Today, what we take for common knowledge on prospecting, wasn't always so. The source of the placer deposits in the Motherlode was of great interest to early prospectors and geologists. Today, we know the source was the ancient tertiary river channels, but still we often ask how these channels managed to be at the top of the Sierras, not the bottom like present day rivers.
Prospecting is an endless process of learning. Each trip is another opportunity to learn something. No matter how good our current claim is, we're always willing to give up on it and head over the next ridge where there's supposed to be even better gold.
Not surprisingly we convince ourselves the next canyon has better gold because there is a reason for it. We hear the tertiary channel crossed right there, or there was a big vein that bled into the creek and huge nuggets have been pulled from there. There's always a reason.
A few weeks ago I had the opportunity to go prospecting with a professional geologist. It's not often you have this chance so I looked forward to learning from someone who had a lot more knowledge than I did.
We decided we were going to go find an old drift tunnel which was still open. The year before I had found this tunnel, then lost it again but I knew about where it was. We spent some time hiking the area wondering why a drift tunnel would be where I found it. This area wasn't well known for drift or hydraulic mines but based on the bearing the drift was running it looked like it was tunneling into a ridge which further south had a tertiary channel beneath it.
We didn't find the tunnel that day, but as we were walking the side of the hill my friend mentioned how the country we were in was a perfect example of Lindgren's Theory of Inverse Topography.
"The what?" I asked. Like most prospectors of the Sierras I had read Waldemar Lingren's paper "The Tertiary Gravels of the Sierra Nevada" published in 1911, but I didn't recall any theory of inverse topography.
"Simple theory," my friend replied, "What's up used to be down, an what's down used to be up."
And there it is in a nutshell. The Theory of Inverse Topography. As simple as it sounds it forces you to think differently. The theory makes an awful lot of since when you start looking at where the tertiary channel ran. Almost at the crest of the Sierras.
I went back and re-read Lindgren's paper. He lays out his theory really in the first few pages so you don't need to read the whole thing, although it is one of the better papers on placer mining in California. He believes, and there is considerable proof to back him up, what are now the ridges used to be the rivers, and the modern day rivers used to be the ridges. It's as if everything was flipped upside down.
As Lindgren Explains;
Toward the close of the Mesozoic era the sediments were compressed in heavy folds, and the intrusion of granitic magmas forced them upward to lofty summits; after the intrusion the fissures and joints of granite rocks and altered sediments became filled with veins and seams of gold bearing quartz... then a long period of erosion pland down the new-born mountain.
The concentration of the gold from the veins began in countless streams..gradually the mountains were reduced to gentler slopes and the canyons widened to valleys.
Rhyolite flows filled the valleys, covered the auriferous gravels, and outlined new stream courses in the old valleys. The streams immediately began to cut their beds deeper; they repeatedly crossed their old courses and the concentration of gold in the new canyons proceeded under less favorable torrential conditions.
At the close of the Tertiary period a steaming, desolate expanse of volcanic mud covered almost the whole of the northern Sierra.
These lava flows covered the old tertiary river channels up to the rims of what was then the ridges. The later erosion of the softer ridges wore the ridges down to valleys, leaving the old tertiary channels as the new ridges.
If you spend any time at all in the northern Sierras you'll find anesite nearly everywhere. You'll also see along the roadways the exposure of the old mud flows embedded with boulders. Some good examples can be seen enroute to Alleghany. Andesite looks similar to granite in appearance, but it is much softer and decomposes readily. It is also a "hot rock" which will set your detector off.
The source of these andesitic flows were a particular type of volcano which was found near the present day crest of the Sierras. The remnants of these volcanoes can still be seen today,
...andesitic volcanoes were mainly located along the crest of the Sierras - in fact, almost continuously from Thompson Peak, west of Honey Lake. Farther south the eruptions diminished greatly in intensity. In the Downieville quadrangle imortant eruptive centers are found at Mount Ingalls, Grizzly Peak, and the group of old vocanoes around Mount Fillmore.
The reason we, as prospectors, study the historical reports is to point us towards the areas which were highly enriched with gold. You could spend a lifetime learning what the old geologists have already studied.
A good example of a modern day operation which relied on the old reports is the Ruby Mine in Sierra County. The Ruby was located specifically because it was at the bend of an ancient tertiary channel which had been mapped out by the old miners and geologists. The people who re-opened the Ruby in the 1940's did considerable research looking for just the right spot, and they found it.
If you plan on propsecting the Motherlode, Lindren's report is a must read. You can find it by by clicking here.
Reading Lingren you learn while the lighter andesite was cut away, on occasion the new rivers hit harder material such as serpentine which caused significant enrichment to occur. These areas occur on the Yuba River near Downieville and have resulted in some really rich deposits. Lingren explains areas which were significantly richer than other areas,
...the concentration of gold would be greatly facilitated by both sinking of the particles through the gravel and by a continous, though slow, downstream movement of the detritus of the rivers...it is worthy of note that the lower narrow valleys through the greenstone ridges must have acted as barriers tening to hold the gravels in the middle reaches of the streams. Along the Tertiary Yuba River, where these conditions were emphasized, we find both the richest, and deepest, gravels.
For the modern prospector, these old reports can be a Motherlode of information. By looking for the old greenstone (serpentine) dikes you can find the areas behind them which would have been much richer due to the ability of these old dikes to stop the gold from travelling further.
The gold is still out there. The old Tertiary channels are still very rich, and they're are large portions of the channels which have never been worked. Feel like opening an old mine? It's expenive. The old reports give some very good estimates of the cost of opening a drift mine. But whether you're going to use a dredge, tunnl, high banker or trommel your first step should be reading the old reports. Find the rich areas. You can start by envisioning the landscape as Lindgren saw it...upside down. Good luck out there.