have
rotated schist foliations to steep southwest dips and resulted in moderate northeast dips for two maficintermediate dikes. One mafic dike exposed 120 m up Manker Creek displays significant sinistral
offsets. Farther up the road (2.8 mi past the gate) the main trace of the San Antonio fault is exposed
beneath the ski lift Kinematic features here indicate pure left-lateral movements
Figure 35. Upper San Antonio Creek Watershed Map with springs and headwaters above Manker Flats and San
Antonio Falls (from CalPoly, Pomona, Senior Thesis.)
Return to your car in the Manker Flats Parking Area.
5.7 (0.5)
The Snowcrest Inn is on the right, continue driving south
7.7 (2.0)
Drive past the turnoff for the Icehouse Canyon Road.
9.0. (1.3)
Cross San Antonio Creek bridge at Buckhorn Lodge parking area.
9.4.(0.4)
Glendora Ridge Road turnoff on right. Continue south on the Mt. Baldy Road, passing
road cuts with landslide deposits from the Hogback Landslide (Figures 36 and 37).
Figure 36. Hog Back Landslide Map showing the head scar and the toe intercepted by Mt. Baldy Road (map from
Herber, 1987).
13.9(4.5)
Turn left on Shinn Road, going towards San Antonio Heights
14.2. (0.3)
Cross San Antonio Creek and park across from the USForest Service Fire Station, on the
terrace of the San Antonio Creek.
Figure 37. Cross-Section across Hog Back
Landslide from A-A’, by Herber 1987.
Figure 38. San Antonio Creek during the1969
Floods at Shinn Road crossing area (Photo from
USFS)
STOP B5: 1969 FLOOD TERRACES AND SAF OUTCROP
Field presentation by Jon Nourse.
Figure 39. San Antonio Creek in the vicinity of Shinn Road, with terraces, alluvial scrub, and riparian vegetation
consisting of willows and other phreatophytes.
Figure 40. Geologic Map of San Antonio Dam Area, showing the location of faults at Stops A5 and A6.
(Map from Diblee, 2002).
Return to your car and to Mt. Baldy Road stop, staying in the left lane. Turn left on Mt. Baldy Road.
15.6 (1.4)
Park on the left into the parking area overlooking the San Antonio Dam
Figure 41. U.S. Army Corps of Engineers Sign at San Antonio Dam overlook, detailing the dam statistics.
STOP B6: SAN ANTONIO DAM OVERLOOK
Field Presentation by Jon Nourse (Notes from Nourse, 2003)
Also see Diblee Map, Figure 40, for field relationships.
San Antonio Dam (Figure 41) was built by the Army Corps of Engineers in 1956 to prevent a repeat of
the high waters that inundated Upland, Claremont, and La Verne during March of 1938. It played a
crucial role in reducing damage during the 1969 flood. In addition to flood control, the dam also serves
the purpose of groundwater recharge. Its porous and permeable substrate supplies renewable well water
to the City of Upland. San Antonio reservoir has never exceeded the height of the spillway, but its water
elevation reached 2193 ft and 2226 ft during January of 1969 and February of 1980, respectively (Figure
42).
The earth-fill design of San Antonio Dam (Figure 43) is prudent given its location near the intersection
of three active faults that conceivably might rupture a concrete structure. The dam is positioned across
an apparent left-lateral deflection or tear in the Sierra Madre-Cucamonga thrust. Coinciding with this
deflection is a prominent left-step between the San Antonio Canyon fault and the inferred northeast
projection of the San Jose fault (Herber, 2001). Given that both of these latter structures record leftlateral displacements, the fault geometry suggests that the thick alluvium beneath San Antonio reservoir
occupies a pull-apart basin.
Figure 42: Hydrograph of San Antonio Creek Flow through San Antonio Dam, as gaged by the U.S> Army Corps of
Engineers, from 1958‐2003 (from Nourse, 2009)
Figure 43. Geologic Cross-section across San Antonio Dam, with personal notes by Professor Larry Herber,
2001, Geology Department, CalPoly, Pomona.
The epicenter of the 1990 magnitude 5.4 Upland earthquake was directly south of San Antonio Dam
(Hauksson and Jones, 1991). This earthquake and a similar event in 1988 (M=4.5) are believed by these
authors to have ruptured the San Jose fault. Detailed analysis of the 1990 Upland event and its
aftershocks indicates left-lateral oblique reverse displacement on a fault with an orientation of
N26E/77NW (Dreger and Helmberger, 1990). Rupture initiated at a depth of 6 km.
The Sierra Madre-Cucamonga fault has not ruptured in the immediate vicinity during historical time.
However, the entire frontal fault system is classified as active and capable of generating a magnitude 7
earthquake (Dolan et al., 2005). Its youthfulness is implied by prominent escarpments along the range
front, and evidence for accelerated erosion that we will view later today. Also, both the east and western
abutments of San Antonio Dam preserve dramatic examples of north-dipping thrust faults that break
crystalline rocks. These faults, synthetic to the modern-day Sierra Madre-Cucamonga thrust, have
facilitated uplift of the eastern San Gabriel Mountains. There is an active sand and gravel quarry
operations as shown in Figure 44.
Figure 44. San Antonio Earthfill Dam, in April 2010, with a partial-filled reservoir and access roads for sand and
gravel operations
The spillway is now the site of a lot of grafitti, or urban artwork. So, we’ll end the trip for you to enjoy
an expression of local urban creativity.
END OF THE TRIP.
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