True Bedrock vs. False Bedrock (Part 2)
In my first post on this topic I covered some of the characteristics of true bedrock. In this post I will discuss false bedrock and how it differs in overall character from its underlying layer of support, true or real bedrock.
The Function of True or False Bedrock is the Same
Unlike true bedrock which is composed of the predominant host or country rock found in a given placer gold locale or area, false bedrock is invariably composed of various types of clays or successive clay layers. In dry desert placers these clay layers (sometimes called caliche [kah-lee-chay]) often reach the consistency of cement or hardened concrete. This latter occurs because the intense heat of desert summers and the very alkaline constituency of dry placer clays combined with assorted gravels (and a bit of moisture) react chemically to form an extremely hard and dense material (i.e., "desert cement").
Regardless of whether false bedrocks composed of clay are in wet or dry placer areas, their function is essentially the same. They both act as impermeable layers that prevent the transition of gold to lower points in a given streambed or drywash.
Gold Moves Ever Downward
Always remember the single-most fundamental rule of placer gold (or gold in general). Gold is a very heavy and dense metal that will continue to "work" its way ever downward until it can travel no further. As long as gold DOES NOT meet an impermeable layer like true or false bedrock, it will continue this transition to depth either continually as it does in a running stream, or intermittently as it does in dry or desert placers.
False Bedrocks Can Exist in Multiple Layers
Unlike true bedrocks however, false bedrocks can sometimes be found in multiple layers at various depths in both wet and dry placers. This doesn't mean however, that each clay layer (or layer of caliche) will have gold resting atop it. Sometimes this is so and sometimes not. It all depends on when and where the false bedrock layers were laid down in time and space.
The false bedrock pictured in the two photos at the beginning of this post is a classic "pinkish" clay bound together with stream gravels and rocks. The river-borne material resting atop this section of clay layer was auriferous and produced 3 small nuggets as well as numerous flakes and fines.
Gold Atop Additional Clay Layers?
This is because the clay acted as a typical impermeable layer (false bedrock) to all the gold deposited upon it since the layer itself was laid down. However, digging into the false bedrock itself and panning or processing that material itself produced no gold whatsoever..and trust me, I dug fairly deeply down into it to establish this fact.
However, every possibility exists that in this particular location (California's North Yuba River), that other clay layers or false bedrocks exist much farther down than this topmost layer. Depending upon when those additional layers (false bedrocks) were laid down and how they were positioned depositionally to capture gold, they might contain even better and more extensive gold values.
See what I am driving at here? Unlike true bedrock which exists (no matter how far down) as a single impermeable layer, false bedrocks can actually exist in multiple impermeable layers in one location. Finally, all of these false bedrocks will be resting atop...yep, you guessed it......true bedrock.
Other Types of False Bedrocks?
Are there other types of false bedrocks out there in gold-bearing areas? Probably. I've even heard of very densely packed river or wash gravels acting as a form of false bedrock but I've never come across this in more than 30 years of small-scale mining.
To tell you the truth, I have a very hard time believing this one. No matter how densely packed they are, stream rocks and gravels are going to have a hard time acting as a false bedrock since they cannot be truly impermeable to gold's downward movement.
Hopefully these two posts have helped explain the difference between true bedrock and false bedrock. Good luck out there!
If you liked this post, you may want to read: "True Bedrock vs. False Bedrock (Part 1)"
(c) J.R. 2010
Questions? E-mail me at firstname.lastname@example.org