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?
Oakland Geology 
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.