I live in the Santa Clara Valley, which sits at the south end of the San Francisco Bay in California, USA. Most people are more familiar with this place as “Silicon Valley” because of all the high-tech work done here. And as you can see from the Google Earth snapshot below, it is pretty built up.
South and west of the valley are the Santa Cruz Mountains; the Diablo Range is to the east. We’re not talking high mountains here, think about 4000 feet (1200 meters) or thereabouts at the highest points. Most of our rain comes in storms that travel west to east, so there’s a significant orographic effect across this area. You can see it in the colors; the Santa Cruz Mountains are quite heavily forested, but only the western edge of the Diablo Range gets enough rain to support a lot of foliage. Further east is grassland with occasional oak trees. That’s because the Santa Cruz mountains grab those incoming rainclouds and squeeze as much as they can out of them, before letting the remnants sail over the valley to the Diablo Range. (Okay, that’s not the most scientific of explanations, but you get the idea.)
But I digress. Here’s a very simple geologic map of the area, shamelessly stolen from my MS thesis:
GUAD, CCOC, STPK. WLLO, and MGCY are the names of wells. I studied sand cored from those wells for my thesis, examining sand samples taken from different depths and looking at spatial and temporal differences, and asked what kind of story they told about the last 800,000 years of valley formation. But that isn’t the subject of this post, so you can ignore them.
“Quaternary alluvium” are alluvial deposits mostly dumped by the streams that cross the valley. Of course, these deposits are pretty much lying under asphalt and concrete and buildings now. Streams mostly run in concrete or agggregate-lined channels. “Bedrock” isn’t necessarily raw exposed rock; if it were, none of that greenery in the first picture would be established. These are places where the soils are thin, there actually are a lot of exposed rocks, and while stuff can grow it’s not growing on meters or kilometers of alluvium. I’ll talk about that more in a bit.
I do want to point out the red lines. Faults: we’ve got ‘em! The San Andreas Fault is pretty famous, but the entire valley is riddled with faults. And this is a simplified map; if I’d plotted ever minor fault there’d be very little but red lines all over the place. Some like the San Andreas Fault, the Silver Creek Fault, and the Calaveras Fault, are primarily right-lateral strike-slip faults. That means the rocks to either side of the fault are sliding past each other. Being right-lateral, if you stand on one side of the fault, the rocks on the other side are moving to your right. The other faults are primarily thrust faults; that is, rock is pushing up over other rock. Now, just to complicate things, most faults have both left-right and up-down motion. So the local strike-slip faults also have a little bit of up/down motion going on, and the local thrust faults have a little bit of slipping going on. Nature absolutely refuses to be categorized simply.
So what’s that line across the picture, with the ends labeled A and AA? It’s the line of a geologic cross section. This picture is in my thesis, but I shamelessly stole it (with correct attribution of course) from a USGS paper. (Don’t worry, I’ll put all the relevant references at the end of the post.)
To fully explain everything that’s going on in this diagram would take a whole ‘nother blog post. But it’s really instructive. (Elevation, by the way, is based on sea level.) That skinny yellow bit at the top? That’s no older than 1.8 million years, and lots of it is a good sight younger than that. But the Cupertino and Evergreen alluvial basins are much older, on the order of 5 or 10 million years for the Evergreen Basin, and maybe as much as 20 million years for the Cupertino basin. I’m guessing a bit here, I haven’t researched the basins, and I’m not sure there’s much literature on them anyhow. But the point is that they’ve been having streams run through them and dropping sediment for a fairly long time. The other rocks, the Franciscan Complex, the Coast Range Ophiolite, and the Great Valley Group (GV) are much older, and formed during the Mesozoic when the Sierra Nevada mountains were volcanoes, dinosaurs pottered about, and this whole area was under water.
(I did tell you that the Silver Creek and Calaveras Faults were primarily strike-slip faults, didn’t I? But here in this picture I show arrows indicating up/down motion. This is the “bit of up/down motion” I was talking about.)
That middle section in the picture, between the Cupertino and Evergreen basins, is informally known as the Basement High. There’s a lot of evidence that 800,000 years ago it was exposed rock happily being rained on or snowed on and then being eroded into that Quaternary alluvium. But by 400,000 years ago it was covered up by alluvium. There’s only one remnant of it left exposed today, a hill in the middle of the valley bristling with transmission towers.
Welcome to my valley. Some day I may show you a real geologic map of those mountains, and explain where the rocks came from. But that will have to wait for another day.
Brabb, E.E., Graymer, R.W., and Jones, D.L., 2000, Geologic map and map database of the Palo Alto 30′ X 60′ quadrangle, California: U.S. Geological Survey Miscellaneous Field Studies 2332, scale 1:100,000.
Bryant, W. A. (compiler), 2005, Digital Database of Quaternary and Younger Faults from the Fault Activity Map of California, version 2.0: California Geological Survey Web Page, http://www.consrv.ca.gov/CGS/information/publications/ QuaternaryFaults_ver2.htm, accessed March 21, 2011.
Wentworth, C.M., Blake, M.C., Jr., McLaughlin, R.J., and Graymer, R.W., 1998, Preliminary geologic map of the San Jose 30 X 60 minute quadrangle, California: a digital database: U.S. Geological Survey Open-File Report 98-795, scale 1:100,000.
Wentworth, C., Williams, R., Jachens, R., Graymer, R., and Stephenson, W., 2010, The Quaternary Silver Creek fault beneath the Santa Clara Valley, California: U.S. Geological Survey Open File Report 2010-1010.