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Author Topic: Evidence of an ancient river bed?  (Read 1173 times)

Offline Zorak03

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Evidence of an ancient river bed?
« on: 26/11/2015 18:47:24 »
Hello,

am trying to learn more about the geological history of the area I grew up in. I am from North-Central Alabama and the area I live in is within the Pottsville Formation so the rocks should be mostly Pennsylvanian age.
Directly under my land the Rock is almost entirely sandstone, but in one section I suspect the rock is conglomerate. I am attaching photos. If you could help me identify and confirm the type of rock I would appreciate it very much.
If it is in fact conglomerate would it possibly be evidence of an ancient stream channel? The reason I ask is the rock is composed of several rounded pebbles.
One last question, about how long does it take rock, such as sandstone and shale, to become rounded in a steam bed? I know it would depend on many factors such as flow rate, but is it a process that could be very recent or almost certainly take several hundred/thousand years?

Thank you so much for taking the time to review my question.












 

Offline Bass

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Re: Evidence of an ancient river bed?
« Reply #1 on: 28/11/2015 18:12:55 »
According to the Geological Survey of Alabama the Pottsville Formation is composed of "Interbedded sandstone, siltstone, shale, conglomerate and coal. The lower part is dominated by massive, conglomeratic orthoquartzitic sandstones..."
The rocks in photos are conglomerates. I am surprised by the amount of iron oxide, especially in the second photo.
As to your question about how long it takes to round the rocks in a stream bed- I'll give you the standard geological answer- it depends.
Rounding depends on the erosion resistance of the clasts, the size of the clasts, the other rocks in the streambed, the rates of erosion, the gradient of the stream, the length of the stream, etc.  I've been surprised by how quickly rocks can become rounded in steep, mountainous terrain- especially those areas that experience episodic, torrential rains.  Many of these rocks remain angular but a majority can be rounded in a matter of a few storms.  The Pottsville Fm probably represents near shoreline, deltaic sediments with occasional flood events (conglomerates). I suspect that it took thousands, if not millions of years for most of these rocks to erode down to their well-rounded state.
 

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Re: Evidence of an ancient river bed?
« Reply #1 on: 28/11/2015 18:12:55 »

 

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