All About Flood Layers (Part 4)
(California's Yuba River in massive flood stage.)
OK, it's time to get back on track and talk about flood layers some more. In this post I'll provide general information for you as well as additional tips concerning flood layers and placer gold.
I've mentioned many times here in Bedrock Dreams about how heavies like lead and iron tend to end up where the gold is (and vice-versa). These non-auriferous heavies (iron, lead, etc.) also get carried along in flood conditions much in the same manner as does placer gold. When heavies and gold are in suspension in a raging flood (desert or wet placer) they will "drop out" first while the lighter rocks and materials around them continue to be scoured, dislodged, and carried downstream. As paystreaks and gold pockets are being formed in low-pressure areas of a stream or wash, those other "lighter" materials will continue moving downstream even as the gold is finding its resting place. Get this image fixed inside your brain and then visualize what's transpiring in terms of stream hydrology and gold deposition. That's where the gold will be, especially in locations where old hard pack has been exposed or re-charged with gold. New hard pack layers are a different story, however. They may or may not contain much gold on their surface. This depends on too many variables to count but rest assured of one thing...the next time a flood occurs in that stream or wash you best get to sampling that newer flood layer.
(Flood layer strati-graph of the gold-bearing American River.)
Tip 6: Small amounts of fine gold will be deposited close to the surface along the main path where gold is traveling. If an older flood layer lies just below the overburden of that path then greater amounts and larger-sized gold will rest upon it.
Note the words "along the main path" here. If you start finding small but consistent amounts of placer gold following a line downstream (or even upstream for that matter) then that's like a two-by-four alongside the head. That's where the good gold is being deposited. If the overburden is shallow or even non-existent in that spot, your next step is accessing that hard-pack layer. Ditto for gold pockets which tend to be measured by how far out they extend from their central core or deposition point. Pockets will not follow a linear trace like a paystreak. Bear that in mind and understand that either/or is a good thing.
Tip 7: Flood layers form as the last phases of a flood occur.
For a flood layer to form into a solid hard-pack the flooding that gave birth to it has to subside. This is common sense and requires no Einstein-type theoretical considerations. Once flooding subsides the material composing that new flood layer can settle, consolidate, and harden. This is especially true in desert or dry pacer washes, gulches, and arroyos.
Tip 8: Gold can be "sandwiched" between old flood layers and new ones.
I alluded to this earlier in this series but it's an important point to retain in your memory. Even if not much gold is found atop a new flood layer hard pack, that same gold can be plentiful below it and just above an older flood layer. In other words, I don't care if the flood layers you come across are old or new, or stacked on top of one another like the contents of a layer cake. You must check each and every layer for placer gold. In wet placers the deeper and older the layer, the better your chances of finding good gold and lots of it. In dry placers the gold deposition will be much more erratic in general and good gold might be found atop a new hard pack while farther down older flood layers may yield little yellow. That's the basic nature of dry placer deposition.
(A slice of strati-graphic "layer cake.")
Tip 9: Most flood layers in wet placers are formed from hard-packed rocks or clays.
Remember, true bedrock is not a flood layer but the underlying rock structure ("country rock") of a given region or locale. You can identify a wet placer with one or more hard-pack rock flood layers because the rocks in those layers will be resting along a horizontal plane with maybe just a slight tilt upward on the ends facing the stream flow. This sort of rock hard pack is difficult to contend with simply because the rocks in that flood layer tend to "interlock" and are a bitch to loosen up. But if you want the gold, you'll do the work necessary to get it. I've written about clay layers numerous times in Bedrock Dreams. Clays are usually formed from the weathering and gradual breakdown or decomposition of other rocks and minerals into a finely grained (and often "gummy") mass. Clay flood layers can be excellent hard packs with placer gold typically resting atop them or in the 6-10 inches above them. Most dry placer clays get hardened pretty quickly by the excessive heat and dryness of the desert. Once this happens, they form a totally impermeable layer to the passage of gold, so look for the yellow on top of them or in the material resting just above them. If you can work your way through a desert flood layer hard pack to what lies beneath it, you may hit some excellent gold as well.
That's it for now. I wish you all peace, success, and happiness in your lives.
(c) Jim Rocha (J.R.) 2016
Questions? E-mail me at firstname.lastname@example.org