Looking at the area toward the top middle of yesterday's exposure, you can see the widely variable colors of the clay here. A characteristic of hydrothermal alteration is that it is very patchy, as it's dependent on the permeability of its host rock, and on channels such as joints and faults. Near the bottom middle of this photo, you can see some essentially unaltered remnants that look like basalt (behind the scotch broom). This was a feature I would point out on field trips, but again, with middle school students, not one I felt worth the time to actually stop for. But on a trip Hollie did with her college-age students, she found a nice pull-out with a good exposure across the road. So this isn't a stop I'd made before, but it's a quick and easy one, and worth while, IMO.