Misunderstanding of grainsize

greenspun.com : LUSENET : Introductory Geology, Oswego State : One Thread

While I was studying for the exam I came across what seems to be an error. The topic is on weathering in chapter six. My notes state that fast moving water and wind selectively pick up material while leaving behind the larger material and therefore the grainsize is small and well-sorted.

My notes also state that river channel deposites are frequently exposed to sand bars and due to a high flow and velocity in the channel, the deposites are coarse grainned.

These two seem to contradict themselves. Could I have misunderstood the lecture on this topic or am I misunderstanding this information?

-- Carla Scott (smileycar@yahoo.com), April 02, 2001


The grainsize of a deposit reflects the energy of the flow that DEPOSITED the sediment (not materials left behind after other materials are taken away). In the case of wind-blown material, the wind can't pick up gravels. The only way to form a dune is to have a lot of SAND exposed at earth's surface that the wind can blow around and sculpt into dunes. Finer material (silt and clay, or "dust" in layman's terms) can be picked up by wind, but tends to be transported larger distances, whereas sand isn't transported as far. So sand builds up in some places and silt in other places. But gravel is never transported, so it's not part of aeolian deposits.

Rivers can have strong currents during flood periods and transport larger particles like gravel. The water still sorts the sediments out by size, as can be seen in many rivers where gravels are found only in the deepest parts of channels, and finer sediments are found in shallower areas.

-- Sharon Gabel (gabel@oswego.edu), April 03, 2001.

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