Along the south shore of Chabot Reservoir, in the mudstone of the Joaquin Miller Formation, I spotted this exposure of tiny, enigmatic mineral crystals last year.
They’re sprinkled across what looks like a bedding surface in the mudstone. This shot shows an area just a couple inches across. Because I was just taking a walk, I didn’t have my hand lens with me and I didn’t try to bring some home, so this is all we have to work with until I, or someone else, finds the spot again.
A medical maxim that’s just as useful in geology is, “where you see hoofprints, look for horses and not zebras.”
Most of the time, large grains in a mudstone are quartz. The crystals are elongated and appear to be prisms with points, just like quartz. But the honey color appears to be part of the mineral and is most unusual for quartz. Any quartz in this rock would be detrital, not authigenic—by which I only mean that quartz would not grow here, but would come in with the other sediment that turned into this rock. And if it were detrital, it wouldn’t have this clean sparry shape. Instead it would be ordinary sand grains. So I’ve ruled out quartz.
My working hypothesis is that these are calcite crystals in the typical “dogtooth spar” shape that have grown here, or very near here. I could confirm that with a quick acid test, and one little tiny piece of a small puzzle would be clarified.
Oakland Geology 
24 April 2011 at 11:05 pm
Andrew:
I seem to remember from past posts that you’re against taking samples, but I think prying ONE broken shard out & doing that acid test is indicated, in the name of science!
12 February 2011 at 11:53 am
JH, 511.org shows a bus route almost all the way to the lake from the Hayward BART station. You access the lake from Lake Chabot Regional Park in Castro Valley. I agree that Oakland/San Leandro’s end of the lake is harder to access, but 511.org shows a route from San Leandro BART. You have to specify Lake Chabot Park, San Leandro.
Thanks for showing your fossil. I’m not sure, but I think most of the rocks around Pinole are marine, which points to it being a shell. A tree seed would be pretty unlikely in a sandstone, and the veining pattern in the fossil is very regular. So much depends on the missing piece!
11 February 2011 at 11:47 pm
I looked into finding a way to get there to check these out for myself. But I am disabled (can’t drive but can walk and hike and bike fine) and to my dismay have found that although there are many great regional parks in the east bay, there’s essentially no public transportation to get to them.
Which means that a 6 mile hike within a park might require another 6 miles round trip added to the hike to get TO the park entrance.
I did find this strange unidentified fossil in Pinole though:
