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.īut 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. The degree to which the grain size of a rock is uniform is also a really important clue to its sedimentary history. I am sure that settling velocity would be lower on Mars than on Earth.) I didn't take a sedimentology class in college. I would think that it'd be easier to get sediment moving on Mars, but that's just a guess. (Some of you may be asking whether these numbers hold true for Mars, where the gravity is lower. And now you're measuring the size of little sand grains and using them to paint a picture of what kinds of things water was doing on Mars back when this rock first formed. Flow rate enters into estimates of flow volume. I am not enough of a sedimentologist to know the difference between the two estimates, but the point is that grain size serves as a proxy for the speed of the flowing fluid that transported the sediment in the first place. I particularly want to draw your attention to the last two columns, titled "Threshold velocity for traction." It tells you how fast a stream has to be moving to shift grains of that size. Here's an expanded version of the Wentworth (1922) classification scheme that makes some of this a bit more clear. It tells you about how the rock's sediment formed in the first place, and it influences what can have happened to the rock since it formed. The answer is that grain size is an extremely important clue to the history of a rock. So what we're looking at here is a "slightly gravelly" something according to Folk's classification scheme, but it's hard to tell on this weathered and dusty surface whether it's a slightly gravelly sandstone, or a slightly gravelly silty sandstone, or a slightly gravelly sandy siltstone, or whatever.įine, you might be saying, but what's the point of all this hair-splitting? It's a good question. Most of them are under 2 millimeters in diameter, but a very few are more than 2 millimeters across, which puts them above "sand" and in "gravel" territory. Here's a recent image from MAHLI that clearly contains quite a few fairly coarse grains. I'd love to see more examples.)Īnyway, back to Mars. I am sure there are more very silly or funny ternary diagrams among some geo prof's slides somewhere. (Aside: I looked really hard yesterday to find funny ternary diagrams, and that's the best one I could find, thanks to a tip from Patrick " Poikiloblastic" Donohue. If you're not quite sure you get how a ternary diagram works, here's an amusing exploration of the phase equilibria of pie crust that employs a ternary diagram. A pure sand would plot at the "sand" corner, but if there's some clay in your sand but not very much silt, it would plot in the "clayey sand" region. In a ternary diagram, three endmembers are represented at the corners of a triangle, and you represent the proportion of those three endmembers in something by plotting a point somewhere inside the triangle. So you can have gypsum sand like at White Sands National Monument, or olivine sand on certain Hawaiian beaches, or any other composition where mechanical and chemical weathering bust rock of any composition into sand-sized particles.īoth the Shepard and Folk classification systems involve a graphical tool beloved by geologists: a ternary diagram. But the term "sand" does not specify composition, it only specifies grain size. When we think about sand, we usually picture quartz sand, because that's what most sand is made of on Earth. Gravel is anything larger than that silt and mud are finer. Wentworth defined "sand" in a publication in 1922, along with "gravel," "silt," and "mud." Sand-sized particles range in size from 63 microns to 2 millimeters. Just as with "rock," for geologists, "sand" has a definition that is far more precise than the way that the word is commonly used in conversation. How can the word "sand" possibly be confusing? Well, the situation is a lot like the one with the word "rock," which I talked about in this blog entry. I had one of those "A-ha" moments last week where I suddenly realized that I had run afoul of a common problem in science communication: when the words I'm using mean something different to me than they do to almost everyone I'm talking to.