Friday, February 5, 2010

True Bedrock vs. False Bedrock (Part 1)



(Photos from top to bottom: 1. True bedrock entering a section of the N. Yuba River; 2. True bedrock entering both sides of Lone Jack Creek.)

I still receive occasional e-mails requesting that I elaborate a bit about the differences between "true" or real bedrock and "false" bedrocks such as clay layers. This is, for the most part, a very straightforward task to complete and I am happy to do so.

True Bedrock

The simplest explanation of true or real bedrock is that it constitutes the predominant, underlying host or "country" rock of any given gold-bearing area. That area can be wet or dry; it really doesn't matter since surface materials will rest atop typical bedrocks such as granite, serpentine, gneiss, gabbro, and so on.

Gold-bearing rivers or streams and their desert or dry counterparts throughout the world are inevitably underlain by true bedrock. This is the solid body of local country rock that poses a complete barrier to the downward movement of placer gold or to put it another way, the final resting place of gold in a given stream bed or wash.

Underlying Foundation

Sometimes true bedrock can be found close to the surface with just a few inches of overburden (i.e., sand, dirt, rock, and gravel) covering it or it can be completely buried under overburden as deep as 10, 20, 30, or even hundreds of feet. Regardless of the depth of overburden covering it, true bedrock will be the underlying foundation of most (if not all) gold placer deposits.

If you examine the two true bedrock photos above, you can easily see it's structural solidity. In Photo 1 the existing country rock (again, the predominant underlying rock structure of a given gold-bearing area) enters the N. Yuba River at a fairly steep angle and continues down to a point underneath the river. The overburden at this particular point is many feet deep, but just downstream from this point the bedrock is quite shallow and riddled with good gold traps such as fractured shelving and cracks and crevices.

Not Always Visible

In Photo 2 (the home page photo for "Bedrock Dreams") similar country or host rock constitutes bedrock for this section of Lone Jack Creek as it enters both sides of the creek. Here the overburden is quite shallow, particularly near the margins of the creek. Even in the center of this small gold-bearing stream, the overburden is less than 6-8 feet in depth (if that).

Gold Pans
Gold Panning Kits
Mining Equipment

In many instances where wet and dry gold placers are found, true bedrock is not always readily visible in the stream beds or washes themselves. That's because most of the existing true bedrock has been covered by various layers of overburden that can include "false" bedrock layers composed of clays or caliche ("desert cement") as well as successive layers of rock, sand, dirt, and gravels. In areas like these you may have to search some distance above existing streambeds or washes to spot the country or host rock that constitutes true bedrock in the immediate area.

This should get you started. In my next post on this topic I'll cover "false" bedrock so you can easily understand how it differs from true bedrock.

Good luck out there.

If you liked this post, you may want to read: "Prehistoric Rivers of Gold (Part 4)"

(c) J.R. 2010

Questions? E-mail me at jr872vt90@yahoo.com

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