Every schoolchild is taught that mountains are formed when one continental plate pushes up against another, causing wrinkles. The Pacific Plate pushes into the North American Plate along the San Andreas fault in California; this is why the state has a whole string of north-south mountain ranges along its length. The general profile of California goes, from West to East: the Pacific Ocean, the coastal plateau (where Pacific Coast Highway, US-1, runs North and South), the Coastal Ranges, the Central Valley (where I-5 and US-99 run North and South), the High Sierras, and the Owens Valley (where US-395 runs North and South). Every schoolchild in California is also taught that rainclouds form over the ocean and then move inland, to the East, rising and growing colder as they reach mountains and so dropping rain on Western faces of the ranges. The valleys to the East tend to become deserts.

But what about the anomaly in Southern California, where a string of East-West mountains stretch from the desert to the sea: the San Bernardino, San Gabriel, and Santa Monica mountains that form the northern border of the Los Angeles Basin (and help make it so smoggy there)? How did this happen? For a succinct visual explanation, go the web site of Tanya Atwater of the University of California at Santa Barbara (UCSB): www.geol.ucsb.edu/faculty/atwater, click on "Animations" and then select the movie that shows "Southern California plate tectonic history from 20 million years ago to present day."

(It will require a free registration, which is low-hassle.) What you will see is a mind-blowing animation of how a chunk of plate got stuck on the North American side and twisted around to form the East-West mountains in Southern California. This made a bigger region, and drier region, for the deserts to the North and East, bounded by mountains forming a rough "L" shape: the upright being the Sierra Nevada, and the bottom being the San Bernardino.

At the angle of the "L" is a small gap formed by the shifting of the San Andreas Fault which separates the two continental plates: Cajon Pass. Almost all passages through the desert use this gap, including the Old Spanish Trail, the Southern Pacific and Santa Fe Railways, old Route 66, and Interstate-15. Even the newest fiberoptic cables go through the "blue cut," as Cal-Trans nicknamed Cajon Pass when they built the freeway through it and found blue rock.

Snowfall from the San Bernardino Mountains forms the Mojave River flowing north into the desert. It provides sustenance to Victorville and Barstow, and all the towns between on old Route 66, and then a little east of Barstow, near Forks in the Road, disappears into the desert sands, providing water for wells in Newberry Springs and at one time feeding watering holes along the old trails, before ultimately evaporating in the Mojave Sink.

The rocks in the East Mojave show extremely vivid evidence of volcanic and sedimentary activity (see photos above), making the area popular today with geology teachers.

See Tours and Detours "Tour #2: Geological Wonders" for a sample geology tour.

Last update 12:34 PM Fri. 27-Feb-2004 by ABS.