DESERT EXPLORER

California Desert trips,interesting places to visit: ghost towns, old mines, lost treasure, personalities and bits of the old west

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The desert is sort of my second home. The places described below are special and worth visiting provided you know the story. That's what this blog is all about. Now I have tried as best I can to be factual but sometimes you hate to let facts get in the way of a good yarn.

Sunday, June 14, 2015

THE CAJON PASS INTERLUDE





 
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”   

Anticipating strong 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.

 Wow.

Discovery of that mechanism arrived almost 40 years after the publication of Levi Noble’s landmark article.

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 Cajon Pass 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 BC.

The San 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.