Murieta Rock, El Cerrito

In Gold Rush days, the Bay area was as wild as the rest of California: depopulated of Indigenous people and a free-for-all of frontier characters. One of those characters was the legendary outlaw Joaquin Murieta. His story, at least the version we have today, had all the makings of legend — a handsome, peaceable Mexican, viciously victimized along with his wife and family at the hands of Americans, who turned desperado and came to a bad end. As befits a good legend, every crime in California was added to his name — and this fine outcrop too in the hills of northern El Cerrito.

The rock stands out in early photos of El Cerrito, back when the hills were still bare, but today it’s unobtrusive in surroundings of trees and homes at the intersection of Cutting and Arlington Boulevards. It’s also smaller than it used to be; a rectangular quarry pit has been carved into its southwestern side.

Supposedly Murieta’s gang would watch the main road from up here and swoop down on victims. Or this would be their lookout when they hid out in Wildcat Canyon. That may have been. I think the name stuck because it looks like a broken-down haunted house made of a rare, unearthly-looking blueschist.

The area is geologically interesting. The rock is just south of Cutting, below the large “L” at the center (part of the name of the old San Pablo rancho).

All the bluish rocks are Franciscan, the orange (Tor) is the much younger Orinda Formation, and between them is the Hayward fault zone. “KJfy” is a metamorphosed sandstone and “spm” is the melange. Regular readers may recognize “Jsv” as the Leona volcanics, but this little pod is actually the northernmost occurrence of Northbrae rhyolite, the stuff of Berkeley’s rock parks. (Thanks to Karl in the comments for flagging my oversight.)

Murieta Rock is a high-grade block in a melange of serpentinite — a rare outcrop within a rare setting — and for background I refer you to this post from the last time I was up this way. Notice the large areas of the map labeled “Qls”; these are gigantic, slow landslides all of which originate in that melange. More of them are in north Berkeley (see my 2017 walking guide to the area, from the Berkeley Path Wanderers Association site).

Enough of that. You can reach Murieta Rock on the 7 bus line, from either the El Cerrito del Norte or Downtown Berkeley BART stations, or drive there yourself of course, but I enjoyed walking there through Canyon Trail Park — if I were Murieta, I’d swoop that way to carry out my robberies. The view from the top of the rock over San Pablo Bay is superb.

And since this is Valentines Day, why not consider the rock for a romantic geo-outing?

5 Responses to “Murieta Rock, El Cerrito”

  1. mpetrof Says:

    What is the light blue between the serpentine melange (spr?) and the Orinda (Tor)? The whole ridge and back in to Wildcat R.P. is rife with high grade blueschist. I gather that much of the KJfy is blueschist fascies (just to confuse matters via confusing terminology). Or is the lower grade blueschist fascies of the quarry to the south also in the light blue?

  2. Anna Korn Says:

    I enjoyed this post about the Murieta Rock outcrop. I am curious about other East Bay outcrops, such as Albany Hill– Is it a knocker that floated up out of the melange? If you have posted about it earlier, is there a way to access earlier posts?

  3. Karl Hans Says:

    A response and a comment based on Wakabyashi’s 2015 International Geology Review article (V57 No 5-8) “Wither the Megathrust: Localization of a large scale subduction slip along contact of a melange:

    Wakabayashi identifies the light blue as Angel Island Nappe (blueschist facies graywacke). Above this nappe and extending to the Hayward Fault, the Tiburon melange contains high grade blueschist and serpentine rocks readily seen along the Arlington.

    Regarding the rhyolite, it is apparently younger Northbrae rather than older Leona, based on lin Murphy’s evaluation. Wakabayashi writes:
    “Rhyolitic volcanic rocks unconformably overlie the Franciscan rocks. These have been mismapped in the past as Jurrassic rocks of the Coast Range Ophiolote but detailed field studies and geochemical and geochronological analysis have shown similar rocks to be Miocene postdating Franciscan subduction.”

    He maps them as “Miocene Northbrae Rhyolite”.

    The exact origin of the Northbrae rhyolite (indian rock and others) has not yet been determined as I inderstand, and there there might be some association with Quien Sabe formation to the south.

    Regarding Albany Hill, it is mapped as part of the Novato Quarry Terrane, extending north west through Point Richmond, Red Rock, China Camp and into Novato. So not what would be considered a “knocker”, but what makes it so prominent and subject to less erosive trimming has not been explained to me.

  4. Andrew Alden Says:

    Thanks, Karl. I should’ve checked Wakabayashi’s mapping and gotten the rhyolite right. (The paper has a coauthor, Christie Rowe of McGill University.) The geologic map I use (see its page on this site) was published in 2000, before the correct identity of the Northbrae rhyolite was widely known. I haven’t seen those particular exposures in person yet. The nomenclature of the Franciscan is a mess and not worth the attention of most readers. Albany Hill is simply one lump of the dismembered Novato Quarry terrane. “Knockers” are large blocks in the melange that protrude from the ground; I’ve retired this old field term because some men think it’s funny and have embarrassed their women peers.

  5. Mitzy Valdes Says:

    great informative stuff

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