The degree to which the grain size of a rock is uniform is also a really important clue to its sedimentary history. As a rule of thumb, a poorly sorted sediment (one in which the grain sizes vary wildly) has not been moved very far from its source; it's some kind of mass movement, a sudden episode like a landslide or a flood in which a bunch of sediment was grabbed all at once from a source, carried quickly to a new place, and then dropped.

But there are lots of natural processes that can sort out sediment by size, and most of these are controlled by the local speed of water flow. Imagine a mountain stream; it moves fast (and is helped along in moving large cobbles by gravity), so you see a stream bottom made of large rocks and very little sediment. Downstream, where the slope is less and flow slower, the stream bottom can be sandy, or even silty. When large rivers overflow their banks and debouch onto broad, flat floodplains, the water suddenly slows down. It can't carry boulders or even, usually, much sand far from the channel; instead, everything that was flooded is covered with mud, which, if you read the Wentworth diagram, has a grain size under 3.9 micrometers. The same thing happens when a mountain stream exits a valley onto an alluvial plain. All the big stuff gets dumped right next to the mountain, but the finer sediment gets carried farther. That makes a classic progression in alluvial fan deposits, where it's coarser-grained near the valley wall and progressively finer grained as you travel down the widening fan.

One last note about grain size and Curiosity. The MAHLI camera -- that's the hand-lens imager on the end of the arm -- can achieve a maximum pixel resolution of 14 micrometers per pixel. when it's held at its closest possible distance of 2.1 centimeters away from a rock. But it can't really see a grain in a rock if it's only one pixel across; you need three or four pixels to tell one grain from another. Look at the Wentworth scale, and you'll note that if MAHLI can see grains in the rock at its very closest distance, the grains must be at least 50 or 60 microns across. The boundary between silt and sand is at 63 microns. So where MAHLI sees grains, you're probably looking at a sandstone; if MAHLI can't see individual grains, the grains are essentially silt-sized or smaller.

So next time you hear any geologist talking about "sand" or "silt," now you know what we mean!