All About Flood Layers (Part 1)
(Looks can be deceiving...the relatively mild appearance of this desert flooding masks deceptive power...a new flood-borne deposition layer is being created before your eyes.)
In this series of posts I'll be talking primarily about desert or dry placer flood layers, how they're formed and how to recognize them, and where placer gold tends to be deposited within them. Some of the information I'll supply you will be relevant to wet placers as well, so read on if you're interested in learning more. Be advised, however, that I'll be laying down a lot of detailed information on flood layers from both the geological as well as the mining standpoint. The reason my main focus is on dry or desert gold placers in this post is due to the fact that flood layers and flood-borne gold pockets/paystreaks are, to a great extent, easier to identify and locate in dry contexts than in wet. Still, you'd do well to remember that the basic principles remain the same for both. I'll talk more about these similarities and the differences later on. With these things in mind, let's continue.
What's a Flood Layer?
What's a flood layer? It's a layer of material laid down or re-arranged and/or re-deposited in a wash, arroyo or streambed by the force of a large volume of rushing water. Ditto for wet environments where perennially running rivers, streams, or creeks exist. Like deposition layers, flood layers can be composed of a range of material sizes and types containing coincidental placer gold concentrations or values. Unlike most "ideal" wet placer contexts where the best gold lies on bedrock (existing "country rock") at depth, the greatest concentrations of desert flood-borne gold are found on and in hard-packed layers formed of any material or combination of materials at any depth. Note my use of italics here and file this point of information away for future reference. Ditto for this point: Again, most wet placers are largely dependent on true or false (i.e., clay layers) bedrocks for concentrating gold and the most productive wet placer bedrocks or false bedrocks are typically found at depth. Any shallower bedrocks in common wet placer districts have been worked hard...again and again in certain instances.
(The force of recent flash flooding has stripped the hard pack in this wash. I'd be crevicing and checking those rock and gravel piles.)
Here's another point to remember about flood layer identification and deposition in wet placers versus dry ones: The visual and physical evidence associated with wet placer flooding will be remediated with time as the river or stream drops from high flood stage back to "normal" flow. The constant flow of water will sort, move, and "organize" those materials moved (no matter how violently) in flood stage and eventually return the physical characteristics of a given river or stream to a non-flood appearance. The visual evidence of flood layers churned up or laid down during high water flow will soon disappear, as will flood evidence left higher up a stream's banks once new growth takes hold. Getting the picture here? Granted, so-called "100-year floods" take place in both wet and dry placer regions on occasion, but the frequency and distribution of these major flooding events that can "re-arrange" entire topographies is best correlated to the nickname given them. In other words, they happen damn infrequently. However, the flood layer evidence associated with the hydrological forces of flash flooding in desert regions does not remediate itself to a significant extent nor disappear until the next major flash flooding event. File this point away as well.
(The advent of a "100-year" flood in the Utah desert.)
Gold Disposition in Flood Layers
As I've already alluded, the specific type of flood layer and when it was laid down has a lot to do with how much gold it contains. Again, hard-packed layers containing clay, caliche, breccia, conglomerates, or other hard-packed materials will prevent gold from traveling deeper and constitute a firm or semi-firm surface where placer gold will concentrate. In dry or desert placers these sorts of flood layers present you with a "golden" opportunity once you've learned to find and recognize them. The same can be said to be true for wet placers but spotting and finding those old flood layers is problematic at best. Once a dry placer flood layer has been identified, you should approach sampling or prospecting it with typical parameters in mind such as large obstructions, any visible depressions, cracks, or crevices, and low-pressure spots. Benches are one type of location in a recent desert or wet placer flood layer you should NOT pay too much attention to. Why? Simply because the force of heavy flooding tends to scour gold-bearing material loose from sidewalls, banks, and benches and deposit it downstream below the current bench height. Instead of deposition, we get removal and re-deposition in this context. (J.R. Note: Pay attention. Each time I use italics I'm giving away useful information [tips] that I've gained over the course of 36+ years of dry and wet placer mining.) On the flip side, in a non-recent flooding scenario without readily identifiable flood layers bench and terrace gravels should ALWAYS be sampled for the presence of placer gold, especially if they appear to be part of ancient water flows or segments of the current watercourse left high and dry. This remains true for both wet and dry placers.
(Finely grained and tightly bound desert breccia. No gold is moving through this but some may be embedded within it.)
(Gold-bearing desert conglomerate. This type of flood deposition layer can hold gold within as well as on top of its surface.)
Placer gold can be mixed into or through the materials composing clay or caliche flood layers but not as frequently as it is deposited on top of these types of layers. Remember this point because it holds true in most wet placer locations as well. Flood layers composed of tightly compacted rock or gravel (breccia or other tightly bound conglomerates, for example) will often allow smaller pieces of placer gold to be bound up within the gravels themselves but, if they are fine-grained enough those conglomerates can pose an impermeable barrier to the downward passage of placer gold. One deposition area often ignored by small-scale desert or dry placer miners are terraces. Again, terraces are those rises or small plateaus that rise above existing washes, arroyos, or gulches. They are easily identified by their flat, table-like or bread-loaf appearance. In heavy flooding terraces can be inundated by rushing flood waters and flood-borne materials ranging from very light to heavy. Terraces don't accumulate flood layers as much or as frequently as washes or arroyos themselves, but should be checked for evidence of recent flooding. This evidence will include limbs, branches, bushes or other foliage recently washed atop the terrace as well as rocks, gravel, dirt, sand, pieces of old cars or equipment, and the like. In instances such as these placer gold is strewn about the top of that terrace at shallow depth. Seldom does gold concentrate atop terraces but it will be there in the top six-to-nine inches or less, usually strewn about in a seemingly haphazard way. One thing the old timers used to do in dry placer areas is sink hand-dug test holes in various locations on the top of terraces. They did this to determine if older flood or deposition layers could be found deeper within a given terrace. The latter was often the case. However, your first step is to skim the top of that terrace. Yes, I hear you. You're telling me I've given you hell in the past for skimming but in this case it's warranted. Remember, when it comes to Ma Nature's hidden treasures the rules get thrown out the window at times.
I think I've given you plenty of food for thought for this round. We'll continue forging ahead with this issue in the next post.
Be safe until then.
(c) Jim Rocha (J.R.) 2016
Questions? E-mail me at email@example.com