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This breaking down and winnowing of material as rocks are weathered and transported is an important sedimentary process, and introduces two important concepts for geologists – sorting and roundness.
As sedimentary materials are transported and deposited, usually by water and wind in the case of sands, there’s a tendency for the particles to be sorted into different sizes. This degree of sorting depends mainly on how particles of different size and density settle through the current carrying them along (whether wind or water). The more uniform the conditions in the environment of deposition, the better sorted the sediment will be.
This is due to two important factors – water and wind only have the power to transport material up to a certain mass. What’s left behind is the material which is right on the limit of what the fluid can carry – any lighter and it will be carried away, and any heavier and it won’t be there in the first place.
In sedimentary systems, the grain size of the material left behind can tell us how much energy was in the fluid at the time of deposition, and how well sorted the material is tells us whether the deposition happened quickly, or slowly.
