The shot above, made on Sunday out the window of a plane on approach to Las Vegas, comes three and a half years after this shot, which I took from the ground at Hoover Dam. Here’s a whole set of the fly-by. Not much of the dam shows. The Colorado River gorge is easier to see.

Two things stand out for me in this scene. One is the remarkable engineering involved in building the Mike O’Callaghan-Pat Tillman Memorial Bridge, better known as the Hoover Dam Bypass. The other is that, from altitude — far more than from the ground — you can see the volcanic nature and origin of the rock supporting both the bridge and hte dam. I’ve been looking around for source docs online that detail the provenance of this rock, which needs to be of a competence sufficient to anchor one of the world’s biggest dams, while also supporting a bridge over a gorge. As I recall from the visit, it’s rhyolite. But, not sure. Looks like it. Maybe Arizona Geology can fill us in.