Why are rocks on top of light sandbar? (Pic)

[IowaRiverWalker]

I was working a new small creek and found this sandbar with softball sized rock on JUSt the top layer. I dig about 2.5 inches down and its light sands for several feet and I assume clay about 6 feet down from looking at the surroundings.. I did find some very fine gold from the very top layer nothing below. I'm used to the opposite....as I dig deeper I usually run into bigger rocks and heavy material.

Anybody have experience with something similar?

 

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[Assembler]

A flood deposit that dropped that size rocks.

 

[et1955]

Very simple, water flow and duration is the reason gold is moved on your creek, no flood is the same. As you have seen and tested, a flood capable of moving cobble also moved gold yet a flood that move sand didn't.

 

[Assembler]

Very simple, water flow and duration is the reason gold is moved on your creek, no flood is the same. As you have seen and tested, a flood capable of moving cobble also moved gold yet a flood that move sand didn't.

Better said.
If the duration of a big enough flow to carry that size rocks ended or was reduced enough then the rocks drop out there.

 

[Laz7777]

the right conditions moved the sand and exposed the cobble, then concentrated the gold.

 

[Golden_Crab]

I was working a new small creek and found this sandbar with softball sized rock on JUSt the top layer. I dig about 2.5 inches down and its light sands for several feet and I assume clay about 6 feet down from looking at the surroundings.. I did find some very fine gold from the very top layer nothing below. I'm used to the opposite....as I dig deeper I usually run into bigger rocks and heavy material.

Anybody have experience with something similar?

View attachment 1502976

Go upstream and find a gravel deposit instead of a sand bar

There is the slim possibility flood started cutting through an old channel washing some gravel into the mix while leaving nothing but sand up stream. In that case investigate anywhere you see the water cutting the bank, look for gravel.

 

[bobw53]

the right conditions moved the sand and exposed the cobble, then concentrated the gold.

I've run into this in the desert A LOT.. The cobble didn't get deposited on top of the sand.. The lights moved off and left the heavies.
Concentrating the gold RIGHT ON THE SURFACE.


You said you found some right on the top.. Its already concentrated for you.. scrape it and get some quick easy gold, even if it is small.
Though those rocks are bigger than gravel size... Might be some nice pieces in there.... And it should be EASY digging.

 

[Assembler]

I've run into this in the desert A LOT.. The cobble didn't get deposited on top of the sand.. The lights moved off and left the heavies.
Concentrating the gold RIGHT ON THE SURFACE.


You said you found some right on the top.. Its already concentrated for you.. scrape it and get some quick easy gold, even if it is small.
Though those rocks are bigger than gravel size... Might be some nice pieces in there.... And it should be EASY digging.

This is like the case of the glass being half full with the lights being slimmed off for you and just the heaver good stuff left for you. Better dig before the next flood puts some finer gravels over this. Good luck

 

[KevinInColorado]

I've seen this around metro denver. The two posts above have it exactly right!!

 

[Bandmenter]

I would be more interested in the type of cobble that has been deposited. Are these rocks native to Iowa, such as lime or sand stone or non native such a granite. As was pointed out before, no flood is the same and I would as well suspect the lighter materials have been washed away. If the rocks a local material it may be only worth sampling and moving on. Best of luck, dig deep and keep the color.