Once upon a time I agreed to collect five matched sets of rocks for my local science center, to be used for teaching kids a bit of geology. I had a few extra pieces when the project was done. One leftover specimen of slate served me as a makeshift coaster on my desk, but it always seemed kind of forlorn. After a while I felt sorry for the rock. It seemed like it was reproaching me, and I decided to put it back the next chance I got.

Why should someone care so much about a particular stone? Well, I cared because I'd spent a lot of effort to collect the rock specimens, and I'd wastefully gathered one too many. I didn't pick it to use as a coaster. I already have stone coasters—doesn't everybody?

But that was only part of the story, and there were two other things. First is that my home has a limit as a repository of stones. I've tried to hold the line against bringing in more without a clear purpose for each new rock. The extra slate specimen simply didn't belong here. The second thing, which I found the most compelling, is that it would be an excuse for a road trip to revisit the place where I got the stone.

The slate came from a locality two hours' drive away. It was good teaching material, with strong slaty cleavage and a bit of color. It looked like a regular rock rather than commercial slate, but you could see its potential. I found the spot by consulting the geologic map, then keeping my eyes peeled as I drove through the area.

And there at the intersection of Horseshoe Road and Route 4, in the Sierra Nevada foothills, was an old borrow pit. I got out of the car and sorted through the clattering chunks of slate, picking five hand specimens and then one more for good measure (which was my mistake).

Borrow pits are quite common, especially in hilly country. Road builders use large amounts of crushed stone as road metal, and unless there's something chemically wrong with the rock, every roadcut creates free stone to use for building up the roadbed somewhere else. After the road is finished, the borrow pits are still there, looking like extra-large roadcuts. The road designers may turn borrow pits into something more useful like turnouts or scenic overlooks, or they may just leave them as wide spots along the road suitable for geologists. That was the case with this exposure of slate. I enjoyed seeing the structure of the bedrock on the sides of the pit as I picked out my specimens.

Exactly two years and one day later, I had occasion to drive this way again. I made sure to bring along the stray stone, and when I arrived it was as if no time had passed. After a few moments standing there, kicking the stones (and NOT picking any up!), I put back the piece of slate and moved along.

That's my kind of rest stop. And I have a few more rocks that I should give back. One of them needs to be returned to Hawaii, just to get rid of any bad luck it might be causing me.