by John Dvorak
Temblor is the Spanish word for “shaking,” and so one should not be surprised that the San Andreas Fault runs along the base of the Temblor Range. From the top of Overlook Hill, one can see about 40 miles of the fault trace, or about one-twentieth of the total length. It is best to climb Overlook Hill in the early morning or late afternoon when the sun is low in the sky and the shadows are long. At such times, the eyes are guided from the snow white of Soda Lake across a large grassy patch to a row of barren hummocky hills that mark the western edge of the Temblor Range. The edge of those hills is remarkably straight and is the San Andreas Fault.
To reach the fault from Overlook Hill, one drives southeast several miles, then backtracks to the northwest a similar amount, skirting the southern edge of Soda Lake. Take note of the occasional giant kangaroo rat, Dipodomys ingens, that seems to pop up out of nowhere and race across the road. The road may be barred temporarily by rolling tumbleweeds. At Elkhorn Road turn right, and after another mile arrive at Wallace Creek.
Before seeing the main attraction, I suggest walking a quarter mile to the base of the hills. Stand facing the southeast. To the left is a steep slope. To the right is the edge of a nearly level plain. Where the base of the slope meets the level plain is the San Andreas Fault. Now start walking.
No matter where you begin, eventually off to the right you will find a small dry streambed. With your eyes, follow the course of the streambed eastward to where it ends abruptly at the base of the slope. This is a beheaded stream. Where the stream is beheaded is the San Andreas Fault.
Next, walk a half dozen or so steps to the southeast, then look left. More often than not, on the adjacent slope and continuing up the hill is a straight and deep gully. Try it again. Find another beheaded stream, trace its course to the base of the slope, march off a half dozen or so paces to the southeast, look to the left, and voilà, there’s another gully! You can do this over and over. What has happened here?
Each streambed-gully pair was once a continuous feature, formed by outbursts of rainwater from occasional heavy storms that left deep erosional scars that ran straight down the hillside and out onto the plain. The outwash and erosion happened repeatedly. Then in 1857, an earthquake—the Fort Tejon earthquake, the largest so far in California’s brief history—shifted the ground horizontally at Carrizo Plain by about 20 feet. But this was not the only earthquake to occur along the San Andreas Fault at Carrizo Plain. To see what multiple earthquakes sliding along the same segment of the San Andreas Fault can do, walk back to Wallace Creek.
Standing on the hillside, about 40 feet above the plain, one sees a deep gully—at this scale properly called an arroyo, again, a Spanish word—about 100 feet across and 30 feet deep. With your eyes, follow its straight course down the Temblor Range as far as the San Andreas Fault. At that point, the creek makes an abrupt turn to the right, then after another straight course, this one 400 feet, the arroyo turns abruptly to the left and opens onto the plain. Geologists have known of this peculiar feature since 1909, when they were led to it by local ranchers, but its exact relationship to the San Andreas was not understood until the late 1940s when Robert Wallace—the namesake of Wallace Creek—began a lifelong interest in the offset streams.
Wallace began his geology career as a student mapping a 20-mile segment of the San Andreas Fault near Palmdale—the segment immediately west of where Noble worked 20 years earlier in the 1920s. Wallace was doing his initial geology work at the same time that Hill and Dibblee were successfully finding oil in Cuyama Valley. Being a student, he had almost no money, so he slept under the stars in the desert, listening to coyotes howl, urging them on with a violin he played, and surviving on cans of Campbell beef soup that he bought from the sole store in Palmdale. He chose to eat the soup cold.
It was along that segment of the San Andreas Fault, near Palmdale, that Wallace realized that many stream courses were abruptly offset at exactly the place where each stream crossed the fault—and always in the same direction: If one stood on one side of the fault, the other side had moved to the right.
That simple observation naturally drew his attention to Carrizo Plain, where eventually he documented dozens of stream offsets—again, always to the right—speculating that hundreds of examples probably exist along the entire fault trace from Humboldt County to the Salton Sea. Later investigators found those other offset streams, hundreds of miles away. They are in the Mecca Hills near Indio close to the Salton Sea, along the popular Sawyer Camp Trail that runs along the center of San Andreas Valley, and a right step in the source of Alder Creek just before it enters the ocean near Point Arena far north of San Francisco. An offset has even been found at the head of Noyo Canyon on the floor of the Pacific Ocean where the San Andreas runs under the ocean between Point Arena and Shelter Cove near Eureka. So Wallace’s original estimate of hundreds of stream offsets should not seem that outrageous. They are the most obvious clue—at least to the nongeologist—as to how the fault works. And the most accessible and clearest example—and the one mentioned most often and illustrated in textbooks—is Wallace Creek itself.
To see a more subtle—though, when one realizes what one is seeing, a more startlingly—example of large horizontal movements along the fault, from Wallace Creek drive southeast on Elkhorn Road to the intersection with Panorama Road. Now start walking downhill to the west, paying attention to the rocks lying on the ground surface.
At first, the surface is covered with sand. Eventually, boulders are seen scattered across the surface. Examine them. Some are granite, a hard rock with visible crystals of white quartz and plagioclase, pink feldspars and black biotite and hornblende. Where did the granite come from?
To the east, behind you, are the rocks of the Temblor Range, mostly white shale. To the west, 10 miles away across Carrizo Plain, is the Caliente Range, which is mostly marine sediments. So where did the granite come from?
Now look to the south. You can see a flat-topped mountain. That is San Emigdio Mountain and it is composed of granite. And it is on the east side of the San Andreas Fault. When you walked away from the intersection of Elkhorn and Panorama Roads and encountered the boulders of granite, you were on the west side of the San Andreas Fault.
San Emigdio Mountain is 50 miles away.
A simple calculation, assuming the ground shifted horizontally 20 feet during every major earthquake, shows that about 10,000 earthquakes—each one comparable to the 1857 event—could have slid the granitic boulders 50 miles from their source at San Emigdio Mountain to Carrizo Plain. A remarkable result, but it is only the beginning.
A hundred miles north of Carrizo Plain is Pinnacles National Monument, a place favored by hikers and climbers because of the many rocky spires, crags, and other points of sharp relief, all the product of the deep weathering and erosion of an ancient volcano. Pinnacles National Monument is also a prime place to see the California condor (though don’t make the mistake I made on my first visit and confuse the much more common red-headed turkey vulture, which also soars high on air currents, for the more majestic condor).
A geologic study conducted in the early 1970s identified ten distinct layers of volcanic rocks at Pinnacles, including successive layers of pumiceous tuff, hypocrystalline hypersthene-andesite, and augite-olivine andesite. Based on radiometric dating, the age of the volcanic rocks at Pinnacles—that is, the time of their eruption—is 23 million years. Pinnacles is immediately west of the San Andreas Fault.
Far south of Carrizo Plain are the rocks of the Neenach Volcanic Formation, which has ten distinct rock layers that match those at Pinnacles, including successive layers of pumiceous tuff, hypocrystalline hypersthene-andesite, and augite-olivine andesite. Radiometric dating of Neenach shows that these rocks also erupted 23 million years ago. By every measure, the rock sequences at Pinnacles and at Neenach are identical. And the Neenach Volcanic Formation lies immediately east of the San Andreas Fault.
Clearly, the Pin
nacles-Neenach pair was once part of a single volcano, now sliced through and slid apart by the San Andreas Fault. This separated pair provides compelling evidence that the accumulated slip along the fault exceeds 100 miles—Pinnacles and Neenach are 175 miles apart.
But the evidence is not easy for a nonexpert to see. In fact, it is difficult even for a professional geologist to see, requiring extensive fieldwork, chemical and mineralogical work done in a laboratory and, in the case of Neenach, access to private land. For these reasons, when people ask me where definitive proof can be seen for more than 100 miles of movement along the San Andreas Fault, I direct them elsewhere: I send them to another pair of identical rock outcrops located farther to the south.
A few miles east of where the I-5 freeway passes through the San Gabriel Mountains north of Los Angeles—east of the steep downgrade known to truckers as “The Grapevine”—the San Andreas Fault cuts through the edge of Liebre Mountain. This mountain mass is difficult to distinguish from the other surrounding and equally rugged mountain masses except for a key feature: Within Liebre Mountain is a Triassic monzogranite that can be reached by driving along a barely maintained paved road, the Old Ridge Route Road.
A monzogranite is a type of granite that is the last part of a magma body to solidify. For that reason, it often has unusual mineral and chemical compositions and especially large crystals. The one at Liebre Mountain—which solidified 215 million years ago during the Triassic Period and was reheated 70 million years ago during the late Cretaceous—catches the eye because it contains large well-formed crystals—often more than an inch across—of a salmon-colored potassium feldspar.
From Liebre Mountain in the San Gabriel Mountains, drive 120 miles southeast, through Valyermo and past the Pelona schist in Cajon Pass, to the San Bernardino Mountains, which are on the opposite side of the San Andreas Fault from the San Gabriel Mountains. Follow State Highway 330 toward Big Bear ski resort. Just two miles south of Angel Camp is a high roadside cut where an equally distinctive and attractive Triassic monzogranite is exposed. It also has large salmon-colored feldspar crystals that solidified 215 million years ago and were reheated 70 million years ago during the late Cretaceous.
These two widely separate exposures of a Triassic monzogranite have been subjected to a battery of laboratory tests, and in every case they have been shown to be identical. They have the same mineral content. They have the same chemical composition, which includes an unusual enrichment of strontium. So just as Pinnacles and Neenach can be paired, so can the two Triassic monzogranites in the San Gabriel and San Bernardino Mountains, a clear indication of more than 100 miles of horizontal sliding.
Scores of other pairings have also been made. Deposits of similar rock types are found on the west side of the San Andreas Fault in the northern Galiban Range near San Juan Bautista and on the east side of the southern Temblor Range on the Carrizo Plain. A light-colored sandstone known as the Butano Sandstone, originally deposited as a submarine fan, now located in the Santa Cruz Mountains, has been matched with an identical sandstone in the northern Temblor Range, indicating 200 miles of horizontal movement.
But how did such huge displacements—caused by the action of tens of thousands of major earthquakes over the course of millions of years—come about? Hill and Dibblee had no explanation. In fact, it would be another decade before the answer became clear. It would be provided by a Canadian geophysicist who had never seen the San Andreas Fault but who, fortuitously, had made a trip to the Hawaiian Islands.
*Petrolia enters the story of the San Andreas Fault in another way: It is the northernmost community to sit astride the fault and its few buildings were totally demolished by the 1906 earthquake.
*Steno’s law: Layers of rock are arranged in a time sequence with the oldest at the bottom and the youngest at the top.
*Expectation of facies patterns: The facies, or physical characteristics of a sedimentary rock, will vary within a sedimentary layer because the depositional environment varied; for example, coarse rocks are deposited near a river’s source where water flows fast, while sand and silt are deposited near the mouth of a river where water flows slowly.
Chapter 8
A Transformative Idea
The San Andreas Fault is here postulated
to be a dextral transform fault.
—J. Tuzo Wilson, 1965
In October 1958, a team of scientists from the National Academy of Sciences was planning to go to Antarctica to evaluate the current progress of research there. Merle Tuve, a member of that team, had a heart attack. So on short notice, J. Tuzo Wilson, another member of the academy, went in his place.
One might think Wilson was weary of travel, having completed more than 100,000 miles during the previous 18 months, but this was the end of the International Geophysical Year, and as president of the Union of Geodesy and Geophysics—an organization that was a primary supporter of the geophysical year—he was anxious to meet and organize and collaborate with scientists who had agreed to make simultaneous observations of the atmosphere, the oceans, and the land surface over the entire Earth. It was an unparalleled opportunity to see the planet as no one had ever seen it—from remote scientific stations on ice caps, in deserts, at the tops of mountains, and from the shifting decks of ships that were crisscrossing the oceans probing the deepest parts and, whenever the opportunity arose, seeking out and intentionally sailing into storms.
Wilson began his trip to Antarctica by leaving his home in Toronto, where he was a university professor, and flying to Chile, where he joined the other five members of the team. From there, this sextet of some of the world’s leading scientists flew in a military transport plane bound for a six-week tour of scientific stations on the southern continent, a part of Operation Deep Freeze organized and led by the United States Navy. After those six weeks, the team returned north via New Zealand, then to Honolulu in the Hawaiian Islands, where they inspected a newly installed solar radio telescope near Diamond Head. After that, the team divided. Half went to Maui to see one of the new worldwide stations that had just begun to track the first orbiting artificial satellites. The other half, which included Wilson, made a longer trip to Hilo on the island of Hawaii, where a nearby weather station, equipped with the latest in meteorological instruments, was set to open.
It is a 40-mile drive from Hilo to the weather station, and Wilson recalled that they first passed through fields of sugar cane, then miles and miles of tropical forest that changed suddenly into a field of dark recently congealed lava. They bumped and jolted up and over the stark landscape, climbing ever so slowly higher, the air growing colder and the sun overhead more brilliant.
At 11,000 feet, with a few snowflakes in the air, they ended their journey in front of three metal huts. Wilson was surprised by the absolute stillness in the air. It was cold, and he and his two comrades went inside one of the huts, where they were greeted by three local scientists who ran the station and who immediately began to describe their work.
Today the station is known as the Mauna Loa Observatory. It would be here, just a year after Wilson’s visit, that scientists would announce the first measurements that showed that the amount of carbon dioxide in the atmosphere was increasing at a high rate. But on the day he visited, December 4, 1958, that equipment was still being tested.
After the scientific discussions and after a lunch, the six men went outside. The wind was now blowing. Wilson walked off to look around. Only years later, reflecting on that visit, did he realize that he had arrived at a revolutionary way of understanding the Earth.
That afternoon he was standing high on the slope of Mauna Loa, the world’s largest volcano and one of the most active. To the southeast, out of sight of the weather observatory, was another volcano called Kilauea, much smaller than Mauna Loa though more active. Directly to the north was Mauna Kea, another volcano, that one dormant. And beyond, to the northwest, stood Maui, ris
ing from the sea and on Maui was another volcano, Haleakala, still active though less so than either Mauna Loa or Kilauea, and more deeply dissected by erosion than Mauna Kea and so probably older than that volcano.
And beyond Maui, though he could not see it, was a long chain of volcanic islands, including Oahu, where Honolulu is located. Each successive island in the chain was more deeply eroded—and, hence, must be older—as one progressed northwestward along the chain. Far beyond those islands was a line of islets and shoals, barely rising above sea level and familiar to fishermen, that were also volcanic and certainly older than the main islands.
While Wilson was thinking about this age progression of islands and islets and shoals out in the middle of the Pacific Ocean, he also thought about a discovery announced recently: It seemed that the entire seafloor of the northern Atlantic Ocean was moving, at an incredibly slow though steady pace, away from a long range of subdued mountains down the middle of the Atlantic Ocean and known, appropriately, as the mid-ocean ridge.
And so, Wilson thought, as he collected and ruminated on these fundamental observations over the next few years, might the seafloor of every ocean be in slow and steady motion? Might the entire floor of the Pacific Ocean, extending over a third of the planet, be in slow and steady motion? And if so, what would be the result? Could it explain the age progression of islands and islets and shoals?
He decided that it could, if the Earth’s entire surface was sliding. From this simple realization would come the foundation of the theory of plate tectonics.
Born in Ottawa in 1908, Wilson began to use his middle name, “Tuzo,” early in his professional career to distinguish himself from another geologist named J. T. Wilson, who worked at the University of Michigan. Tuzo was his mother’s maiden name, originally Tuoselle or Touzelle, later corrupted to Tuzo, which came from Huguenot ancestors who had landed in Virginia in the 17th century.
