What to Look for in Bench Gravels (3)
(Old bench/terrace gravels near Placerville, California.)
As we continue on here, I have one more example to relate to you about how good bench placer gravels can be at times so please bear with me. Then it's on to more tips on what to look for. Remember, there's ALWAYS a method to my madness, so take note.
A Solid Wall of Clay
It's not all that unusual to come across desert or dry placer benches that are composed entirely (or nearly so) of caliche but it's much rarer to stumble on bench gravels in a wet placer location that are totally composed of hard-packed clay. That said, during my days dredging along the North Yuba River in northern California I came across a few of these unusual benches when I wanted to take a break from working underwater and just do some prospecting or exploring (usually working my way upstream along feeder creeks in the area). In three separate instances I came across benches on feeder streams that were composed almost entirely of hard-packed clay and that were, at times, located a pretty good distance from the current stream flow. Now don't confuse these sorts of hard-packed benches with the caliche bound types typical of desert areas because the wet placer benches like this are typically easier to break loose and work. "Easier" may not be the best term for me to use here, but I'm speaking comparatively, OK? Anyhoo, one of these clay-bound bench gravel sections exists just slightly upstream from the North Yuba along a fairly well-known feeder stream in the area south of Downieville. How this bench was created I can't say, but my guess is that it was part of the existing creek long ago and at some point was left high and dry (and a good distance away from the modern creek's course). This bench is a solid wall of hard-packed (sun baked?) clay exhibiting all of the good characteristics mentioned thus far. The material was hard to break loose and had to be puddled in certain instances, but it was chock full of placer gold. In fact, the first bucket of material from this bench produced a nugget in my sluice box as long as one of your thumbs' finger nails and about half as wide. You know, suction dredgers can be a bit arrogant at times in terms of seeing dredging as the ONLY way to go when it comes to getting the gold. Hell, I don't judge that sort of attitude because I myself have been there too. But this bench I'm talking about produced more gold in a day sluicing than I typically recovered dredging. This may speak to my weaknesses as a dredger, or alternately it may underline again just how good some benches can be, so take note once again.
(A "monochrome" bench in the Black Hills of South Dakota.)
(This image is useful in that it illustrates the "layered" bench concept.)
Oxidation is a dead giveaway: I can't stress this point enough to you when it comes to learning what to look for in bench gravels. Moreover, the presence of oxidation or oxidized materials is an significant visual clue in virtually everything you do as a small-scale gold miner or prospector. It's Ma Nature's way of telling you that something important is going on, that significant changes have (or are) taking place. Most importantly, oxidation can be an important visual signpost pointing you right to where the gold is. I can't stress this point enough to you, especially the greenhorns or newbies out there. So now that you realize the importance of oxidation (i.e., iron-staining, etc.) when you come across evidence of it in bench gravels it's pretty certain that the good times are about to roll. The way you'll typically spot bench oxidation will be via a reddish-orange coloration to the bench as a whole, or sections or localized areas of "rusty" looking earth or material. Pay particular attention to any layers of clay or black sands ("heavies") containing rusted metal or clumps of oxidation from nails or other metallic trash fused together. These are all signs and you should be looking for them when prospecting or working benches.
(A heavily oxidized glacial gravel bench.)
There's more to come.
(c) Jim Rocha (J.R.) 2016
Questions? E-mail me at firstname.lastname@example.org