Lately I’ve been putting together a rock collection for the Chabot Space & Science Center. The last rock I needed was conglomerate, and I slapped my head and said “Duh! Use the Orinda Formation.” So it came to pass that I was on the Gudde Ridge roadcut, east of the Caldecott Tunnel, admiring that distinctive body of rock. But its exposures were off limits due to the construction.
There were two problems. Finding exposures of the conglomerate was difficult. And once I got close to the rock itself . . .
. . . this magnificent stone turned out to be rotten. You may think of conglomerate as a rugged rock, with all that grit and gravel in it. Indeed it can be. But the young, minimally processed conglomerate of the Orinda Formation doesn’t hold up to sun and weather very well. The pebbles work loose and the matrix turns crumbly in a matter of decades. The stuff in that impressive set of stairstep cuts is actually ready to give way, thudding dull under the hammer and totally useless for my purpose.
In connection with the Caldecott Tunnel work, the Fish Ranch Road offramp looks like it may have some fresh exposures. But it’s fenced off.
In the end, I had to find my conglomerate elsewhere. The fresh stone is beautiful, though, and being able to finish the collection made my day.
Oakland Geology 
5 July 2012 at 5:10 pm
Do you know the roadcut between Fish Ranch and the entry to 24 west – just before the tunnel? i think it might be serpentine – or some other bluish rock. I used to pass by there early in the mornings on my way to work. I’d often see a small flock of white doves feeding on some mineral [i think that’s what they were doing] in this cut. Have you ever seen that? or something similar? i wonder what kind of doves they were and what kind of mineral they were finding there…
[That’s the Orinda Formation. –Andrew]
5 July 2012 at 5:44 pm
The white doves belonged to a kid from Orinda. They eventually got picked off by hawks.
Don’t know if the rock is serpentine in that location. The UC Botanical Garden (200 Centennial Drive, between the stadium and LHS) has a fun little serpentine section, though, with plants adapted to tolerate the heavy metals in serpentine soils. It’s in the California section of the garden. You can inspect the serpentine rocks closely without having freeway traffic whizzing by.
6 July 2012 at 1:53 am
Thanks. I’m pretty familiar w/ serpentine and the plants which grow in them, as well as the uc bot serpentine section. i grow many of these at my native plant nursery. that’s fascinating about the doves belonging to a kid from orinda. i always enjoy your blog. Thanks for all of your posts!