Digger Bob
New member
Ok, you asked for it. Here is my theory on how this particular rock formation occurs. I may be totally off base and a real geologist can chime in and correct me. But I think I'm pretty close based on my research and field observations.
First you have to imagine what things were like 65 million years ago out here. There were no Sierra Nevada mountains. Where they are now was gently rolling hills with massive rivers generally flowing north to south. These rivers eroded the gold out of the viens and concentrated it on bedrock. Then the earth went through a period of volcanic upheaval. In this part of the country, this activity was NOT tall volcanos exploding and spewing flowing lava like you see in Hawaii these days. What happened was that cracks opened up in the crust and hot volcanic MUD flowed out, not lava. This is important. This activity went on for thousands if not millions of years. The mud spread for miles and miles all around, layer upon layer, century after century.
Now, whenever you have a massive amount of liquid flowing, it naturally seeks the lowest point and flows following the laws of gravity. Where were the lowest points? The rivers of course. As the hot mud flowed down the rivers, instantly evaporating the water, it picked up the gravel and rock in the river bottom and carried it along adding to it's mass and weight. Can you guess what else it picked up along with all that gravel? Gold, of course. The gold that was concentrated in the river bottoms was sucked up and added to the the flow.
Eventually the mud stopped flowing and cooled, forming solid rock. This rock now contained not only the original volcanic mud but all the cobble, rock, sand, and gold that made up the bottom of the (now) ancient river beds. Of course it didn't get all the gold, just what was "easy" to grab. It is these ancient river channels that were so rich and sought after by the hydraulic miners. And the subsequent mud flows buried these channels under hundreds of feet of rock. Then the mountains uplifted putting those old river channels high up on the mountain sides.
Through eventual erosion, parts of these channels were exposed, reconcentrating the gold in the modern rivers. But the evidence of the old channels were there if one knew what to look for. This mud flow rock we call conglomerate was a tip off. The 49ers learned that directly underneith the conglomerate was the bedrock of the ancient channel, since that is what it was resting on when it finally cooled and solidified. So, they washed away the hills and mountains wherever they saw conglomerate. There were other clues too, but that's another story.
So, now, with modern metal detectors, we can find the occasional nugget that was imbedded in that conglomerate. The rock is very hard and did not break up well even under the tremendous force of the hydraulic monitors. In some "pits" there are acres of these rock piles most small, but some very large. See the picture attached for what a large piece looks like. Look closely at the picture and you can see the mulitude of quartz cobble, sand, and gravel all imbedded in the host rock. Notice the rock he's standing on. You can see the flow pattern of the hot mud before it solidified.
A specimen like the one shown is not one I would break up just to get the gold. It's unique enough in it's own right to demand a premium if one should decide to sell it. I have seen bigger chunks with bigger nuggets imbedded but most are smaller than that shown.
Digger Bob
First you have to imagine what things were like 65 million years ago out here. There were no Sierra Nevada mountains. Where they are now was gently rolling hills with massive rivers generally flowing north to south. These rivers eroded the gold out of the viens and concentrated it on bedrock. Then the earth went through a period of volcanic upheaval. In this part of the country, this activity was NOT tall volcanos exploding and spewing flowing lava like you see in Hawaii these days. What happened was that cracks opened up in the crust and hot volcanic MUD flowed out, not lava. This is important. This activity went on for thousands if not millions of years. The mud spread for miles and miles all around, layer upon layer, century after century.
Now, whenever you have a massive amount of liquid flowing, it naturally seeks the lowest point and flows following the laws of gravity. Where were the lowest points? The rivers of course. As the hot mud flowed down the rivers, instantly evaporating the water, it picked up the gravel and rock in the river bottom and carried it along adding to it's mass and weight. Can you guess what else it picked up along with all that gravel? Gold, of course. The gold that was concentrated in the river bottoms was sucked up and added to the the flow.
Eventually the mud stopped flowing and cooled, forming solid rock. This rock now contained not only the original volcanic mud but all the cobble, rock, sand, and gold that made up the bottom of the (now) ancient river beds. Of course it didn't get all the gold, just what was "easy" to grab. It is these ancient river channels that were so rich and sought after by the hydraulic miners. And the subsequent mud flows buried these channels under hundreds of feet of rock. Then the mountains uplifted putting those old river channels high up on the mountain sides.
Through eventual erosion, parts of these channels were exposed, reconcentrating the gold in the modern rivers. But the evidence of the old channels were there if one knew what to look for. This mud flow rock we call conglomerate was a tip off. The 49ers learned that directly underneith the conglomerate was the bedrock of the ancient channel, since that is what it was resting on when it finally cooled and solidified. So, they washed away the hills and mountains wherever they saw conglomerate. There were other clues too, but that's another story.
So, now, with modern metal detectors, we can find the occasional nugget that was imbedded in that conglomerate. The rock is very hard and did not break up well even under the tremendous force of the hydraulic monitors. In some "pits" there are acres of these rock piles most small, but some very large. See the picture attached for what a large piece looks like. Look closely at the picture and you can see the mulitude of quartz cobble, sand, and gravel all imbedded in the host rock. Notice the rock he's standing on. You can see the flow pattern of the hot mud before it solidified.
A specimen like the one shown is not one I would break up just to get the gold. It's unique enough in it's own right to demand a premium if one should decide to sell it. I have seen bigger chunks with bigger nuggets imbedded but most are smaller than that shown.
Digger Bob
