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River benches?


wjbell

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I think I've asked this here before but I keep hearing people talking about river benches high above the water line. That would seem to me that the river has eroded it's way down over time leaving benches showing where it use to run. I got a picture from google maps looking downstream at Mineral Bar. Just below the blue line seems to be an obvious place where the water has run at one time. I don't know if that's an old water line or one from modern times at flood stages. Just below the yellow line looks like another transition. Parts of it look like a bench but it also looks like the degree of the hillside changes right there.

So my question is, did the river ever run between the blue and yellow lines? Is it worth it to even explore that area that high up the hillside?

river-benches.png

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Go up to the areas you have indicated and dig a little see if you find stratified ground, in other words as you chisel an area with your shovel do you see layers of gravel,sand, rocks, Rounded gravel , or rocks indicating past movement has smoothed their edges.

Heck if it was me I'd just go ahead and detect it and also take some down to either drywash or wetwash. And see whats there you may be the first to try that particular area and find nuggets or fine gold.

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Go up to the areas you have indicated and dig a little see if you find stratified ground, in other words as you chisel an area with your shovel do you see layers of gravel,sand, rocks, Rounded gravel , or rocks indicating past movement has smoothed their edges.

Heck if it was me I'd just go ahead and detect it and also take some down to either drywash or wetwash. And see whats there you may be the first to try that particular area and find nuggets or fine gold.

OK, thanks frank. Yeah, stratified ground would let you know for sure.

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You might want to pick-up Terry Maley's "Field Geology Illustrated" or Chris Ralph's "Fists Full of Gold", as both discribe river bench geology. They look at where, when, and why of those placers... with pictures. Good Luck.

Keep Smiling... Fritz :cigar:

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You might want to pick-up Terry Maley's "Field Geology Illustrated" or Chris Ralph's "Fists Full of Gold", as both discribe river bench geology. They look at where, when, and why of those placers... with pictures. Good Luck.

Keep Smiling... Fritz :cigar:

Thanks Fritz... I'll probably pick up the second one.

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Old benches are not the only source of "river worn" cobbles. The ocean also wears down rocks and concentrates them. Many parts of the western US were covered by oceans at one time or another. Usually those smooth rock concentrations also have evidence of past sea life [clam shells, shark teeth, coral bits, etc.]. In regard to true river benches, there are two important forces in addition to water runoff and other hydraulic forces to consider when trying to ascertain where a river channel once ran. Earthquakes heave up, tilt, squeeze and rip apart surface features, sometimes changing the course of a river and sometimes forming a new channel for a given watershed. Consider the monster Richter Scale 10 quake down in Chile in the 1950s. That event is thought to have heaved up a solid chunk of land mass the size of the entire state of California 50 feet! Volcanics also have a lot to do with old benches. Lava, pyroclastic flows, cinders and ash can sometimes blanket an entire existing river system, thus freezing the benches pretty much in place for eons. Later, other forces can expose portions of these ancient river beds. Volcanics also generate hot water and acids. Sometimes this water may pass through a buried bench and deposit some of the gold that it carries in solution. Basic small scale sampling performed at various levels of the bench material should yield sufficient initial information for further exploitation.

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