Geology/Size of river rock formula
I was told by a non-geologist that there is a formula for determining the distance from the source of a rock in a river by measuring its size. Is that true? If so, can you tell me what the formula is and explain the variables? Thank you
There are a number or formualas. The first that came to mind was the Rouse equation, which only defines HOW a sediment will be transported, not how far.
It uses a combination of data:
The settling velocity (the speed at which the sediment size fraction settles out of solution. We have techniques to measure that in the lab.
Von Karman Constant which deals with fluid flow dynamics
and shear velocity of the transporting fluid. This is calculated from the velocity of the transporting fluid.
The resulting number tells us if the sediment will be in the wash load (never deposited), bed load, or suspended load. There is another category in between the two that is 50-50 suspended or bed, probably subject to saltation, or bouncing along the bottom.
I checked and yes there is a equation, a few of them, that relate the distance a sediment is transported based on its roundness. These are theoretical predictions based on the fluid dynamics of a particle, not an absolute measurement that would allow you to look a a grain and say, that it was transported X number of miles.
These equations are discussed here:
Look at the material starting on page 22 - 28.
The only way to really tell how far a particle has traveled from it original source is to do a provenance determination. This is done by comparing particle inclusions or isotopes in the sediment and the suspected source rock. For example, matching the quartz grains on a beach in Virginia and matching it to the quartz in a granite in Canada. This was done by a professor of mine in college at Old Dominion in Norfolk. The sand in question was transpored south during the Pleistocene Glaciation.