Last weekend I was at the coast and found these interesting landforms. I took a few snaps and tried to look them up when I got home. I didn’t have much luck (not really my field of expertise) so I thought I’d ask here.
This is not homework or anything that will help me in any way, I just thought they were cool and I’d like to know more.
This was the beach near Hearst Castle. Specifically, N 35° 38.114 W 121° 11.735
Pending more informed replies, I’d say those are beds of sedimentary rocks (perhaps originally limestones) which were heavily distorted by pressure and perhaps heat, so now the bedding planes are nearly turned ninety degrees, and rather curvy in places. The rocks themselves may have been somewhat metamorphosed in the process (transformed into, say, shales.)
I believe that part of coastal CA is one of several “micro-terranes”: small “micro plates” (like, say, one of the major islands of Japan) which collided and fused with the North American Plate perhaps fifty million years ago. That would explain the distorting.
Thanks JKM. One of the things that caught my attention was this from Wiki:
“The name San Simeon also refers to some geologic structures of the area, particularly elements of the coastal Jurassic Age landforms and ophiolite mineral formations.”
I was wondering if this was what I was seeing.
This site : http://hearstcastle.org/wp-content/uploads/HearstSanSimeonStatePark-Brochure.pdf has the geology of this place.
It looks similar to other “wash board” shorelines that feature steeply dipping or overturned sedimentary beds, or “sheeted” intrusive igneous rock.
I know that ophiolites generally indicate deep mantle upwellings, usually along the edges of oceanic crust (in this case, later brought to the surface by tectonic activity) … apparently a pretty rare glimpse into the deeper sources of rocks.
That’s not too specific, but it does seem to be part of that mixed-up melange of older marine terrace deposits.
However, those don’t look like ophiolites to me – they’re usually darker and more igneous/metamorphic in appearance, rather like gneiss but with more dark green in them, with a texture reminiscent of pillow basalt. There must be some nearby, but not right where you took those photos.
The location you’ve given is San Simeon Point, and per my google fu skills, this map reveals the most likely name of the rock unit is the Monterey Formation.
(FWIW, I’ve hiked along the coast further to the south and have seen the same formations along the coastline down as far to just north of Pismo Beach.)
The partial origins of this rock unit are the zillions of diatoms laid down over millions of years in deep ocean water (+3000 ft) during the Miocene times.
Add a smattering of plate tectonics plus time and the result is some pretty awesome pictures. The whole area around San Simeon and to the south is essentially a long-dead subduction zone that has been uplifted and eroded for your pleasure.
Limestone and shale are sedimentary rocks of different composition. Limestone does not metamorphose into shale. Also, neither heat nor great pressure (at the point where it happens, anyway) are necessary to turn or curve the bedding plane. Strata, including surface strata, can get turned or distorted by volcanic activity and earthquakes generated far away from where they actually are.
[Hijack]What ever happed to GeoBabe? She’d know this.[\hijack]
^
Maybe she’s in the field.
The question’s already been answered (basically, these are folded sedimentary rocks) Without chemical tests and a scale in the photo, it’s hard to say whether those particular rocks are sandstone, shale or their limestone equivalents.
njtt, while you are right that strata can be distorted by other activities, in general, the kind of pervasive folding that generatesa fold beltis the result of regional orogenic shear forces. “Pressure”, while not strictly accurate, isn’t a bad description. But it’s important to realise it’s not the overpressure of the rock column that does it, it’s the tectonic forces.
Here’s a sharper image of Monteray Formation
http://0.tqn.com/d/geology/1/0/V/R/1/montanadeoro1.jpg
Text at http://geology.about.com/od/geology_ca/ig/CAcoast/montanadeoro1.htm
The small green patch (near the bottom, center) in photo 2 is about a foot long if that helps.
Given that the San Simeon Point rocks are of the Franciscan Assemblage (not the Monterey Formation as someone else said), I’m going to say something siliceous, with interbedding of chert, shale and sandstone to give you that washboard effect. Metamorphosed a little, but not too much. Not an ophiolite or serpentinite, and doesn’t look like a limestone.
The rocks in the Monterey Formation are highly siliceous.
Yeah, I know, but the rocks in question are not Monterey, they’re Franciscan. I know this because I can read a geological map. All the actual beach deposits at the Point are keyed as KJf - that’s Franciscan sediments.
I wasn’t saying they’re Franciscan because they’re siliceous, I was saying they’re likely that particular combo of siliceous because they’re Franciscan and they don’t look like the other characteristic Franciscan rocks (Ophiolites aren’t sedimentary and are keyed KJfv anyway, serpentinites look nothing like that and would be KJfv too, limestone would be KJf but (especially in a well-watered area) would be all pitted even if interbedded like that, which I’ve never seen anywhere else, etc)
I concur with every word you said.
You thinking of the Farallon Plate? Practically the whole thing got eaten by North America and burped up in the form of volcanism and orogenies all over the West. Rockies, Cascades, Yellowstone, etc. The leading edge of Farallon is thought to have gotten subducted so far down it wound up on the bottom of the mantle.
No, rather with any of several small pieces of continental crust – pretty much every “bump” along the Pacific margin, e.g. Point Reyes, most of the San Francisco peninsula, Palos Verdes, etc., is an “exotic terrane” which rode a Pacific plate and fused to the North American Plate while the oceanic crust around it subducted. Whether you wish to include these bits as part if the North American plate, or else reserve that term for the stable interior craton, is a matter of semantics.
