How can a river bed be on a mountain?

[Bluereg]

Exploring today I came across what was definitely an old river bed, but it was on top of a ridge in the mountains.
Im new to gold hunting, was very excited, extremely curious to know geology of how this has happened

 

[RM Outback]

Millions of years ago the mountain weren't there then they were and the course of the river changed, in simple terms. I have no doubt you will receive a reply from Goldierocks or one of the other geologists who will provide you a detailed explanation.

Welcome to PA :Y: :beer:

 

[Simmo]

Yup. Over time land sinks and rises.
We too have found alluvial washed quartz on top of hills, along with gold.

 

[davsgold]

Exploring today I came across what was definitely an old river bed, but it was on top of a ridge in the mountains.
Im new to gold hunting, was very excited, extremely curious to know geology of how this has happened

Inverted Landscape, Google it, but essentially a volcanic eruption and lava flowed down the old river and capped it, and it being way hard the landscape eroded away from there causing the once river to become higher than the land that was around it.

The water still had to go somewhere and cut a new river system.

Even the Macquarie river in NSW is an example of this (Paleo) and the old river system is way above the present river, and where you see river gravel coming out of a hillside and the old basalt cap (decomposing) above it is a good indication of that happening.

In places along the Macquarie river there are Audits (tunnels) into the hillsides under the basalt cap following the gold rich river gravel.

cheers dave

 

[UnderEmployedGeo]

It would be a very long post to explain completely, it's almost like a microcosm of all of geology. In a nutshell tho:
Rivers don't last forever. They only run while there is higher ground to runoff from. Rivers are the conveyer belt that takes all the rock being eroded off the hills into the sea.
Sometimes though, they get interrupted, maybe by volcanic lava flowing down the valleys (western Victoria - the Newer Volcanics did this and buried old river valleys - these became "deep leads").
Other times mountain building happens - the folding and faulting pushes rocks up, and that can take these ancient valleys with them. What you see today is not the surface gravel like you see in modern rivers, it's the the deeper material - if you drilled down into a deep valley system you would find that gravel becoming a consolidated rock "conglomerate" - and that's often what these uplifted ancient valleys are.
In other situations, river valley systems get buried deeply and hot fluids pass through them to create weird metamorphic conglomerate material that's full of gold! Witwatersrand Basin the prime example - Ventesdorp Contact Reef is a classic of this...
Ahhhh too much to type on my phone lol

 

[user 4386]

Inversion of relief occurs when materials on valley floors are, or become, more resistant to erosion than the adjacent valley slopes. As erosion proceeds, the valley floor becomes a ridge bounded by newly formed valleys on each side.

(2) (PDF) Inversion of reliefA component of landscape evolution. Available from: https://www.researchgate.net/public..._of_relief-A_component_of_landscape_evolution [accessed Jan 31 2021].

It does not have to be a lava flow - sometimes it can be the river gravels themselves that are more resistant.

This is what the area looks like in 3D (the cleared area is the ridge or plateau - because it is flat and the lava soil is fertile, it is the only part of the area farmed)

A 3D view looking east, So the lava that now forms the ridge top once flowed down a valley

 

[UnderEmployedGeo]

Western Victoria is a fascinating example of a whole bunch of geological phenomena. But the Newer Volcanics are particularly spectacular in their coverage

 

[Greenhornet_au]

Wow, great question and great answers, always learning....

O :Y:

 

[aussiefarmer]

Some of the places I have found alluvial gravel makes me believe they are parts of old glaciers before the world changed shapes.

The thing that gets me is how did the old guys know to dig 100ft down into a hill that shows no signs to me of being worth the trouble.

 

[A-team]

Perfect example: Dollys Creek. River wash under a few inches of topsoil at the top of the hills, clay at the bottom of the hills....someone must have tried to turn Dollys upside down to shake the gold out?

 

[user 4386]

Perfect example: Dollys Creek. River wash under a few inches of topsoil at the top of the hills, clay at the bottom of the hills....someone must have tried to turn Dollys upside down to shake the gold out?

Just an old river bed that other streams have cut below, leaving it perched high and dry.

 

[goldtrapper]

Some of the places I have found alluvial gravel makes me believe they are parts of old glaciers before the world changed shapes.

The thing that gets me is how did the old guys know to dig 100ft down into a hill that shows no signs to me of being worth the trouble.

I'd like to know too!

 

[user 4386]

Some of the places I have found alluvial gravel makes me believe they are parts of old glaciers before the world changed shapes.

The thing that gets me is how did the old guys know to dig 100ft down into a hill that shows no signs to me of being worth the trouble.

I'd like to know too!

They were smarter than we give them credit for, and there were usually obvious reasons for those with experience. Although I know of shafts sunk 1000 feet without good reason, and without finding anything, in the 20th century. And in the 1940s people were still sinking shafts using divining rods (and mostly finding nothing).

Mot much glacial alluvial in Australia. a lot in New Zealand.

 

[StoneTheCrows]

I think there is something similar at Tuena
The hill that rises to the east of the Chinese Hut has a lot of river rocks and pebbles 3/4 of the way up

 

[Guest]

G'day

In one place I prospected some years ago I found round river rocks near the top of a hill, they looked exactly like the stones that you would get in a fast river bed all well worn and rounded, not many large ones mainly smallish to medium sized pebbles really, the bigger ones were probably buries a bit deeper, the area has no flowing water other than when it gets a rain storm, and no permanent water holes that I have ever seen there, I did however have a a bit of a detect there but didn't find anything and have not been back to that location since so who knows, but it did display to me that the ground can change quite significantly over millions of years so it also answers the question of why you can find gold in areas that show no features or structures that would indicate a source of the gold as they are long gone.

The old timers were far better at reading the ground than we are, and better understood the indicators that would lead to them sinking shafts to follow ancient river beds for the gold, sometimes when you look at diggings you get to wonder what made them decide to dig there in the first place? the only conclusion I can come to is two things either they could see something I can not, or it was just pure speculation?, but they also sometimes probably followed suit off previous discoveries there and also used other methods such as loaming or as mentioned devining?

Also the situation for the old timers was very different to what we have today, back then you often got to the area after some grueling travel on foot pushing a barrow or horse back or cart if you were lucky, and you just had to try and make it pay or you didn't eat, not like us we can just bug out and drive to somewhere else with no drama, so they had a lot more at stake than we do.

cheers

stayyerAU

 

[UnderEmployedGeo]

The old timers had an appreciation of the geological setting for gold, so they knew what they were looking for. Emphasis on "looking". They were mining visually - following the reef as far as they could see it and as long as gold was coming out of it.

(Still today in mining the mine geologist will mark up the ore zone after the geochem is done - the geo will include likely ore around the samples that the geochem might have missed - that's still a visual technique)

As for locating the reef in the first place ... Panning up the stream, loaming the valley sides, digging costeans across possible quartz veins until one of those structures is carrying gold. A lot of it is visually done - quartz blows are quite obvious things sometimes.

A big reason you can't see what the old timers were chasing in these shafts and pits is that they literally dug all the good stuff out. There's nothing left to see.

To be honest modern gold exploration has a lot in common with the old techniques, we just do it more systematically across bigger areas and with far more sensitive geochemistry and of course geophysics. But "gold finds gold" is still surprisingly true, and let's face it, all this fancy technology has not even found ONE new deposit in Victoria!

 

[user 4386]

I would question the last sentence. Nagambie would fairly much qualify, as well as completely unworked extensions of past ore zones in areas unrecognised by the old timers, discovered by things like soil geochemistry. Others that are completely new have been found that are not yet economic (but which will almost certainly be mined one day). However I agree with the general thrust of what you say about Victoria. However it is not true in other States, with many completely new discoveries being made. The reason is that coarse gold is almost always present in Victoria, and almost everything discovered is in hilly ground with fairly good exposure. There is less potential for things to be discoverable beneath fairly shallow laterite cover etc.

Another factor is that gold is extremely common in most mineralised areas in Victoria, so much outcropping bedrock of early Palaeozoic age contains some gold mineralised quartz veins, even if little more than mm threads. Everywhere from St Kilda to central Melbourne to Waratah Bay beach to along the Hume Highway. And they dug holes on everything they saw - many shallower shafts were complete duds in terms of finding anything economic.

 

[DrDuck]

Another example is the gravels at the top of the Lapstone Monocline, and exposed in places in the cuttings for the Lapstone Zig Zag railway as it cuts into the Monocline, just to the West of Sydney at the start of the Blue Mountains. The gravels (called the Ricabys Creek Gravels) are thought to be deposited by a palaeo-Wollondilly/Nepean River system, prior to the uplift of the monocline. As the uplift happened the beds were stranded as the drainage was pushed east. These, unfortunately, are not gold bearing gravels, but are interesting nonetheless.