Saturday, November 12, 2016

Geologic Fraternal Twins

The red line shows the course of the San Andreas fault.  Tejon Pass and the Grapevine are at the letters TP near the map's lower center.  Cajon Pass  is on the lower right denoted by CP.

Parents of twins are known for naming their children similar names:  Zella and Zelda, Dan and Don, Lacey and Lucy. Southern California has twin mountain passes--Cajon and Tejon--that guide a south-bound traveler into heavily populated basins circled by dramatic rugged mountain ranges. It's only a coincidence that the pass names are twinny.

On the drive south on Interstate 5, traffic climbs out of Central Valley up the steep winding 'Grapevine' through Tejon Pass in the formidable Tehachapi Mountains. Contrary to popular belief, The Grapevine is not named for its knarly twists and turns that resemble a fruiting vine braiding it's way up an arbor. The wild grapes that grew along the original narrow road lent their name to the area. The famous Butterfield Stage line between Los Angeles and San Francisco traveled through Tejon Pass.

85 miles east of Tejon, Cajon Pass leads traffic from the high Mojave Desert down into the Inland Empire communities of San Bernardino and Riverside. A funky formation called Mormon Rocks lies west of the pass, so named for the Mormon wagon train that passed by in 1851.

Four score miles apart as the crow flies, these "twin" passes have a commonality created by the San Andreas fault which has spawned them both. Their imposing, dramatic scenery distracts drivers all the time. If you know when and where to look as you navigate through the two passes, you can detect where that major earthquake fault slices through the mountains to the left and right of the freeway. 

 Nasty brush fires periodically sweep over the passes in the heat of summer.  Both have horrid winter weather that results in regular road closures (during college Craig took a bus home from Utah without hitting snow until 35 miles from home at Cajon Pass).  Most interestingly, the elevation of the two passes is nearly identical: 4160 feet for Tejon Pass and 4190 for Cajon Pass, an insignificant difference in geologic terms, yet it's kind of eerie for Southern California's two main points of entry to be that similar in elevation.

It has always intrigued me that these two important points have similar names, elevations, and features, and even parallels in their history. Earthquake activity from the San Andreas Fault is the chief factor that created the massively steep and spiny mountain ranges, and future fault movement will continue to bend and squeeze and thrust up these ranges.

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