Diminishing dimensions downstream...?

[Pitlamp]

Lots of useful stuff there Graham - thanks. (Note that, regarding your 3rd point, I was focussing on cobbles as a powerful agent of abrasion - not as a covering material preventing limestone removal, which tends to occur in more mature underwater caves which have larger cross sections and therefore lower discharge speed.)

Incidentally anyone who has walked down the Speedwell streamway below the Bung Hole will have cursed and sworn every time they tripped up and fell into the many rockmills concealed below the raging torrent. What few people realise is that these rockmills also exist in the downstream sump (which has never been drained) - albeit less frequently and less well developed. So I suppose one could argue that the still immature vadose canyon in the bottom part of the Speedwell streamway before the downstream sump is both phreatic AND vadose in origin!

(You can tell we got rained off from caving today can't you?!)

 

[graham]

Cobbles can be both agents of abrasion and agents of protection, mate. Circumstance dependant.

The marmites will be less well developed in the sump 'cos gravity will have a reduced effect there - probably!

Caving on a Monday? You reckless boy!

 

[MSD]

Rockmills are created by a whirlpool effect. The axis of rotation of the whirlpool is vertical (with the stones being whirled round and round at the bottom of the passage). What is causing the whirlpool must be some kind of obstruction or change of shape on the side of the passage, at least that's what my instinct tells me. Thus whether the passage is phreatic or vadose seems rather less significant to me. I guess there is more friction in a phreatic passage (the "top" of the whirlpool interacts with the passage roof), which might explain why they tend to be less well formed under phreatic conditions.

However (and maybe Pitlamp has a direct observation to contribute)...wouldn't a rockmill formed under phreatic conditions leave some kind of feature on the roof immediately above? Withe the water whirling around could you expect slightly greater solution due to increased agitation?

Mark

 

[graham]

Mark

Dunno about a connection with rockmills, but certainly roof pockets in phreatic caves will be enlarged by turbulent flow conditions in which preferential solution will take place. They eare an excellent example of a feedback mechanism where as they increase in size the degree of turbulence therin increases, thus causing even more preferential enlargement. Small solution features like rillenkarren and, indeed, hortonian rills on poorly soluble rocks are similar examples of feedback.

 

[Pitlamp]

To be honest MSD I've never really noticed any equivalent small dome above rockmills formed in phreatic passages but I'll make a point of trying to notice in future. Having said that - let's not forget that pioneering underwater science project in Peak Cavern not long after the war. The celebrated Torricellian Chamber (with an air / water interface sometimes ABOVE the level of either end of Buxton Water Sump) was attributed to a naturally occurring vortex in conditions of high discharge, which was replicated in a laboroatory sink with a bit of rubber tube and an upside down beaker.

However, although this is a satisfactory explanation of how the water level varies, the main reason for the existence of this kind of roof dome in sumps is water layering (as mentioned above), not water vortices.

Nowt wrong with a spot of midweek caving Graham!

 

[graham]

Pitlamp

How does water layering produce relatively isolated roof pockets?

 

[Pitlamp]

Will explain but first please can you tell us what you mean by "relatively isolated"?

 

[graham]

Try "distinct" or "individual"?

 

[Pitlamp]

OK, well to save me a lot of typing, if you have a glance at Cave Science, circa 1991, in the forum section - there's an explanation there. If this means nothing to you let me know and I'll either root out my copy or just summarise it here.

 

[Pitlamp]

I just looked it up: Cave Science 18(3), Dec 1991, page 159.

 

[graham]

I just looked it up: Cave Science 18(3), Dec 1991, page 159.

If I take that off the shelf now, a load of stuff will fall on my head, I'll have a look later, thanks.

 

[footleg]

In one of our discoveries in Matienzo we found a roof feature which was an almost perfectly circular dome about 1m in diameter but about 2m tall, going straight up. This was in the roof of a phreatic passage. I was very curious about how it formed. I should try and take a photo of it when I return to that location (it's a bit of a tortuous trip to that part of the cave, but we have some unfinished business up there).

 

[graham]

I would guess turbulence, though Pitlamp may not agree. I would also guess that the height it reached was to the level of the local piezometric surface, after which it could get no higher.

Another possibility is a very big snail (Check out helixogenic cavities in limestone)

 

[Pitlamp]

Graham - I can't really agree or disagree without seeing it. But if Footleg can get one of his excellent pictures of it then I probably will!

 

[Scoff]

As usual, a late entry to the conversation (thinking that it was probably a thread on increasing beerguts making cave s seem smaller).

Anyway - going back a bit - an example of a vadose passage being underwater can be found in Vai Nauri, Mitiaro, Cook Islands. A rise in the sea level since a previous ice age is theorised to have caused the cave to flood. Numerous underwater stals show the cave to have been dry once upon a time, and a splendid rift passage climbs up a series of 'cascades' before emerging in a pool at the base of a daylight shaft, not too far away from a lake. Theory suggests the passage was previously a vadose system carrying water from the lake to the sea...

Like Pitlamp, I am all too aware of the power of the water in submerged passages, and the mechanical erosion this can cause. In upstream Hurtle, for example, a submeged 17m deep shaft has smooth pollished ledges with obvious impact marks & scratches on them - very similar to the 'scoured' rock just inside the Goyden Entrance. I have previously laid ordinary diving line down The Void and it broke. I then tried 11mm Bluewater it broke. Next we tried one inch galvanised stell chain and that eventually broke, and more recently 11mm climbing rope (rigged as near as possible in an SRT style). This was subsequently found to have been cleanly sliced through at a point previously hanging in mid space. Two weeks ago this rope had again been sliced through in various places, and sat in a heap at the foot of the shaft....The point is, debris clearly gets washed down the shaft and causes significant impact - even though the 'weight' of the boulder is reduced by its displacement in water & thus gravity has a reduced effect compared to normal debris on the surface.

Having said that, for the many years since I started potholing I had thought of 'vadose' and 'phreatic' in the classic way. But now thinking about it...

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? On the one hand, a rock roof might slow the flow down (drag, eddies, etc.) but equally there is marginally more pressure on the floor due to wter pressure (weight of water from the water surface....

Again the point re: cobbles or pebbles having a use as an abrasive scourer as it moves, whilst acting as a protective cover while stationary is food for thought. Similarly, would a boulder or cobble which was too big/ heavy to move cause an eddy behind it which would cause additional wear in the floor - a bit like the rim of a scallop then a deep bit caused by the eddy.

And getting down from big, scraping blocks & pebbles, what about microscopic specks of rock hitting microscopic ruggosities in the floor, walls & roof, which have already been subject of corrosion attack and are consequently easier to wear off?

All in all, perhaps for cave divers, there is more of a blur between 'phreatic' and 'vadose'.

That is, until you need to bolt for the surface in haste..... then you know what 'phreatic' really means.

Thanks again for a thought-provoking discussion, chaps.

Scoff 

BPC/ CDG

 

[Pitlamp]

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).

 

[graham]

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:

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.

 

[Pitlamp]

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 . . . )

 

[dl]

Graham,

Nice photos, can you tell us where they are ?

 

[Brains]

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