All About Flood Layers (Part 3)
It's time to move along again. The overall purpose of this series of posts is to make a firm connection in your mind about the importance of flood layers as a carrier and/or resting place for placer gold. Additionally, I'll be laying out a few tips for you in terms of what you should be looking for and how to sample flood layers. Once again, I remind you to pay attention to any text in italics.
Tip 1: Always focus your attention on hard pack any time you're sampling a flood layer or flood-ravaged wash or stream.
Once again, flood-created hard pack comes in a wide variety of types (caliche, conglomerate, clay, breccia, etc.) and can cover a small, very localized area or large swaths of a wash or streambed.
Why is hard-packed material so important? It should be perfectly obvious to anyone and everyone reading this post. That hard pack (sometimes called "hard pan") forms an impermeable (or nearly so) barrier to the downward movement of newly carried placer gold entrained in flood waters. In essence, flood-created hard pack acts as the newest form of false bedrock in a given stream or wash. This can be the case when true bedrock is nowhere to be seen because it's buried under large amounts of overburden. Some of that overburden may include successive layers of gold-bearing hard pack. Note the term "successive."
(A desert road with an underlying hard pack part composed of fragmented breccia.)
Tip 2: Even the largest seasonal storms typically DON'T have the power to remove or replace existing flood layers. They simply lay new ones down.
This is a very important concept that you need to grasp. Only those "100-year" flood events I mentioned in the previous two posts have the hydraulic power to scour away existing hard-packed flood layers in streams or desert washes and arroyos. You and I may witness an event of that magnitude perhaps once in our entire lifetimes. However, new flooding of smaller magnitude has the capacity to create new flood layers or areas of hard pack anywhere along a stream or wash.
Tip 3: Successive layers of flood-created hard pack usually contain successive amounts of placer gold within or atop them.
Here's the best way to look at this. Imagine a length of laminate board viewed from its side where you can see successive compressed layers. In a stream or wash that bottom flood layer might have been laid down millenia ago, with each successive layer heading toward the surface being laid down at various times in the past. Each of those layers from top-to-bottom and bottom-to-top is a potential repository for placer gold or even elluvial gold washed down from slopes, hillsides, and desert terraces. Unlike the usual gold values drawn from sampling surface gravels down to true bedrock in perennially running streams however, the gold values contained on or in flood layers can vary quite dramatically in value. In other words, you can usually expect to recover better gold values in a running stream as you progress downward toward bedrock with the sparsest gold on the surface and the best gold resting on true bedrock. Countless drill sample operations have proven this out. However, when dealing with desert placer flood layers, just the opposite can be true at times. What you want to take away here is that if you work a desert placer and come across various flood layer hard packs at various depths, you'd do well to sample them all thoroughly. Even if the most recent flood layer hard pack shows little in the way of gold, that next layer may be rich with it. And so on and so on with each successive layer you find.
(The geological "macro" view of flooding. This image represents the flood plain of ancient Lake Missoula millenia ago. Somewhere in that vast region there is a flood layer rich in placer gold but you'd play hell finding it!)
Tip 4: Gold being carried by flood waters will work its way down and along the main stream channel much more slowly than the materials around it.
Read and re-read Tip 4 again. Emphasize the following: "will work its way down and along the main stream channel" and "more slowly than the materials around it." Why is getting this concept and visualizing it important? Well hell's bells pard, it's telling you where the gold is going to end up once that flooding stops! Now that you've got an idea of where it's coming to rest in general, you can focus your attention on the best gold traps there. Things like hard-pack deformities or ridges, cracks or deep depressions, or large obstructions that also came to rest on that hard pack or that were not even slightly moved by all that rushing water.
(Running stream [non-desert] diagram. A new flood layer being laid down while an older one remains in place. Note that beneath the old flood layer there is probably a lot of gold between it and true bedrock. Also, the new flood layer will drop gold atop the old layer while forming a new gold- bearing flood layer that will rest on the old, perhaps with shallow gravel intervening. In a "100-year " type of event even the old or ancient flood layer would be scoured and a new layer formed. This is especially true in the case of desert or dry placers. Diagram courtesy the New 49ers.)
Tip 5: The largest pieces of placer gold and the greatest amounts will be deposited atop hard pack in the areas of LOWEST water velocity or pressure once flooding subsides.
Man alive! It doesn't get any clearer than this, does it? If you hit a wash or arroyo (or even some running stream) after a flood event where layered hard pack is visible and accessible and then you e-mail me saying you can't find any gold there, I'm gonna come looking for you in the dead of night and abruptly awaken you from your fitful dreams, and it ain't gonna be pretty...trust me. 'Cause I will grab you by one ear and drag you from your warm, soft bed to that dusty arroyo or muddy stream and shove your nose right into the gold you told me you couldn't find on your own. Sure, I'm joking with you here (well, sort of anyway) but I think you get the message.
OK, that should do it for this round. There's more to come so stick around and learn something.
Best to all.
(c) Jim Rocha (J.R.) 2016
Questions? E-mail me at firstname.lastname@example.org