(Focus on the obstructions, not the scenery!)
Over the course of 35 years plus at this small-scale gold mining thing I've come to realize that there are certain core deposition factors that just can't (or shouldn't) be ignored. That fact stated, I've chosen to focus here on the positive gold potential of obstructions in both wet and dry placers. Now don't let what I just said throw you off since I've chosen to emphasize the differences between wet and dry placers in the past. You see, where obstructions are involved, the similarities between wet and dry gold-bearing areas are more closely aligned than they are different.
I'll Take Experience
What are obstructions? Here's my definition: Any large or very large solid object that exists in the current streambed or wash that deflects or interrupts the water flow in significant enough fashion to create vortices or eddy swirls along its sides or behind it (the downstream side). I don't think I need to belabor this definition because even the greenest greenhorn or newbie out there can comprehend it and you old sourdoughs and journeyman miners out there have seen this sort of thing in action countless times during your mining careers...just as I have. It goes without saying that we focus a lot of our mining attention on the importance of bedrock (false or real) as the Holy Grail of gold deposition and for good reason. But whether bedrock is close by or in shallow depth underneath them or not, obstructions should be one of your main targets for sampling and potential gold recovery. Why? Because more often than not they act as "mini-banks" for the deposition of placer gold. Again, this is true whether we're talking about wet or dry placers despite the significant differences between the two. How do I know this to be true? Through direct experience, that's how. And while we're on the subject, I'll take experience over pure theory any day of the week. Wouldn't you?
You'll notice in my personal definition of obstructions that I use the adjectives large or very large. There is, as always, a method to my madness here. Although there is a case to be made that nearly any reasonably sized obstruction can provide the mechanism for decent gold deposition I don't subscribe to that theory in its entirety. See? Here we are back to theories again! The reason I don't believe this "any size obstruction fits all" theory is, once again, my own decades of experience as a small-scale gold miner and prospector. What I've seen and experienced and the observations and realizations that have arisen from that life's work have shown me all I need to know in this regard. Smaller rocks (less than two to three-feet in diameter) just don't have the overall mass to disrupt water flow enough to create decent vortices and thereby, decent gold traps. Do smaller rocks or even cobble act as traps at times? Probably, but you won't see me out there digging up small rocks or cobble one piece at a time searching for nuggets or large amounts of gold. Again, based on experience I already know that the larger the obstruction and the better its overall placement in a stream or wash, the better the gold deposition potential. Let me revise what I just said slightly. The placement of that obstruction is not as important a deposition factor as the size of the obstruction is. (I'll explain why this is so later.) Moreover, the stronger or more violent the trailing vortices or eddy swirls coming off that obstruction, the greater your chances are for good gold recovery.
(A small piece of obstruction heaven.)
The Twig Test
If you want to personally examine this premise of mine, just pick up a small twig and toss it just upstream of a large obstruction (big boulder, substantial rock, etc.) in an existing stream. Watch carefully as that twig is swept downstream on either side of and behind the obstruction. You will see that in most instances that twig will get spun around (and sometimes around and around many times over) in an eddy current or vortex before it is carried downstream in a line closely behind that obstruction...at least until it clears the suction created by that eddy or vortex. In cases where very large obstructions occur, the vortex or eddy swirl action can be strong enough to actually pull that twig underwater for a brief moment or two before it bobs back to the surface and then goes on along its merry way downstream. Getting the picture are you? I thought so because you're smarter than you give yourself credit for. Now imagine this same obstruction vortex or eddy under strong water flow such as that found in flood or flash flood conditions. If your mental image of this imagined picture is clear enough, you'll recognize that the suction vortices or eddies just mentioned can become quite strong...even violent. So strong and violent that even heavier pieces of coarse gold and nuggets can be swept into them and deposited forthwith. There is nothing magical in all this. It is simply deposition physics as it relates to hydrology.
Not an Exact Science
This said, no two obstructions (regardless of their overall mass or size) will produce the same sort of vortex or eddy swirl. Taking this a step further, each obstruction's ability to "grab" and deposit gold will be different as well. Some vortices and swirls are stronger than others and this is the result of the size and overall configuration of the obstruction and, with less importance, its placement in a stream or wash.The twig analogy I just used works great where flowing water is concerned but it's a total dead end in dry or desert placers. You can toss a twig in the sand and gravel above an obstruction and then sit there like Job in the Old Testament with unrivaled patience and faith waiting for the truth to be revealed to you but alas, that vision you seek will not be forthcoming. Yes, I'm playing the tongue-in-cheek game here, but the real question is how you determine where those eddies and vortices will be under flood conditions in a dry wash or arroyo. There are two answers to this: 1) you put yourself on the scene when the shit hits the fan (flash flooding occurs), or 2) you visualize that taking place and make an educated guess as to where the gold is going to caught up by existing obstructions. Neither of the two options is an exact science but Number 1 is probably the better of the two approaches if you discount the inherent dangers of being close by when the desert turns dangerous with rushing water.
(Desert placers require imagination when "reading" obstructions.)
Just a few things to mull over until I get back to you with more on obstructions. I can hear those gears turning right now and a few of you are thinking, "Well maybe all this is true but how the hell am I gonna sample behind a big boulder in a running stream?" Well, my friend, your options are limited in that regard but there are ways. We'll talk more later...
(c) Jim Rocha (J.R.) 2015
Questions? E-mail me at email@example.com