YOUR OROGENOUS ZONES DEPT.
Look! Fault block mountains, there, and there…everywhere! All around! Closing in –!!
If Southern California is an Island on the Land, as Carey McWilliams dubbed it, the Transverse Ranges are the reason. They seal up the southern end of the Central Valley and separate Northern California from Southern California. The San Gabriels/Sierra Pelona division marks a key split in the northern and southern breeding populations among much of California’s wildlife. All that to do, and they’re still always on my back.
The mountains’ southerly-exposed bluffs and alluvial fans host the Foothill Alluvial Fan Chaparral and Southern Coastal Sage Scrub suites of the California Floristic Province, some of the youngest, rarest and most endangered organisms on earth.
Our microclimates are legion; this breeds diversity. Habitats feet apart, are completely different. Note the “Slope Effect” in Placerita Canyon on the back side (north side) of the San Gabriels:
Click below and fly souteasterly, over the whole Transverse Range Geomorphic in gorgeous relief:
But why do the mountain ranges trend easterly-westerly, when, as everyone knows, most of the other mountain ranges on the continent trend northerly-southerly? I asked a geologist. He didn’t get back to me, so I asked Norah and Ito. Here’s the scoop:
The cats say the reason is plate tectonics: subduction, rift zones, arc accretion, volcanics, uplift, rotation. Click on the animation by the seraphic C.R. Scotese. [Thank Thoth for his work at the Paleomap Project.] Try to spot western North America as it emerges: gasp when it does!
[Point of Awe: this is how far the plate theory has advanced since the 1970s, when the penny finally dropped for geologists and they realized it was the whole show, and had been, all along. Our generation of fifth graders were among the first kids who learned about “Gondwonaland” as basement bedrock of our knowledge. Our poor teachers must have been reeling with it, groping, grasping to understand so they could teach it to us. BLESS THEM.] But what I’ve been groping and grasping to understand lately, is that the Western Transverse Range Fault Block, Home Sweet Home, was formed all the way down by Baja. Santa Barbara and San Diego were suburbs of each other! Then it — we, the Transverse mountains and our Valley — were pushed and pulled and folded and rotated into position, transverse that is. This was quite recently indeed, and it’s still going on. It all hinges — quite literally — on the subduction of the “old Farallon Plate” and the rise of the East Pacific Rise, and the birth of the San Andreas Fault. Enjoy Tanya Atwater’s brief but brilliant video models of how it went down:
To really get an idea of this process at our very local level, consider Malibu Creek State Park. This is how it looks today:
Their resident geologist Don Kovalevsky has done a MAGNIFICENT set of illustrations tracking how this area of Santa Monica Mountains — and everything else — looked in various epochs. Click below for the whole series.
Today the Western Transverse Range Block is half covered by the ocean, with the Channel Islands the only peek-a-boo features. But the whole lump was made anyway from uplifted beach and marine sediments, bunched and folded, so even the bits that are high up in the mountains and miles from the tides, show their marine origin. I’m told you can find marine fossils, seaweed leaves and foot-wide nautilus shells, but I don’t need another solitary well-ventilated hobby…or do I?
The Valley Village View 