Along the entryway to the Leona Quarry development is a tumble of big decorative boulders, ready for deployment in the landscaping. When I drove by I made a mental note to check them out. I assumed they were chunks from the quarry itself. No such luck; they’re exotic blocks full of shell fossils, which are unheard of in Oakland. This is a view of the shell faces.
This boulder is almost what geologists call a shell hash or coquina, but the shells are mostly intact. I might call it a shell marl instead, but the default name would be fossiliferous limestone. I have no idea where it’s from—somewhere over the hills, or maybe in the Santa Cruz Mountains, but not Oakland.
Here’s a closeup of another boulder showing a cross section of the shells.
This boulder is closer to a shell hash; it’s mixed with sedimentary rock clasts. A few of the darker clasts have released iron, presumably from sulfide minerals. This formed in a very active coastal setting.
It’s a fascinating set of boulders, but an arbitrary one. It also includes big hunks of shale that have disintegrated in the sun and rain here. Who picked them, and why?
8 November 2009 at 1:04 pm
Cool. Las Trampas has a bunch of these, are there any quarries near there?
8 November 2009 at 6:55 pm
Las Trampas–that would be over the hills. It’s been on my list of places to visit for years.
But these rocks aren’t the kind you would get from a landscaping company, I don’t think. They struck me as the kind of rocks a big homebuilding contractor might have lying around from another project, like one in Tracy or, as you suggest, in the San Ramon Valley.
8 November 2009 at 9:31 pm
Those rocks with the shells in the gray sandstone/siltstone are locally known as Briones Formation. Indeed shallow marine environment. They can be seen all around peaks about the East Bay. I have seen similar rocks at Mission Peak in Fremont and around the Norris Canyon area between San Ramon and Castro Valley.
8 November 2009 at 10:08 pm
I found a small rock like that along Arroyo del Valle, south of Livermore, in the 1960s. (Don’t know where it is now; I’ve moved enough times that it probably got left behind somewhere.)
It had some recognizable turret shell bits, including one lengthwise cross-section, but was mostly just shell hash.
9 November 2009 at 11:48 am
That shows how parochial I am. I haven’t gotten around to visiting the Briones Formation. Looks like it will be fun.
13 July 2010 at 1:13 pm
Briones formation deposits. i find them regularly around the east bay. there are lots of single boulders of it in the hills above fremont and around sunol. some of them are very nice boulders. i wonder what the age is of the briones formation…??
i have some really interesting fossils from the purissima formation near santa cruz showing the ubiquitous transgender shell – Crepidula princeps. they show this extinct slipper shell growing in a spiral – one on top of the other. apparently, the bottom/oldest one is the female and the rest are male and when she dies, the next one became the female. i have one complete spiral.