Good evening everybody!
We’re back again!
So tonight we’re going to talk a little bit about “why does placer gold pile up?” Why do rocks and cobbles and gold congregate in concentrations? What makes that happen? We’re going to talk a little bit about some hydro dynamics. That’s a big word, but it’s water flow in motion. In fact, really gold is the study of geo-morphology and hydro-dynamics. That’s not general geology, but the geology specific to placer gold, and so you get an awful lot of information or insight into where to look for placer gold and how it concentrates.
If you know a little bit about, you know, fluid physics, (fancy topic,) but the real simple part is there are some things you can understand about flow of water and how it drives gold, and specifically how when it meets obstacles in the water path, it will cause certain flow changes that make the gold and cobbles sort out in such a way that the gold will tend to concentrate, and the cobbles will tend to spread out. Actually, not so much the cobbles as rest of the sands and lighter materials will disappear. You can always hear everybody talk about how riffles are kind of an artificial form of natural concentrator, like a stream bed.
That is true, but the fact of the matter is, the stream bed has always been there and always will be. It’s THE original gold concentrator. And so what you need to understand is how it works. That will help you understand riffles, and all kinds of matting and stuff that can go into your sluice box. But the fact of matter is you’ve got to first start with the stream bed. So let’s take a look at what that means. Also, again, I’ve got the https://SourdoughMiner.com/20-20/ prospecting related to seeing how water flow moves gold.
Let’s see if I got this thing right. So tonight we’re going to be focusing on this. There’s more of this in the 20-20 prospecting thing I’ve been talking about. We’ve got more floods and storms coming in. So guess what, we’re going to be on this topic for a little while longer. Next week, we are supposed to actually, maybe a little later this week even. We’re supposed to get the leading edge of another atmospheric river hitting California and heading on east. So, you know, it’s all connected.
There’s this Arctic blast that’s hitting the east coast right now and freezing everybody and, causing freezing and warming that breaks records for the cycle speed. That same kind of behavior in terms of atmospheric climate behavior drives what we call an atmospheric river. That’s basically some heavy, dense rain storms that come in off of the Pacific, typically coming up from the tropic and meeting cold air, and moist air coming in from Alaska and the arctic, and the two collide and make a real mess. But what they do is they drive a whole lot of water in a big hurry, especially in the Sierra Nevada, or any of the coastal mountains here in California, or anything up toward Oregon and Washington. So they’re getting plastered with snow right now. Same kind of conditions building.
So what we’re going to talk about today is what happens when that stuff comes down at the high altitude elevations and starts to move its way toward the ocean, as we’ve talked about earlier? What exactly is going on there when it comes to gold and how it affects the gold? So I’m going to draw some pictures on our blackboard again tonight, and we’ll spend a little time talking about how gold concentrates, and where you should start looking. So let’s take a look. Go to the blackboard. So why does placer gold pile up? Yeah, well that’s a good one. So if we look at placer gold, we can see that the basic concept is that we have these mountain ranges, a big tall thing.
I’m going to draw it as a cone, and on that mountain will become a stream that will typically come pretty steep. As it starts to level out, it’s going to start winding. Now why does it wind? Well, that has to do with the changes in elevation and what’s going on with the contours or topology or topography of the ground. So when things start shifting around, and also with the geology, when the rocks get softer or harder, the water cut makes a change in the river, and the river will actually cut into the softer material, preferentially, compared to the harder stuff. So it tends to turn a corner. When those corners are cut, there’s some things that go on. I’ve talked about that in the helical flow, you can find that elsewhere.
Basically, the inside edge of these bends is what you want to be focusing on all the time, even up in the mountains, because the water will tend to favor that inside edge. There will be a gold pay streak that will form along that edge, and it will tend to kind of cut across as an s curve. So it’ll start in over here, straight line through the mountains, and then it will cut in as the water flow kind of meets this thing. So we’ll make this s curve zooming across the tips of these bends and forming your pay streaks across these things. You can kind see how I’m really botching this. But you got the idea.
So what we’re going to be talking about is why does it do that? Well that that’s covered in helical flow, but the idea is that it always sweeps towards this inside edge of the bend. Let me give it a new crayon. So the gold will be swept as it makes this turn through the center of the creek, because of the toroidal or helical flow that’s going on here. There’s not toroidal, it’s helical, so it’s like a coiled spring.
It sweeps like a big street sweeper toward this inside here, and toward this inside over here, and so on. And that pattern just continues until it makes its way to the ocean. Now that’s one thing you want to be aware of, and so the principal is primarily you start here. Why would you vary from that? Well, that might happen, if you happen to come across an area that gets cross cut, forms an Oxbow Lake or a cross cut, that is, there’s a piece of stream bed that goes through here.
Now all of a sudden your gold no longer concentrates here, and let’s say it goes and cuts like this, so let’s see if I can do this. Nope, we can’t. So the idea is it may start concentrating over here, because of changes in how the stream is cut in the direction of flow over ancient history. It might show up in a completely different side than you would expect to see, and I hear this all the time. I hear prospectors telling me, no, I know that that business of going on the inside edge of the bend isn’t always true, and the answer is absolutely not, but what is true, is the physics of what drives it, will be that helical flow. That’s a root cause of why! The question is what happened earlier this week, because that’s a root why for changes you may see in the stream bed due to ancient flows, which you can’t see unless you’re looking really closely at the cobbles like we showed earlier, where the cobbles formed that sill a couple of days ago and you had a sill.
Well that sill was from ancient flows, not this year’s flood. It was from floods maybe thousands of years ago. The same thing holds true with any stream bed, and therefore you’ve always got to be open to the possibility that there might be gold on some other place, on some other side of the creek, and you need to go every once in a while sample on the wrong side, go over here and look, because it’s possible that x marks the spot for an ancient flow that concentrated a lot of gold, so be open to that, but just know that in general that gold is going to be much older, or it’s going to be in an area that should show some kind of disturbance from a recent, or more recent, as in the last hundred or thousand years worth of floods.
There might be some indicators that would tell you that, like a high bench, or some kind of a rock and cobble interface that just doesn’t look right. It’s broken up, or there might be some old trees that are now, completely degraded, but there’s some indication that a flood came through that area and now it’s all kind high and dry away from the creek. That doesn’t mean it’s not worth checking. It means it is, especially if it has an indication that at one time the stream flowed through there. Then you want to look again at the inside edge of the bend, but for that flow, not the one you’re looking at today. So keep that in mind as you’re going through this.
Now, let me erase this mess, so I can start again with something a little different.
So now let’s look at our flow and cross section as we go through our creek. And so we’re going to look at the stream bed and what happens inside a stream bed. This is bedrock, and as the bedrock pops up a little bit let’s give it some dips and wiggles that might be due to slates, granite intrusions, changes in rock, or that kind of material. So again, this is all part of the bedrock. So what’s going to happen to the flows as we go down through this area is that we’re going to see something.
I’m going to draw the flows in a color that really stands out, let’s use red.
As the flow comes down here, it’s going to tend to go up, and then, because of hydrodynamics, it’s going to want to curl. Now why does it do that? Because it forms a low pressure zone on the backside of this lump. That low pressure zone, like a riffle, will tend to draw the water back around, and then eventually it’ll work its way out. However, in the meantime, it’s driving material into this zone right here. And that’s where you’ll find rocks and cobbles will tend to concentrate and gold will tend to hide out. That’s one of the hiding places.
It’s essentially a gold concentrator, that you will want to keep your eyes out for. Now, if on the other hand, we go in there and we find that in a similar place, we have now found some big boulders. The boulder has the same basic property, and that is that as we get going, we’re going to look in there, and we’re going to see that this same stream flow continues on down, and now something weird goes on here, and that is it’s going to go over the rock and it’s going to do its thing. Just like we did it here earlier, head on downstream.
X marks the spot, but there’s another property that goes on with hydrodynamics, and that is, as the flow speeds up over the top of this lump, which it will do, it’ll come down here and it has to change direction. Anytime you change direction drastically, there’s going to be a drop in speed, and if you recall, whenever you drop in speed, look for a drop in gold.
So one of the things that can start happening, is this leading edge can start concentrating heavier materials, because there’s another eddie going on here as well. It kind of tucks away a little differently, but the idea is it’s going to be curling up, and forming a little pocket worth of stuff on the front end here, and so what ends up happening is in all of these different places that I’ve shown, you’re going to end up finding that there’s a tendency for gold to concentrate here, here a little bit, very much here, and possibly, very interesting, under here, because both of these will be driving to the middle of this.
That’s where nothing moves until this bolder moves, and if this is a big enough boulder, that means that only 100 year or thousand year floods would have moved gold, and it would have had all that time in between those big floods to concentrate gold. This is a natural gold concentrator. Very important to keep your eyes out for this kind of thing. Sometimes you’ll find these boulders are huge. Sometimes the size of houses! Very carefully look around it, and if you’re in a diving situation, be careful that you have a buddy system, and you use the right precautions, because a boulder can drop right in on top of you inside the hole.
Even the smallest boulder can seem kind of inconspicuous and light, and underwater it feels like it weighs significantly less because the water it displaces helps buoy the boulder up, so larger cobbles are easier to lift in water because of that fact, but they are still pretty darn heavy and they will pin you to the bottom. And then once you’re down there, you’ll lose a limb, or a life, not worth it. Gold is not worth that kind of stuff, but the fact of the matter is, with the right prying, the right safety equipment, and maybe some cables and netting, you can move that boulder enough to make a safe journey out of your hole, to go down and find this spot underneath.
You saw a bit of that happening in gold rush when they went way down under the waterfall. Finally they got to bedrock, and they started finding old boulders and things like that. They were making a mess out of what they were trying to do because they were, as usual, doing it in a funny way. The fact is that the water started collapsing their walls and everything started kind of coming in on them, but the fact is this gold was buried deep underneath those cobbles, those big boulders. They weren’t just cobbles, they were boulders, and so that’s a gold trap. Any one of these can form a gold trap, and it’s worth your investigation to find out.
There’s a lot of other things that I go into in 20-20, and a little bit in helical flow. I’ll be talking about it in rocks and geology, because slates form a natural riffle, and so forth.
So at https://Sourdoughminer/20-20/ , you can go find more on that topic, and get into more detail about where gold concentrates, and what helical flow does to drive gold and all that kind of stuff.
You want to have that 20-20 vision to be able to see where gold is hiding.
So I’ll let you go. Good prospecting! Good Night!