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.