A new set of images from the ground of Mars has been a sensation: they show an outcrop of gravelly mudstone, that is, conglomerate. This is really not a surprise; it's what we meant to find when Curiosity's itinerary was planned.
News stories have called this image "definitive proof." It is not definitive proof. But I'm still OK with the scientists' conclusion that it represents streambed deposits, much like those we can find in any desert wash.
Let's look at the picture like geologists do in the field. We see rock beds, with the look of consistent layers, that are tilted out of the ground. We see that the rock is full of clastsfragments of older rockswith a wide range of sizes. They are not jagged pieces, like the crushed stone used in roadbeds, but are rounded. To be more precise, they are subrounded rather than the fully rounded "river rock" sold in landscapers' yards. On Earth, there are three basic settings that can create such a rock: streambeds (conglomerates), underwater landslides (turbidites) and volcanoes (pyroclastic deposits). How to choose the most likely of these?
To me, the most telling detail is in the gravel lying below the outcrop, which I will assume is stuff that has weathered out of the rock. It's very nice gravel, by which I mean that it has a pretty consistent grain size. It is sorted. This is where I rule out pyroclastic flows, which don't sort out clasts by size to this extent. Pyroclastic flows carry everything with them regardless of their density.
The choice between streambed and turbidite is less clear at the outcrop. We don't see any bedding features or sedimentary structures beyond the crude layering. The Curiosity science team looked to the regional setting and the evidence from other outcrops to conclude that this represents an ancient river channel where water (salt or fresh? we can't yet say) once ran around knee deep. I think it's time to accept a working consensus that a genuine river was here and not something else.