What to Look for in Bench Gravels (Part 1)
(A small section of bench gravels in New Zealand...note the angular and rounded rocks.)
The nice thing about bench gravels is that they can be found in both dry (desert) and wet (stream) placer gold locales. Depending on where they're located and how they're configured, some benches can be very good gold producers and fairly easy to work (while others are not). These things said, let me give you a few tips on what to look for in benches.
Listen Up Newbies
Before I get started, let me address the following to you newbies or greenhorns out there. Bench gravels are those stream or wash banks that contain gold-bearing rock, dirt, and gravel but are no longer an active part of the current stream or dry wash you're checking out except, in certain instances, where flooding or flash flooding reaches them. Some bench gravels can be directly adjacent to a current stream or wash, while others can be some distance away. Regardless of the latter point, each and every bench you come across was once part of an active stream or wash gold deposition channel. That's all you need to know about benches to get started. As for the rest, I've written about benches elsewhere in Bedrock Dreams so make use of the "Search" tool in the right sidebar to learn more.
(There's something I don't like about this bench...can you guess what that is?)
OK, here's what to look for in bench gravels:
Rock size and shape: Just like the active streams or washes near them, good bench gravels will contain a variety of rock sizes and shapes. These can range from rocks the size of a pea, on up to fist-sized, and even small boulders in certain instances. There should be a good mixture of rock sizes scattered throughout and along a bench and those rocks should be both smooth (water worn) and angular. Trust me here...the very best benches will contain both water-worn and angular rock in a variety of sizes. In desert or dry placers the general trend for good benches will be less water-worn rock and more angular, but I've worked desert benches that contained large amounts of smooth rock, indicating that bench was, somewhere in the past, an active stream or ancient riverbed. Conversely, wet placer benches will often contain more water-worn rock than angular, but fractured rock with defined edges will still be there...sometimes in quantity. The tip to take away from this is, again, the best gold-producing benches tend to have a mixture of water-worn and angular rock of varying sizes.
Dirt, not sand: Let me tell you something right from the get-go. If you happen to stumble across a bench containing good rock size and shape indicators but containing lots of loose sand or "blow" sand (also called "blonde" sand) as opposed to darker, rich-looking dirt then you're barking up the wrong tree my friend. Benches configured in this manner are rarely good gold producers. In fact, about all you're gonna find in them is an occasional flake and some very fine gold. This point is also true in most placer mining situations, whether you're working benches or not. The very best benches will have dirt or earth mixed in and around the rock and gravel and that dirt will not be light or powdery, but dense and often firm to the touch. Even after all these years, it's hard for me to describe the sort of dirt I'm talking about but when I see it I know it. If you could walk alongside me I could point it out in an instant but writing about it is not as easy. Anyway, I hope you get the drift here just the same. If nothing else, you now know to avoid benches containing copious amounts of blondish-looking "blow" sand.
(Note the color of the dirt in these gravels...what I call "good looking dirt.")
Clay is your best buddy: The old timers used to call clay "gold robber" due to its elasticity and propensity for binding gold to it and even within it at times. These very same same old timers didn't have much good to say about clay and they hated it when they came across it in "normal" placer mining operations because it generally caused them a great deal of additional pick and shovel work, not to mention a laborious and time-consuming process called "puddling." Puddling involved soaking large amounts of clay in tubs of water to dissolve it and release any gold it contained. There's your daily mining history lesson. Now let's move on to a bit of small-scale mining heresy on my part...clay is your BEST BUDDY when it comes to spotting good gold benches. Clay in bench gravels can exist in a layer or layers, it can be bound up in small sections of a bench in conglomerates, or the entire bench could be composed of soft or even hard-packed clay (called caliche in desert placers). When clay is layered within bench gravels that means those layers act as a form of false bedrock that prohibits the passage of gold downward in a stream or wash.
The very first thing you want to do when coming across clay layers in bench gravels is to remove (via force or finesse...take your pick) the material sitting in the five-six inches just above that clay layer or layers. Don't dig into the clay layer itself...there won't be anything there, or very little. Be careful when you do this so you don't get buried alive or bonked on the head by rocks or boulders above. In fact, in taller benches, you may want to take the safe approach and collapse the gravels and rocks above your head before you get down to the heart of the matter. In situations where only sections of clay are present in a bench, you want to run the material surrounding the clay (or caliche) and then puddle the clay itself to see if it too contains gold. Unlike layers, sections or conglomerations of gravel and clay can contain good gold values. The third type of clay situation you'll want to keep an eye out for is hard-packed clay. Hard packs or caliches that form the basis for bench gravels in ANY CONTEXT are some of the best gold producers you'll ever run across. I know this because without the slightest bit of humility on my part, I'm the bench gravel and clay "king" and I damn well know how to get the gold out of them too. You can take that to the bank with you because I'm not given to idle boasts.
(I've used this image before but it tells the tale. Note the section or conglomerate of clay bound bench gravels in the right-center.)
Anyhoo, back to hard-packed clay and benches. These sorts of benches will kick your ass, pure and simple. In dry placer areas they don't call caliche "desert cement" for no good reason. Ditto for stream placers where hard-packed clay dominates a particular bench. With these types of benches there is no softer, gentler approach. If you're gonna work one of these puppies you want to work the entire bench from top-to-bottom and side-to-side. And unlike those prima dona commercial miners you see on reality TV each week, you aren't gonna have large excavators or bulldozers to do your dirty work for you. No sirree...no way, no how. You'll be pick and shoveling your way through that shit like an Egyptian slave building the pyramids of old, no mercy, no slack, and no easy way out. In fact, if you come across a bench like this and are itching to get at it, that'd be a perfect time to drag along that lazy assed son-in-law of yours who always likes to brag about what great shape he's in. Hand Mr. Loudmouth a miner's pick and tell him to show you his stuff. I guarantee he'll be whining and whimpering like a whipped puppy by the time the day is done. Trust me on that.
So why go to all this effort with hard-packed benches (even if it is gratifying to see the son-in-law finally get his come-uppance)?
We'll talk about that next time...
(c) Jim Rocha (J.R.) 2016
Questions? E-mail me at firstname.lastname@example.org