This is My Valley

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.

Santa Clara Valley and surrounding mountains
Google Earth snapshot of the Santa Clara Valley with the surrounding mountains.

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:

Santa Clara Valley with bedrock/alluvium and faults
Here’s a very simple geologist’s view of the Santa Clara Valley and the surrounding mountains.

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.)

Santa Clara Valley geological cross section
Here’s what’s going on underground along line A-AA from the previous picture.

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, 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.




This is My Valley

16 thoughts on “This is My Valley

  1. 1

    Very cool. Even tiny chunks of CA are more variegated and interesting than large cross-sections of OH. I’m not quite sure if it is the physical geology (some, certainly) or the differential in the amount of geological study given to the areas. Also, of course, the lack of access to things behind paywalls does not help.

  2. rq

    Thanks for that small tour!
    Sometimes it’s strange to think that we build our cities on such old and venerable and unpredictable foundations.

  3. 3

    California is a damned mess, geologically. mostly due to accreted terrain, if i remember correctly. all the faults and stuff don’t really help to make things much easier to figure out.

    Accreted Terrain> there used to be a subduction zone along the coast, where the Pacific plate dove under the North American plate. Island chains would get carried along, and then instead of getting dragged down the trench, they would just get smushed up into the west coast of north america. So there is all kinds of lumps of rock from all over the place, all glued together to make california.

    Things changed a bit, and it’s more sliding along each other then sliding under now. but there are still a mess of faults and volcanoes.

    Ohio is pretty much on the continental shield, so there isn’t too much in the way of tectonics directly affecting it. mostly remnants from the old inland sea (why there are lots of areas to find brachiopods, trilobites, etc.). and some recent stuff from glaciation.

    It’s been a while since I’ve studied anything, and I wasn’t exactly a great student, so I’m sure I’ve got some stuff wrong. please correct whatever you notice. :)

  4. 4

    Plate tectonics is a quite recent theory, besides if it wasn’t for the hills it would probably be all desert and no-one would have settled there.
    Look on the bright side, all that fault line jiggling stops the place being dull…

  5. 6

    Re the difference in complexity between Ohio and California: If you stripped off all the post pre-cambrian sediments in Western Ohio I suspect the same complexity might arise. For example it was always assumed that the Precambrian in western Ohio was igneous, but recent seismic work shows layered returns, and at least one drill hole found sandstone in the Precambrian. There is an interesting feature there called the East Continent Rift Basin that dates from 950 to 1020 million years old. A link to a piece about this feature:

    Note that this covers western Oh, as well as parts of In and Ky.

    Of course eastern Ohio has remnanats of the collisions that occured in the formation of Pangea as well.

  6. 7

    Great post, Karen!
    Since I am a resident of the Evergreen District, I found this especially interesting. But, don’t you think it’s a bit rude to expose our faults to the rest of the world?

  7. 8

    It’s not just accreted terranes, but stuff that’s been subducted and exhumed, and lots of scraping off of the subducting plate, and erosional stuff from those long-ago Sierra Nevada volcanoes… and that’s all just in my little piece of California. Anyone who wants a good introduction to the variability of the geology of California should spring for “California Geology” by Deborah Harden. It’s a bit pricey because it’s a textbook (for non-majors), but it isn’t written like your standard textbook; it has a conversational tone, and all of these asides and such that make for fascinating reading. It’s one of the books that convinced me I needed to go back and study geology. Look for it used.

  8. 10

    Regarding stuff behind paywalls, it’s enormously frustrating. For the last couple of years of working on my thesis (yes, an MS thesis should take a few months, but I had other things going on at the time) if I wanted access to a paper in my school’s library I had to use my advising professor’s computer and search under his name. Didn’t have to do that often, because my advising professor has a tremendous library of his own, and 80% or more of the time I could mention a paper, and he’d go to one of his many filing cabinets and produce it.

    The USGS encourages it’s scientists to publish in Open File Reports, which are freely available on the web. Bless them.

  9. 14

    After a bit more thought, Easten Ohio is a foreland basin for the Appalachian collisions, where all the sediments from the mountains went to re-lithify. Note that there are apparently earthquakes on the Fort Wayne rift since Anna Oh has been known for a while to have such events.

    Putting my wild speculation hat on, I wonder what the link between the East Continent Rift Basin and the Illinois Basin might be, and if similar type of structures exist elsewhere. For the Michigan basin, at least some say the events leading to the Mid Continent Gravity high (continental break up which stopped) are related to the cause of the basin in the paleozoic. (My suspicion in both cases is that the mantle cooled off and the land subsided)

  10. 15

    You don’t need mantle changes for basin subsidence; it does it very well on its own, as sand converts to sandstone and sometimes (with an influx of converting fluids) to graywacke. Basins can subside quite substantially just under their own weight.

  11. 16

    I remember tracking down the age of some rocks in the Santa Cruz Hills for someone who lives there. Those rocks turned out to be Miocene sandstones, about 10 – 12 million years old.

    I also entertained myself with describing where everybody was at the time — where the land masses were, what the flora and fauna were like, etc.

    I think that that’s a good exercise – it’s nice to see what our planet was like at some previous time.

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