THE MAKING OF CALIFORNIA--PLATE TECTONICS VS. MOTHER NATURE’S WEATHER AND EROSION, PART 2: THE SAN ANDREAS FAULT AND THE CARRIZO PLAIN
This is Part 2 of the story of the making of California and takes us to the Carrizo Plain, a dry, mountain bound valley in the central California Coast Range mountains, for a very up close and personal visit to the prime mover in the making of California: the San Andreas Fault.
If you have not yet read the section of this page titled MAKING OF CALIF. PT. 1,you should read it before this Part 2. This article about the Carrizo Plain will make more sense if you read about the Panoche first.
The ground shook violently. People and animals lost their footing and fell over. In buildings things fell off shelves, and around Fort Tejon, California, entire buildings collapsed. It was January 9, 1857 and there had just been a magnitude 8 earthquake on the south-central portion of the San Andreas Fault. It was graphic evidence of plate tectonics engaged in mountain building on the west coast of North America.
Along a 225-mile section of the San Andreas, the ground west of the fault had moved north by a substantial amount and in only a few seconds. In places like the Carrizo Plain, a valley lying between the Caliente Mountains and the Temblor Mountains in Central California the movement was dramatic. Wallace Creek, which flows westward into the Carrizo Plain from the Temblor Mountains and crosses the San Andreas Fault scarp at the base of those mountains, was displaced 30 feet in this one quake. This is dramatic, but not unique. Over the last 3,700 years, the creek has been displaced over 1,100 feet by movement in the fault. This aerial photo shows the creek above and below the fault scarp and the portion of the creek actually flowing in the fault scarp itself connecting the two disconnected segments of the creek uphill and downhill from the fault.
Pic 1: Wallace Creek Displacement Seen From The Air
Pic 2: Wallace Creek Displacement Seen From The Ground
Click the link below for a one minute video of the Wallace Creek displacement from top to bottom. This link to a video clip, as do others on this page, opens the video in a new window from Flickr. Please close the Flickr window when you are done to redisplay this page on The Adventures of the Real Mr. Science. Click Here for Wallace Creek Video.
The San Andreas Fault is called a slip-strike transform fault. It is a slip-strike fault because the fault surface itself—the interface between the rock on one side of the fault and the rock on the other side—is almost vertical, and the motion of the two sides of the fault is side to side, not up or down. So the sides of the fault mostly slide past each other. Slip-strike faults are frequently found at the boundary between two tectonic plates, and that is true of the San Andreas. More than 20 or 30 million years ago, the boundary between the North American Plate and the plates underlying the Pacific Ocean was a subduction zone. The relatively small Farallon Plate was rammed against and then under the North American Plate by the motion of the far larger Pacific Plate behind it. Melted by the Earth’s heat at great depths under the North American Plate, the rock of the Farallon Plate rose through fractures in the overlying plate and became the granitic plutons of the Sierra Nevada, the volcanoes of the Southern Cascade, the Sutter Buttes, the Pinnacles, and more.
When the Farallon
Plate was completely subducted the Pacific Plate itself rammed into the North
American Plate, but instead of being forced under, it made a left turn and
began to slide north along the border between the two plates. In places its motion broke off pieces of the
North American Plate and carried them along.
The interface between these plates became the San Andreas Fault. A fault that separates two tectonic plates
sliding by each other is called a transform fault, so the San Andreas is a slip-strike transform fault.
Because the rock to the east of the fault moves to the
right, the fault is called a right slip-strike fault.
Pic 3: Map of San Andreas Fault, labeled "SA" on map.
Pic. 4 The Triple Junction
The San Andreas
Fault is over 800 miles long and begins in the vicinity of the Salton Sea in
California’s Mojave Desert. It leads
westerly, but north of Los Angeles, makes a turn to the north, paralleling the
California coast until, north of San Francisco, it goes “off-shore,” eventually
reaching the Mendocino Triple Junction.
The Triple Junction is the place where the San Andreas, the Mendocino
Fault and the Cascadian subduction zone come together. These faults separate three tectonic
plates—the Pacific Plate, the North American Plate and the Gorda Plate—and the
local earth movement action near the Triple Junction is extensive. Earthquakes are common, as are volcanoes
resulting from plate subduction. Local
mountain ranges such as the King Range in Humboldt County piled up where the
plates collided. It is an interesting
area, and we will explore it in a subsequent article.
The movement of the fault created Tomales Bay and Bodega Bay, just to the north, in each case leaving an isolated strip of land on the west side of the fault, the main land on the east side and a sheltered bay open in between. This is a link to a very interesting interactive map of the San Andreas Fault, showing these bays. Link to San Andreas Fault Interactive Map. The bays created by the fault's movement fostered marine environments that now harbor remarkably beautiful and interesting creatures and plants. This is a link to a wonderful blog that features images of these plants and animals. Link to Images of Marine Life.
The effects of the movement on the fault can also be seen in the Cholame Valley, not far north of
Carrizo. There the United States Geological Survey maintains an extensive earthquake
study facility at Parkfield, a tiny community in the Cholame Valley. (In fact, it is the most closely
observed earthquake zone in the world.)
Earthquakes are as common as rain in winter in Parkfield. And naturally enough there is significant displacement
along the segment of the San Andreas Fault in Parkfield. Like Wallace Creek at Carrizo, the fault has
produced a low-lying channel through which a creek runs.
Pic. 5 In the Parkfield area, the San Andreas Fault lies in a distorted creek bed.
Pic. 6 Displacement of the sides of the fault show up in a
bridge over the creek channel. The far end of the bridge, which is on the Pacific Plate, curves to the right, showing that the Pacific Plate is moving northward (to the right in the picture) relative to the near end of the bridge, which is on the North American Plate. This amount of movement happened over just a few years. Displacement is averaging about two and a half inches per YEAR.
Pic. 7 Signs at
either end of the bridge emphasize that the fault defines the intersection of
the Pacific Plate and the North American Plate. The photographer of the first picture was standing on the North American Plate looking west across the fault.
Pic. 8 This sign is on the other end of the bridge, and the photographer is standing on the Pacific Plate looking east.
Evidence of long
term creep along the San Andreas appears in many places.
For example, the volcanic spires in the nearby Pinnacles National Monument
started off life 20 million years ago and 195 miles to the south of their
present location near Hollister, California. They were moved north, riding along with the
Pacific Plate as it slipped north. About half of the original Pinnacles were left behind in Southern California
near Lancaster in the high desert, where they originally formed.
Pic. 9 These are the Pinnacles in the Coast Ranges of Northern California--a volcanic intrusion into the basically sedimentary mountains elevated by the plate collision and carried north as the plates slipped by each other.
Pic. 10 These are the Neenach Volcanics, formed by the same volcanic events that created the Pinnacles, but have been eroded by rain and wind. They were formed along side the Pinnacles in the preceding picture 20 millions years of ago, but were left behind since they were on the edge of the North American Plate, and the Pinnacles were on the edge of the Pacific Plate being carried northward.
The Cholame Valley where Parkfield is located is, like the Carrizo Plain, a
dropped fault block or "graben," which is the German word for "trench," and is discussed in more detail later
in this story. The Cholame graben is bordered on the east by the Gavilan Mountains and on the west by the Cholame Mountains. To leave Parkfield on the floor of the graben the roads must climb over these mountains. First, however, the Parkfield-Coalinga Road, also called the Parkfield Grade Road, travels north through the floor of the beautiful valley, paralleling the San Andreas fault scarp. The scarp can be seen clearly from the road.
Pic. 11, just below, shows it as a relatively low vertical bank at the foot of the hills east of the highway.
Pic. 12 Volcanic Intrusion. Further up the road, on the shoulder of the Gavilan Mountains, there is evidence of volcanic activity. Pressure ruptures caused by fault motion offered a path to the surface for molten rock under pressure from the fault movement. This volcanic intrusion was probably formed below the surface of the local sedimentary rock and was then exposed by erosion.
Pic. 13 The Parkfield Grade Road. To leave the Cholame Valley graben to the east the road must climb
over the Gavilans, and its climb over the mountains is a spectacular
journey. Click the link below for a one minute video of the climb and summit
approach of the Parkfield Grade Road in a new window from Flickr. The Climb of the Parkfield Grade Road.
Pic. 14 This is the view of the Central Valley from the summit of the Parkfield Grade. Well worth the trip.
There are many
places in California to observe the San Andreas Fault and the effect on our
landscape of movement on the fault, including especially Parkfield, but perhaps the most exciting and, in its own way, the most beautiful, is at the Carrizo Plain National Monument. There visitors can truly get up close and personal with
a long segment of the San Andreas Fault and the geological results of its motions. Click Here To Visit The National Monument's Web Site In A New Window.
So it is at Carrizo
that we continue the story of the making of California begun in “The Making of
California—Plate Tectonics vs. Mother Nature’s Weather and Erosion, Part
1: Panoche Road,” also here on
The Adventures of The Real Mister Science website. Click here if you wish to jump to Part 1.
The Carrizo Plain
is a huge, dry, almost 400-square-mile valley of unconventional beauty and
astounding solitude. Large enough to
hold about 10 San Franciscos if they were tucked in cheek-by-jowl, today only a
handful of man made structures exist there.
It is only about 100 miles from the Los Angeles Metroplex and perhaps 70
miles from growing Bakersfield, but Carrizo has virtually no residents. You may see a map that shows the north end of
the plain covered in a crosshatch of streets and roads. Before the plain became a national monument,
the county agreed to a massive subdivision being developed there, but nothing
significant has been built and few roads were even graded.
Pic. 15 This is a panorama of the south end of Carrizo.
Carrizo did have
early settlers. They came to ranch
cattle and some sheep and to dry farm wheat and barley. At first, the settlers called the valley
“Llano Estero”—Salt Marsh, but after a while it became the “Carrizo Plain,”
Carrizo being the name in Spanish for a local, especially tall grass that grew
in the area. The name morphed again to
the Anglicized “Carrisa Plain,” but when the State of California, the Nature
Conservancy and the Bureau of Land Management began to assemble the national
monument, a decision was made to return to the Spanish roots of the
Even though there
are few facilities and almost no human residents, Carrizo can be visited in a
family automobile. California Highways
58 and 166 cross the Plain on its north and south ends respectively. Soda Lake Road, which is paved for most of
its length, traverses the Plain along its north/south axis between the two
state highways. If you have access to
4–wheel drive, old ranch roads and the dirt/gravel roads crossing the
surrounding mountains can also be explored.
More on those later.
Pic. 16 Each season at
Carrizo presents its own magic. Late
winter and spring produce wildflowers almost beyond imagination.
Pic. 17 More wildflowers
Summer produces an arid, brown landscape with
Soda Lake as its centerpiece and solitude as its surrounds.
Pic. 18 This "dust devil" shows how creative Mother Nature can be.
Dust, rising hot air and the Coriolis Effect are all she had to work
Fall is a
season to visit and walk along the
San Andreas Fault and the strangely crooked Wallace Creek.
Carrizo is geology in action. In fact, Carrizo is geology in action right under your feet. Between 10
million and 20 million years ago the area around Carrizo and the southern end
of the San Joaquin Valley (the south part of California’s Great Central Valley)
was the sea floor of the Pacific Ocean.
Shales and fossil plankton were deposited on the sea floor and eroded
rock washed into the sea from the newly elevated Sierra Nevada Mountains. They consolidated and became rocks now known
as the Monterey Formation. Only a few
million years ago the sliding collision of the tectonic plates caused the
elevation of the Temblor Mountains to the east and the Caliente Mountains to
the west, leaving the flat and now dry sea floor between them to become the
Carrizo Plain. The two ranges
effectively come together at both the northern and southern ends of Carrizo,
enclosing the valley on all sides.
The Temblors and
the Calientes are both syncline mountain ranges. That is, they were folded up much as a rug
might develop a fold when its edge is pushed toward the middle. The folding, of course, was caused by the
tectonic plate collision. Storms blowing
in from the Pacific dropped more rain on the western faces of the folded up mountains
causing more erosion there than on the
eastern side, which makes the mountains look more like wedges with a steeper
eastern face than a rug fold.
Cuts through the upper layers of the ground in these mountains show the
slope of the sediment layers and reveal the characteristic folding. The same tectonic forces that folded up the
surrounding mountains also elevated the area which became Carrizo to more than
2,000 feet above sea level. Then,
further movement of the fault resulted in the rock between the Temblors and the
Calientes dropping down a bit. As a
consequence, the Carrizo Plain was left as a depressed dry valley with a flat floor—a graben, as described above.
Pic. 21 A typical graben from the air.
Much of the great Basin and Range country of Nevada and California east of the Sierra Nevada consists of mountain
ranges and graben basins between them. This relief map of Eastern California and Nevada shows the regular basin and range structure of the geology of this region.
Pic. 22 Map of Nevada and Eastern California Basin and Range Province
mountainous enclosure of Carrizo also meant that there is no place where the
rain falling on Carrizo and the valley facing slopes of the Temblors and
Calientes can run off to. Instead, on
the floor of Carrizo, just as on the floor of similar mountain bound, flat
plains, a large seasonal lake or playa forms.
In Death Valley it is called Badwater Basin. In the south end of the Panamint Valley it is
called Searles Dry Lake. In the Mono
Basin and the Owens Valley, the playas are called Mono Lake and Owens Lake
respectively. In Carrizo it is called
Soda Lake. In this case, “soda” refers
to the sodium-containing salts that line the lake bed. Soda Lake is mostly sodium sulfate with
perhaps 10% sodium chloride, or table salt, mixed in. The lake forms when there are several inches
of rain over a period of a month or so.
The water dries out from evaporation in the spring. As a water lake it covers 3,000 acres and
harbors brine and fairy shrimp. Birds,
especially seagulls, fly in to feast on them. Click this link to view an Audubon Society brochure on the birds of the Carrizo Plain.
geological terms, all of these mountain girded valleys with seasonal playas are
called endorheic basins—closed drainage basins that retain water and allow no
outflow to other bodies of water such as rivers or oceans.
Look at the images of these other playas to understand how endorheic basins form and develop.
To the northeast
from the Soda Lake Overlook just off Soda Lake Road lies the north end of the Temblors and the remarkable
displacement of Wallace Creek caused by the San Andreas Fault. To the southeast lay the Elkhorn Hills. The
Elkhorns formed as a fractured fault block bordered on the west by the San
Andreas and on the east by an unnamed thrust fault created by pressure from
movement on the San Andreas. The
pressure forced the Elkhorn Hills fault block upward where it eroded and
softened into the hills we see today.
The flat plain that can be seen east of the Elkhorn Hills is generally
known as the Elkhorn Plain. It is a
small graben produced between the Temblors and the Elkhorn Fault on the eastern side
of the Elkhorn Hills. Farther south
along the San Andreas a similar formation resulted in the Panorama Hills.
the line of the San Andreas Fault all the way to the South end of Carrizo the
visitor will notice that Calientes joined the Temblors. The Carrizo graben is defined on the west by
several small faults: the Big Spring
Fault, the San Juan Fault and the White Rock Fault, and, although movement along
these faults is nowhere near as dramatic as on the San Andreas, they still have produced their share of folding and elevating followed by erosion.
Pic. 24 Jumbled Hills Created by the Minor Carrizo Faults
The Carrizo plain
extends many miles north of Soda Lake, and the effect of movement along the San
Andreas is visible there too. In
addition to causing the Carrizo graben to drop, fault movement has forced up
bodies of rock called pressure ridges.
Some are quite dramatic.
Pic. 25 This is
an aerial photograph of Dragonback Ridge, a distinctive, eroded fault block,
pressed up by movement on the San Andreas.
Pic. 26 Dragonback Ridge from the floor of Carrizo Plain. Image
copyright 2011 by Jack Elliott. Used
with permission. This and other excellent images are viewable on Jack Elliott’s
Santa Barbara Adventure blog at http://yankeebarbareno.com/
Carrizo is the
perfect example of the way that the California Coast Ranges, which stretch from
Santa Barbara north to the beginnings of the volcanic Cascades, were
formed. Subduction and the pressures
created by it scraped up, folded up, and tilted up rock that formerly lay on
the continental shelf. Some rock was
pressed deep enough under the North American Plate to melt and show up as
rising magma plutons which carried mountains up with them. Some showed up as volcanic eruptions and
flows. The pressures squeezed some rock
formations up out of the surrounding rock but without accompanying volcanic
activity. And then, as pressures were
relieved, and as the plates began their long, slow slide past each other instead
of their head–on collision, huge fault blocks of rock settled down between the
rising mountains: Carrizo itself, the
dropped fault blocks under the Cuyama, Cholame, and Salinas Valleys, the block
underlying San Francisco Bay created between the San Andreas Fault and the
Hayward Fault to the east, and the tilted fault block under Clear Lake.
is also an example in miniature of the forces that created the Basin and Range
province. Basin and Range country lies
mostly in Nevada, but also partially in eastern California. Along California’s eastern border the White
Mountains, the Panamints, the Inyos, the Monos, the Amargosas, the Funerals and
other smaller mountain ranges have been folded and tilted up, and the basins
between them—the Mono Valley, the Owens Valley, Death Valley, the Panamint
Valley, and others—have dropped down. In the north the Warner Mountains have been pushed up and the Surprise Valley dropped.
By driving one of
the ranch roads from Soda Lake Road east across the plain to the fault zone,
the visitor can reach the surface trace of the San Andreas Fault. In several places it runs in a displaced
creek bed. There, you can stand in the
fault zone, with one foot more or less on the North American Plate and one on
the Pacific Plate. Hopefully an
earthquake won’t occur while you are in that position.
Pic. 27 Ranch Road Toward San Andreas Fault
Pic. 28 "Riding The Fault"
more than earthquakes to the visitor. Carrizo was also inhabited for a long period
by Native Americans, and they left a large number of painted figures and
symbols on “Painted Rock,” which can seen by arrangement with the National
Monument office. Click here to visit a fine web site with detail and photos about Painted Rock. Please close the window when you leave it. Painted Rock.
you have a high clearance vehicle and do not mind driving on steep dirt roads,
you can get spectacular views of Carrizo, the San Andreas Fault and a sense of
remoteness hard to find in California by entering Carrizo by Hurricane Ridge
Road from the section of Cal 33 between Cal 58 and the city of Taft. Although you can find your way across the
Temblors by a bit of trial and error, topo maps and a good GPS would be
useful. Be warned there are several
ranch roads that wind through the area.
Hurricane Ridge Road connects with Elkhorn Road, running along the
eastern side of Carrizo, which is also dirt, but in better condition.
Ridge Road, however, has its advantages if you are comfortable navigating it. It begins on the eastern side of the Temblor
Mountains in a gigantic oil field. Well
pumps, called “grasshoppers” bob up and down all around the road. As the road climbs its winding path to the
crest, it gives wonderful views, both looking back into the Central Valley. Over the crest the panorama of Carrizo is
laid out before you. The large official looking
entrance sign greets visitors on this modest dirt road part way down. These are some views from the road.
As you emerge from the Temblors at the entrance to the national monument, there is lovely panoramic view of the plain.
For a slide show of the drive over the Temblors on
Hurricane Ridge Road click the link. The slide show opens in a new
window on Flickr. Please close the window when you are done.
However you visit Carrizo, please remember that
the plain was almost turned into a massive suburban housing tract that would
have obliterated most of its significant natural resources. Many species of both plants and animals that
have been displaced by farming and development in the Central Valley, such as
the San Joaquin Kit Fox, Blunt-Nosed Leopard Lizard and the critically endangered
Giant Kangaroo Rat, now survive almost exclusively at Carrizo, as do the
Pronghorn Antelope and California Rattlesnake. For more on Carrizo’s unique
wildlife, click this link to a BLM web page. BLM Web Page.
For a slide show of pictures of some of the endangered of Carrizo clink the link below.
The story of how joint efforts by the Nature Conservancy, BLM and California state agencies came together to preserve Carrizo deserves much more attention than The Real Mr. Science can give it on this web page, but the work of two women who energized that effort to preserve Carrizo should be acknowledged. As
development efforts began to roll, Ann Chadwick and Marlene Braun, two women
with the foresight to see the dangers that were developing and the will to
stand in the way of the destruction of this incredible place, became a moving
force behind engaging the Nature Conservancy and other public interest groups
and the Bureau of Land Management in the effort to preserve Carrizo. Ann came to Carrizo in 1976 and worked
inventorying thousands of species of flora and fauna. After the National Monument was declared, she was also the long time Goodwin Education
Center Coordinator. Marlene was the
first Monument Manager of the Carrizo Plain National Monument and worked tirelessly to preserve the natural environment of the monument.
the modest visitor center (and I do mean modest), there is a memorial to
them. If you are there during your visit
to Carrizo, and are as awed as almost everyone who visits, be sure to pay your
respects to their memory with a moment of silence and a slow visual panorama of
one of the most interesting places on Earth—the Carrizo Plain.
Pic. 29 Memorial Plaque
Pic. 30 Leaving Carrizo to the south.
Do come back!
Part 3 of our adventure in exploring plate tectonics and the making of California will focus on the California part of the great Basin and Range geological province, which covers much of the western U.S. Look for it soon.
Pics #s 5, 7, 8, 14, 16, 17, 24, 28, 30, and the images in the Hurricane Ridge Road Slide Show are copyright 2012 by E. Ahonima, and used by permission. All rights reserved. More excellent pictures of California by E. Ahonima are available on
flickr. Click here to go there.
Pics #s 10, 21, 23, and 25 made available though the courtesy of Wikimedia Commons, and are subject to
Wikimedia Commons Creative Commons License.
Pics #s 1, 3, 4, and 22 and the images in the slideshow of the endangered animals of Carrizo are kindly provided by the USGS or the BLM, which manages the Carrizo Plain N.M.
Pic #26 is kindly provided by and subject to the copyright of Jack Elliott as described above in the caption to the pic.
All text, videos and pics not otherwise attributed above are copyright 2012 by
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