A few weeks ago, someone asked me why we have mountains. I explained to him about tectonics, continental drift, and faults. I'll now pass that on to you, dear readers.

Mountains occur for different reasons throughout the world. The Hawaiian Islands are actually volcanic mountains, or the remains of them. Most of the islands in the oceans are volcanic in origin. Others, like the Himalayas, the Rocky Mountains, and the Appalachians, were formed because one continental land mass collided with another. The San Gabriel and San Bernardino Mountains are like these.

Many millions of years ago there was a vast inland sea on the West Coast of North America. Ocean waves from the Pacific Ocean actually crashed ashore against the Rockies. We know this because of various geologic evidence, not the least of which is the presence of sea shells and limestone on the High Desert. Limestone usually forms below sea level and sea shells on the seashore.

The Pacific plate collided with the relatively stationary North American plate, thrusting beneath it and raising it up some four thousand feet. That's why we are distinctly High Desert. And as explained in my last column the San Andreas Rift Zone (or fault) is largely the division between the Pacific and North American plates, particularly in Southern California. In the process, a weakness in Earth's crust formed the San Gabriel and San Bernardino Mountains and some of the local mountains as well.

These transverse (east-west) mountains, the Sierra Nevada and Tehachapi Mountains (approximately north-south) are all fault-block mountains, caused by uplift along fault zones. Although erosion keeps tearing them down they continue to rise in altitude. Mount Baldy (10,064 feet) is the highest peak in the San Gabriel Mountains. Mount San Gorgonio (11,500 feet) is the highest peak in the San Bernardino Mountains. The so-called Little San Bernardino Mountains, east of the regular San Bernardino Mountains, the High Desert's Granite Mountains, Ord Mountain, and Quartzite Mountain (north of Victorville) are also the result of uplift along weaknesses in the earth's crust. Some of them are volcanic as well.

Southern California has other mountain ranges, also the result of uplift and most are block-fault mountains. The Santa Ana Mountains of Orange County, the various mountains in San Diego County, and the coastal ranges are all examples. The mountain ranges south and east of the High Desert result in blocking weather fronts, making us mostly a "rain shadow" desert. We'll be discussing this next time.