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Stevens Canyon Road in the Santa Cruz Mountains (near Cupertino, California) passes right next to a large rock formation that, to my untrained eye, reminds me of the inside of a limestone cave. Here's some photos I snapped from when I was there:

Top of the formation

The full formation

Top of the rock formation

Toward the base

This is right off the road; you can see other pictures here on Google Maps.

The rock surface usually has water dripping over it, even in the summer when nearby creeks are otherwise dry, and it's covered in rippled ridges that remind me of the inside of the few limestone caves I've visited. This spot is right at a bend in the road and has no markings indicating what it is.

My current guess - based on absolutely no expert knowledge at all - is that there's a spring at the top of the rock that's slowly dripping over the surface and slowly depositing calcium carbonate when it lands. The mountains here have a bunch of old limestone kilns (and an active cement plant, and at least one active quarry), so it seems at least somewhat plausible that rainwater is dissolving the minerals and depositing them here.

Any guesses as to what, geologically, is going on here? Is there a name for this sort of formation?

Update: Based on the comments that this looks like travertine, I climbed up to the top of the formation. I found a small cave carved into a rock just above it that had a steady drip of water coming down. The water was cold and clearly had a bunch of red minerals dissolved in it. (Sorry about the quality - it was getting dark)

A cave!

There was a clear path the water from there was taking that produced the drip over the surface:

The path the water takes

And finally, I noticed some largish rocks pushed off to the side of the road. They look like this:

Rocks near the side

I noticed that the rocks near the water path up top look a lot like these, and these look a lot like random pictures of travertine that I've been Googling for.

So based on this, I'm going to make the educated guess that I'm looking at travertine that's deposited on the surface of the rock. I don't think there are any nearby hot springs, but maybe this is simply water flowing through the limestone in the rock above, dissolving the minerals, and slowly depositing them on the surface?

I see that questions of the form "identify this rock" are off-topic here, and I'm not sure if this counts. If so, let me know where else I should ask this.

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    $\begingroup$ reminds me of Thermopolis in Wyoming: 1, 2, 3 $\endgroup$
    – uhoh
    May 27, 2021 at 1:00
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    $\begingroup$ Looks like travertine? $\endgroup$
    – Gimelist
    May 27, 2021 at 2:20
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    $\begingroup$ the water does not have to be hot when it reaches the surface, that just makes the deposition faster. $\endgroup$
    – John
    May 28, 2021 at 5:01

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Well given that travertine springs trail is right next door, in castle rocks state park I am going to guess travertine. Which is a calcium carbonate mineral deposited from geothermal springs, hot ground water brings up the dissolved travertine which is deposited as the hot water evaporates, but can also slowly be dissolved and redeposited by rain. Thy can form very quickly, on the span of years or decades. There are many travertine geothermal springs in the area. If you want more information try this link for a start.

https://en.wikipedia.org/wiki/Travertine_Hot_Springs

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  • $\begingroup$ Travertine is certainly looking like a winner here. FYI: Travertine Hot Springs is in the Sierra Nevada range, which is on the other side of the Central Valley. Though I didn't know that Castle Rock State Park had that trail, and I'm definitely going to need to check it out! $\endgroup$
    – templatetypedef
    May 28, 2021 at 3:47
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    $\begingroup$ travertine can be deposited from cold springs due to degassing of CO2. You just won't have as much calcium carbonate in solution as with a hot spring. $\endgroup$
    – haresfur
    May 30, 2021 at 23:55

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