Cajon Pass

Have you ever driven from Las Vegas to Los Angeles? Two thirds of the way there, you are driving along I-15 in the high, dry Mojave Desert at 3,000 to 4,000 ft, and suddenly you begin a steep curving descent, reaching very high speeds, hoping to retain control of your car. The topography is tortured with impossibly steep mountains and hills running across your path, but... you look far below and see a large wash cutting a path through the mess, and that is what the freeway follows. You see that the freight trains too run through this steep canyon. If you knew a little more, you'd also know that all the infrastructure for gas and oil to Las Vegas runs through here too. A few minutes of this wild driving and then you are dumped out into a low plain; it's cooler, more humid; the coastal air is tinged with both Pacific moisture and the exhaust of 15 million cars.

The drop, the canyon you pass through, is Cajon Pass. It is a critical transportation, infrastructure, and drainage corridor. It is also entirely a creation of the San Andreas Fault.

On your left side as you head towards L.A. are the San Bernardino Mountains. On your right, and more impressive in appearance, are the San Gabriels. They, and some other ranges to the west, constitute the Transverse Ranges, a east-west set of ranges. They are really one mountain range, formed from the compression along a kink in the San Andreas slip-strike fault. The motion of the Pacific plate, normally slipping smoothly, rides hard against the North American Plate at this location, compressing the land up into an unstable, steep, untenable range.

Why there is a usuable (although steep) pass right in the middle of the mountains can't be ascribed to luck--the San Andreas Fault runs NW-SE right through the pass. You can see a diagram of this from here: http://www.laep.org/target/fragile_habitats/images/mtns_calif.gif (The link was down today but it's been up before).
Also, I've annotated a TERRA image of Southern California:



The full image showing the whole region is here:


This is a nice block diagram showing the region from the USGS.

Imagine sliding the left side of the satellite image down and to the right, and now the San Gabriels and San Bernandinos line up.

Not only does the fault create the range, it also slices the range up.

University of Chicago and Hyde Park types will be amused by the roads in the small settlement near the base of the canyon. Google Hybrid link.

Another point of interest is the strongest plume of sediment you can see in the ocean. The Cajon Pass drains through a convoluted path of washes, creeks, and dams to the sea right there. Is some of that silt from the rapidly eroding rocks of the Pass? Absolutely.

I originally started writing this entry in August, trying to start out with a broad comment about how geology completely controls topography. I am amazed at how every geographic and topographic structure can be explained geologically. That approach to the entry didn't work, so instead I wrote about a specific instance. And what's amazing is I am finding these explanations everywhere.