Comments on: Rocks of Leona Quarry Con’d.

By: pete veilleux

pete veilleux — Tue, 13 Jul 2010 20:13:34 +0000

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.

By: Andrew

Andrew — Mon, 09 Nov 2009 18:48:03 +0000

That shows how parochial I am. I haven’t gotten around to visiting the Briones Formation. Looks like it will be fun.

By: P J Evans

P J Evans — Mon, 09 Nov 2009 05:08:15 +0000

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.

By: Jacob Lee Fink

Jacob Lee Fink — Mon, 09 Nov 2009 04:31:21 +0000

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.

By: Andrew

Andrew — Mon, 09 Nov 2009 01:55:07 +0000

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.

By: Jef Poskanzer

Jef Poskanzer — Sun, 08 Nov 2009 20:04:26 +0000

Cool. Las Trampas has a bunch of these, are there any quarries near there?