Author Topic: Diminishing dimensions downstream...?  (Read 9941 times)

Offline Pitlamp

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Re: Diminishing dimensions downstream...?
« Reply #75 on: January 23, 2009, 07:16:50 am »
An interesting one that Scoff - caves changing from vadose to phreatic (in your example due to sea level rises). In England it's more common to think of caves first being phreatic then changing to vadose.

There is one unusual example - in a British cave system which you are extremely familiar with. The downstream sump in GG's Booth Parsons Extension is 45 m long and surfaces in the "big stuff" in the original Hensler's passages. It's has the morphology of a phreatically formed bedding plane tunnel but it's got underwater stal pillars in it. So here's an example of a passage which formed phreatically, was drained to become vadose (when the stals grew) - then reverted to phreatic again (for reasons which are not as yet understood).

I reckon Footleg's roof dome is likely to be due to water layering Graham - just like the fine examples in the Five Arches passage in Peak Cavern for example (one of which is close to being symmetrical).


Offline graham

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Re: Diminishing dimensions downstream...?
« Reply #76 on: January 23, 2009, 08:49:16 am »
So, nobody liked the idea of giant snails then ...

I haven't managed to unearth that Cave Science yet, Pitlamp, sorry (I shall do so) but I must reiterate my support for the understanding of the power of mechanical erosion in caves.

However:

Quote
Doesn't the water in the bottom few centimetres a metre deep stream, and the debris it carries, behave in the same way - irrespective of whether the uppermost surface of that stream is in contact with air or in contact with a rock roof?

Of course it does! the difference between vadose and phreatic is that in the latter, the roof can be eroded as well. Whilst it is important to look deeply into these things, it is worth pointing out that this



would not have formed except if wholly underwater and this



formed due to downcutting under an air surface.

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Offline Pitlamp

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Re: Diminishing dimensions downstream...?
« Reply #77 on: January 23, 2009, 09:42:36 am »
Indeed - and in fact the roof can be eroded MORE THAN the floor under phreatic conditions, hence all the high joint guided roof domes (without corresponding floor pits) just around the corner from where your fine picture of The Tube in Peak Cavern is taken. Water layering is key to understanding this. It can all be directly observed by divers. (I do a fair bit of diving in caves and I have to confess I've never met many roof boring snails . . . )


Offline dl

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Re: Diminishing dimensions downstream...?
« Reply #78 on: January 23, 2009, 10:45:41 am »
Graham,

Nice photos, can you tell us where they are ?

Offline Brains

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Re: Diminishing dimensions downstream...?
« Reply #79 on: January 23, 2009, 12:10:14 pm »
Graham,

Nice photos, can you tell us where they are ?

The first is Peak Cavern as Pitlamp says, the second is in Mammoth Cave, USA

Offline Rob

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Re: Diminishing dimensions downstream...?
« Reply #80 on: January 23, 2009, 01:44:16 pm »
The first is Peak Cavern as Pitlamp says...
And it should be noted (preferably by the original poster) that the photographer was Brendan Marris.
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Offline graham

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Re: Diminishing dimensions downstream...?
« Reply #81 on: January 23, 2009, 03:17:55 pm »
And it should be noted (preferably by the original poster) that the photographer was Brendan Marris.

Is it? Sorry, I simply found it by a Google image search and the Flickr page does not give the photographer's real name.
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