You can see a lot of calcium sulfate veins in the lower, Murray formation rock, which are truncated by the overlying Stimson rock. That alone isn't enough to be sure that Murray and Stimson formed at very separate times; it could just be that the Murray formation has different properties to the Stimson that allow veins to propagate easier in it than in the Stimson. But there's something peculiar about the Stimson unit where it contacts the Murray; it's full of little rock fragments, whereas higher in the section it's a much more uniform fine-grained sandstone. (Enlarge the MAHLI photo above to spot those grains.) The newly capable ChemCam shot at the Stimson unit right above the contact, profiling the composition across matrix and grains. And when ChemCam struck a particularly bright grain, one team member told me, "Bam! Calcium sulfate."

What does it mean for tiny little grains of calcium sulfate to be incorporated into the base of the Stimson? It means that before the sands of the Stimson were laid down, the Murray formation had already been buried and turned into rock. The Murray rock had already been shot through with calcium sulfate veins. Then it had already been unburied and exposed at the surface, where the calcium sulfate veins had eroded away into calcium sulfate pebbles. When the Stimson sands blew through, the bottom-most sand layers incorporated broken up bits of Murray. In other words, a whole lot of time passed between the Murray and the Stimson. Millions of years. Maybe tens or hundreds of millions.

Kevin Lewis presented a poster at AGU that argued for the Stimson being the youngest rock that Curiosity has encountered. As Curiosity has driven along, it has found the elevation of the base of the Stimson to march up the mountain; the base of the Stimson unit tilts about 4 degrees to the northwest. It's not an originally flat-lying rock that has been tilted, he argued; rather, the topography of a central mountain in Gale crater was already present when the Stimson sands were draped across its northern flanks. They looked at the direction of the crossbeds, and determined that the flow of wind or water that laid down the Stimson sands was moving from west-southwest to east-northeast -- which is to say, basically across the slope. That, in turn, means that the fluid that transported the Stimson sands was almost certainly not water (at least, not most of the time); they're windblown sands, much like the modern Bagnold dunes. Except they don't contain olivine. And they're blowing in a different direction. And nobody knows what causes the topographic expression of east-west trending ridges in the Stimson. So perhaps they're not at all like the Bagnold dunes! As Curiosity keeps driving, Lewis told me, it will be able to investigate isolated outcrops of Stimson that the rover can drive around and view from all sides, which should help the team understand its depositional environment a little better.

It's a pleasure to see the rover using all its instruments together, as intended, on the rocks that the rover was sent to Mars to study. Getting ChemCam back to full function, with the new-and-improved calibration, has been particularly helpful. It took a long time to get going, but the science mission is really underway now.

Curiosity is now exploring the modern sands of Bagnold dunes, and will continue to do that for several weeks. I'll post a writeup of that campaign after the holidays.