As the Pacific Plate moved
to the northwest, it slowly unhinged the mountain chain that separated the
coastal plain of Southern California from the Mojave Desert. As it happened, it
created Cajon Pass,
a gap between the San Bernardino and San Gabriel mountain
ranges. Little was known about this
unusual event until the 1920s when a young geologist took an interest in the San
Andreas Fault -- a rift that extends almost the length of California.
Levi Noble carried
credentials of a qualified geologist with PhD and extensive field experience
with the U.S.
Geological Survey. His study of rock
formations in Cajon
Pass led him to conclude
that a “disconnect” existed across the fault zone. Terrain features and rock
composition on the south side of the fault differed significantly from terrain
features and rock composition on north side as if there had been lateral movement
between the two sides.
In a landmark article
published in 1927, Noble stated, “Scarcely anywhere in the fault zone are the
rocks on the opposite sides of the master fault similar”. He goes on, “The distribution of certain
Tertiary rock masses along the master fault affords a suggestion that a horizontal
shift of many miles has taken place along the rift”
disagreement from the establishment, he closed the article with, “The evidence
just cited, however, is not convincing, and is certainly not definite enough to
amount to proof”.
Noble makes reference
to two examples of the horizontal shift in Cajon Pass:
Blue Cut -- An
exposed slab of Pelona Schist (a
greenish blue crumbly mineral), located on one side of the rift faces a very different tan colored sandstone on the
other or opposite side of the San
Andreas Fault which runs through the canyon in a northwesterly direction.. (See Google Earth at: 34, 16.078; 117, 27.257. In the 1920s, a café, gas station and
campground existed at Blue Cut. It was
called Camp Cajon.)
Hidden Lake – The second example, lies within the main strand of
the fault a short distance from Blue Cut.
The shore on one side of the elongated lake is paved with pebbly Pelona
Schist and the shore on the other side of the lake is a brownish, eroded sandstone.
(See Google Earth at: (34, 16.399; 117, 27.952)
Dr. Robert Wallace
Professor of Geology at U.C. Berkeley led a chorus denouncing Noble’s
suggestion calling it “poppycock”. The time-honored
and sanctioned view held that earthquakes along fault lines cause the earth’s
crust to move vertically creating mountains and valleys. He further stressed that until science finds
a physical mechanism to support sideway movement this view will stand.
Discovery of that
mechanism arrived almost 40 years after the publication of Levi Noble’s
James T. Wilson, a Canadian
geologist, came up with the strange notion that continents and sea floors are
not permanently fixed in place but move about the planet laterally spreading
apart and bumping and scraping into one another possibly creating fault-lines
where they meet. This hard-to-imagine phenomenon Wilson called “plate tectonics”. According
to this belief, the San Andreas Fault formed
when a sliver of the Pacific plate began moving against the North American
plate starting about four million years ago.
Plate tectonics, once
accepted by those who decide such matters, opened new opportunities for PhD
candidates majoring in geology and related fields. Some began analyzing organic
material embedded in the fault zone; looking for evidence of past seismic
events and the amount of lateral movement caused by each.
One study centered in
in a swampy area located at the southeast end of Lost
Lake and appropriately called Lost Swamp.
A trench across the fault-trace exposed
a tapestry of embedded mud, peat, pink colored sand and bits of plant and
animal forms caught in crevices created by past earthquakes.
The exposed wall of
the trench is layered -- each layer representing a period of the past. Bits of
charcoal and peat within or at the edge of a layer can be carbon-dated giving an
approximate age which can be translated into an estimate of the event-year.
Before an earthquake
or seismic event occurs, the surface of the fault-trace or zone is sandy or
pebbly and relatively smooth. During a
seismic event cracks appear on this smooth surface. In time, the cracks fill with mud and debris
fed by intersecting streams and storm runoff.
Organics that can be carbon-dated also flow into the cracks and become
embedded. Size and extent of these
incisions (cracks) indicate severity of the seismic event.
So what can be learned
from this excavation? The last major seismic
event within the Cajon segment of the fault occurred around 1805, two-hundred
and ten years ago. Prior major events occurred in 1290, 1450 AD; 5900 and 6350
BC. The earliest deposit in Cajon Pass was found in river gravel at the junction of Cleghorn Canyon and Cajon Creek dated at 23,000
Francisco earthquake (1906) was the last major event along the San Andreas Fault.
Estimated at between 9 and 10 on the Richter Scale, it centered on the
northern segment of the fault and cause no significant seismic effect on the
southern Cajon Pass sector.
Levi Noble came from
a wealthy eastern family. That fact allowed him to work for the U.S. Geological
Survey without compensation. It also permitted
him to pick and choose which projects he wanted to pursue. Noble’s major contributions included the mapping
and interpretation of geology in the Grand Canyon and parts of Death Valley in
addition to the southern sector of the San Andreas Fault.
The Noble’s lived on
a fruit orchard near the edge of the San Andreas Fault in the San
Gabriel Mountains. Levi wanted to experience an earthquake first
hand. But that never happened. The Great Fault remained fiendishly quiet
during all of the years that they lived there.
After retirement he
and his wife moved back to his family’s estate in Auburn, New York.
Levi Noble died in 1965. Hopefully, Levi became aware of Wilson’s discovery that supported his own 1927 conclusions
about seismic movement along the San Andreas Fault.
POST SCRIPT I checked Google Earth for the location of
the Pink River
that flowed into Lost
Lake eons ago and noted
that the stream currently shows an offset of 222.09 meters (lateral movement
caused by earthquakes). According to information
from the Lost Swamp excavation, the initial disconnect
occurred about 9,000 years ago. That
gives an average slip rate of about 2.4 centimeters per year.
Now, the evidence just cited, however, is not
convincing, and is certainly not definite enough to amount to proof. It may well be poppycock.