All My Children: Making and Donating a Rock Collection

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The search for a good slate specimen took me across the Central Valley, but once there it was a moment's work. Photos by Andrew Alden

The search for a good specimen of slate took me across the Central Valley, but once there it was a moment's work. Photos by Andrew Alden

A few months ago, I accepted an assignment to gather a set of California rocks for the Chabot Space & Science Center in Oakland. I thought I could putter around the local hills and have it done in a few afternoons. Didn't turn out that way. Finding good teaching specimens is an exacting project. Now that I'm done, it's a little hard to let them go.

I was given a list of 15 rock types—five each of igneous, sedimentary and metamorphic rocks—but some of them were clearly nonstarters. For instance, coal was on the list, but California's coal is too soft for presentation, and besides it's off limits in park lands. I made some substitutes and then set forth to find the following:

Igneous rocks: basalt, granite, obsidian, pumice, scoria
Sedimentary rocks: breccia, chert, conglomerate, sandstone, shale
Metamorphic rocks: gneiss, marble, schist, serpentine, slate

Almost everything was available in loose material by the side of the road. Indeed, picking up roadside rubble is doing the public a favor. Conglomerate and sandstone were available within the Oakland city limits; in fact Chabot authorized me to collect from its own property, but the sandstone there is too soft. Chert came from my own Oakland yard, the same red Franciscan chert that makes up the Marin Headlands. I was using it in the landscaping, but giving up a few choice pieces was all for a good cause.


With other rocks, I knew where to get them; it was just a matter of scheduling a trip. Obsidian came from the Napa Valley, shale from the Great Valley Sequence north of Fairfield, schist from the Franciscan Complex in Glenn County.

A teaching specimen needs to be clean and sound, naturally. It needs to be the right size. More difficult is that it has to be typical, showing what the textbooks describe and nothing more. Once I found the right spot, it took a lot of pawing, picking and pondering to assemble a satisfactory group, like this basalt.

Toward the end, I had to go pretty far afield. Take the metamorphic set, shown below. The schist (upper left) I already mentioned. Going clockwise, the slate came from near Copperopolis, the marble from near Columbia, the gneiss from near Murphys, and the serpentine from near Clear Lake.

That left two igneous rocks: scoria and pumice. I could have made an overnight visit to the volcanic districts of northeastern California or the trans-Sierra, but I elected instead to check out the local landscape suppliers. I finally found what I wanted at Morgan's Masonry Supply, in San Ramon: fist-size lava rock (scoria) and a boulder of pumice. From that boulder I carved five hand samples like this one.

Now I have added respect for the people who make and market those boxed rock collections. They have to keep truckloads of rock types on hand, most of them imported from somewhere else, then pick through the chips and waste to get what they need. On the other hand, they don't know the pride I take in delivering this custom collection to my local science educators. And I can visit my rocks any time to see how they're doing.