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  • Writer's pictureLori Dee

High Benches and Ancient Riverbeds


I have discussed places to look for gold in rivers, streams, and creeks. Those are the most popular places to look for gold because that is where placer gold is concentrated.


Think about it, the easiest and most common places to look will be the places where most people look. Places that just might be prospected out until the next major flood. I like to look for out-of-the-way places where it is unlikely that everyone has brought their family for the weekend to do some panning. After all, if I hit a big strike, I don't want a bunch of people watching me and wondering why I am dancing in the middle of the river.


Looking for signs of ancient rivers and high benches above existing rivers can turn out to be quite beneficial. Most people think of prospecting as panning, sluicing, or dredging - all of which happen in the water. Many people do not want to prospect in the winter for this reason. It can be cold and miserable work. But some high benches and ancient rivers may not be anywhere near water. First, another look at how river valleys are formed. Valleys with a rounded U-shaped bottom were likely cut by a glacier. An example of that can be seen in the topographic map of the state of South Dakota. Notice that huge gouge in the east? That was caused by a glacier from the Ice Age. Not all valleys are small or narrow.


If the valley is narrow, with steep walls like cliffs, then most of the erosion happens at the bottom of the river channel. That indicates fast-moving water, which also tells us the river bottom will have a steep slope. Fast-moving water has a lot of energy and can carry heavy sediments, gravel, and rocks.


If the valley is more V-shaped, then the sidewalls of the valley are also eroding down into the river. This happens during floods when the river channel is filled or overflowing with fast water.


As the river flows downstream, the river bottom slope becomes less steep from all the rocks and gravel that got pushed from upstream. This causes the water flow to be a bit slower. Slower water causes streams to meander from side to side, causing bends. Slower water has less power so will drop its heaviest materials. Water is slowest on the inside bend of a turn, so more material will be deposited there, forcing the river to flow around the obstacle.


As the river travels still further downstream, the slope becomes flatter, sometimes almost level. This is where the water will be the slowest, and bends will be plentiful, with almost no erosion happening. This is where most of the sediments will settle.

Sometimes rivers changed course or dried up, but later came back and began cutting a new channel in the bottom of the old valley. We can tell much about how the river is flowing by the type of channel it is cutting.




We are not interested in slow meandering streams because they lack the force needed to transport something as heavy as gold. However, we cannot judge the river by how it is flowing now. Look carefully at the topography of the area. If gold was found nearby, it might be worth a look. If the evidence is something like historic gold mines in the hills above the valley, we will definitely want to look to see if any gold washed downhill. Try to imagine how the river flowed during a massive flood, with water 20 or 30 feet overhead.


Here is an example of a local creek that flows a short distance from several old gold mines. Notice how the creek just sort of wanders back and forth as it slowly meanders downstream. The light green grassy area on either side is where the water flows during a flood. Look at how wide that valley is: from the limestone cliffs to the foot of the hills to the West. This is a clue. Maybe there was a massive flood and this whole valley flowed with water. Or maybe this creek has been changing course throughout its lifetime. Let's look at what we know.


The limestone cliffs were eroded early in its history. That is the highest elevation with erosion, so at some point, the creek flowed at that height. Notice that if the creek followed the cliffs, it would be flowing in almost a straight line due south. There was some kind of disruption, and it started dropping rocks and gravel, forming a gravel bar. Maybe large rocks eroded and fell from the limestone cliffs and dropped into the water flow. The creek had to flow around the obstacles and that created a bend. The creek continued to deposit material on the inside of the bend, so the bar kept getting bigger and bigger. Eventually, the creek was pushed over to its current location. So far, all of that fits with what we know about river flows.


Now, look at all the material that was deposited on the inside bend. That area, from the creek over to the cliffs, is now high and dry with trees growing out of it. Think about how much water had to be flowing in that creek to move that much dirt. The water had to be deep enough to cover that area, at least during the annual Spring runoff. If the water was not that high, it could not deposit anything up there. The water had to be moving with enough force to deposit that material. There may be sand on top from when the water slowed, but I would bet there are some large limestone rocks underneath. Also, notice the bench where the dirt road is along the west side. That bench is about the same height as the base of the limestone cliffs. So maybe the water flowed at that height, and then during a major flood, it was higher still. That is a lot of water that can be washing gold out of those hills. And we know there is gold in the hills because there are gold mines there.


Sometimes you may find an ancient riverbed where there currently is no water at all. Prospectors have found ancient riverbeds on top of mountains. If you are scouting a dry area, even in the desert, use what you know about how rivers form valleys. In the desert, the main erosion is from wind, but it may not have always been a desert. And it does rain and snow in the desert.


Use your maps and Google Earth to find valleys that look like they may have been caused by water erosion. Then check the valley walls for any sign of river rocks -- smooth rounded stones like you would find in a river with water flowing. Always pay attention to soil that has a different color or texture from the majority of the surrounding soil and rocks.


Always watch for exposed bedrock. Very often, ancient riverbeds were tossed up by geological processes and left the bedrock exposed. See if it is smooth like it was polished with moving water. If so, check any cracks to see if they contain gold. Finding even a little gold might make it worthwhile to bring a pry bar, sledgehammer, and chisel to break open the cracks and thoroughly clean them out. If the bedrock is at an angle, be sure to follow it downhill, cleaning as you go. Gold will concentrate at the lowest level.


I have heard stories of prospectors who located ancient riverbeds and exposed bedrock in remote areas and spent years cleaning out every crack, breaking open the rock, and cleaning even more. Just take your time and be thorough.


Good luck!



 

All content Copyright 2021- 2024 - Lori Dee


 


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