Passage size - Phreatic vs Vadose

[graham]

I presume the passage is larger for a good length, and not just on that corner? Certainly downstream where the vadose incision begins the phreatic roof tube seems back to 'normal' dimensions.

It's larger for a good length. The present stream cuts left into a steeply descending trench & after that teh size of the passage becomes more and more obscured by sediment, but remains larger than it was in the upstream vadose section.

 

[Pitlamp]

Just one thing to bear in mind; the idea of equal solution in all directions under phreatic conditions was alluded to above - this simplistic model usually doesn't apply. Our problem is that most researchers (understandably) only have the opportunity to study DRAINED phreatic passages. Us sump floppers regularly come across layered water in active underwater caves; the layers probably have different chemistry (yet to be studied in the UK) and often different temperatures, resulting in sufficient density differences to cause the stable layering. Obviously this results in unequal dissolution rates.

I like Langcliffe's useful comment above but I'd also point out that many phreatic passages are actually far bigger than is needed to accomodate the flow even in extreme conditions, so perhaps his observation isn't quite as useful as you might at first think (but still useful).

Rob - I'm not convinced that you can draw any completely reliable conclusions from what you're trying to understand here - but perhaps you can recognise at least a shift in the balance of probablities? In case it helps though; I'm familiar with all the lower level passages in Giants and also with the underwater part of East Canal. I'd say (without being able to quote hard data) that the size of East Canal underwater is bigger than the combined development of the two approach passages. This in itself is actually less useful than it might appear due to the involvement of mineral vein cavitation in that area. There really is a large number of variables . . . . .

 

[Fulk]

Pitlamp: By langcliffe's comment above do you mean:

It's almost inevitable as a phreatic passage only needs to be as big as is needed to accommodate the water flow

I don't think that this is (necessarily) true. In the case of a Vauclusian rising the water cannot 'escape' as it were, except by overflowing, so it will continue to erode the passage for as long as it is 'trapped', and thereby could easily form a passage far bigger than that needed merely to accommodate  the water. You, indeed, have stated that the downstream phreas from Lancaster Hole sump is huge ? there's no way that the stream in LH 'need's such a big passage.

 

[graham]

Assuming that the chemistry of the water doesn't change, surely passage size in the phreas is purely a function of discharge and time?

 

[Fulk]

Assuming that the chemistry of the water doesn't change, surely passage size in the phreas is purely a function of discharge and time?

Of course, you are right; the point I was trying to make above concerns the length of time the water is trapped in the phreas. If it has to go uphill for some distance under pressure in order to resurge, then it may be trapped there for a long time, whereas if the uphill section is only short, then a surface channel could be eroded relatively quickly, which would make the phreatic phase of development much shorter.

 

[langcliffe]

It's almost inevitable as a phreatic passage only needs to be as big as is needed to accommodate the water flow

I don't think that this is (necessarily) true.

Actually, I think that it's a truism!

I was deliberately proposing a very simplistic model to make what I believe to be a valid point. Vadose passages will weather backwards from a base level, leading to an inevitably increase in size. Mechanical weathering is obviously an important factor in this process. Phreatic passages have the advantage, of course, that they continue to weather in low water conditions.

 

[Fulk]

OK ? I concede that in theory a phreatic passage only needs to be big enough to accommodate the water flow available . . . but in practice they're often much bigger!

 

[Pitlamp]

I take your point Langcliffe.

Always remember folks that "mechanical weathering" is also an extremely important contributory factor in the development of phreatic passages as well as vadose ones. (I seem to remember we've been through this within another topic in the past.)

 

[danthecavingman]

Hello,

Going to jump in here with something maybe / maybe not relevant...

Vadose development is as a result of the lowering of the water table - for example the downcutting of the Hope Valley. Think about Jacob's Ladder just Upstream of Suprise View - this very immature canyon takes water from a relatively smaller phreatic tube (The Trenches) and has down-cut into the much bigger phreatic tube of Upper Gallery, which has been subsequently modified by the same stream to have a vadose canyon incised in it. This is a case of a vadose canyon forming a hydrological link between two very different levels of inception probably of very different ages geologically. I'm not sure there can be any direct comparison between passages we perceive to be one and the same where there is a transition between vadose and phreatic or vice versa and a lot of time has passed geologically.
The volume of water involved and the speed of downcutting must also have an effect. Slow erosion of a valley would would allow bigger vadose passage to form than say the unwatering of a valley in phreas by the breaching of a terminal morraine which would result in very rapid erosion due to the sudden drop in water levels. You must also consider in cases like this that small misfit streams may modify passages formed by much larger volumes of water.
I think what I'm trying to say is that there are an awful lot of factors to consider. Caves be where you find 'em.

Dan.

 

[Brains]

Probably not that helpful, but could a fossil example be where the Giants Crabwalk meets the relief passage? I am thinking of where the roof phrea descends to stream level before reascending in what would have been a sump. The vadose canyon virtually disappears in what would have been the sump before reappearing towards the junction to eating house...
There are inlets further upstream, but are they far enough away to be discounted?

 

[Rob]

...could a fossil example be where the Giants Crabwalk meets the relief passage?...

That sounds like a good example of vadose leading to phreatic, although not really proving much relating to my original puzzle because i wouldn't have said the cross sectional area of the Relief Passage is bigger than the cross section of the crabwalk (~30m high in places!).

Surely there's loads of prime examples of vadose leading to phreatic up in the Dales? None of these get bigger in the phreatic section???

 

[danthecavingman]

What about Ireby Fell Cavern - Duke Street is way bigger than the streamway that now feeds into it....

 

[Rob]

Isn't the main feed into Duke Street on the right as you enter, and now totally full of sediment? If so, I don't think the 'entrance series' has much to do with the formation of Duke Street.

 

[cavermark]

Timescale is important too. Rob is defining his vadose passage as the currently dry one (presumably vadose in character). and the sump as Phreatic (but of unknown character). The whole lot could have been phreatic for the last thousands of years until a recent water table change, flow direction (or change due to sediment, earthquake slips, glaciers etc.)

Changes over short sections of the same passage could also be local variations in rock chemistry, structure, hardness etc.

Hard to make useful predictions

 

[Les W]

Is now the right time to chuck Paragenesis into the mix?  :-\

 

[Rob]

Is now the right time to chuck Paragenesis into the mix?  :-\

I wouldn't mind if it was done with the use of examples.

A lot of people have misinterpreted my querry as one that requires ooddles of science to be chucked at. I don't really care (for now) about all the different variables and methods possible. This is mainly because I understand that cave formation is extremely complex. But hard examples like Graham's at least show that it can happen, even if we don't understand all the methods at work.

 

[graham]

... hard examples like Graham's at least show that it can happen, even if we don't understand all the methods at work.

I'm still not convinced that something else wasn't happening in that cave at that point. It's very difficult to resolve a change in passage size to just one specific cause.

 

[Pitlamp]

Graham's right of course. You can't draw any meaningful conclusions without fully understanding all the factors that could potentially make such conclusions wrong.

Back to your vadose / phreatic size differences question; the streamway (vadose) in Ingleborough Cave below Giants Hall is significantly smaller in cross sectional area than the sump it feeds into (the 210 m long Lake Pluto to Beck Head Stream Cave sump). Downstream of this sump the vadose streamway in Beck Head Stream Passage is smaller again. (To keep Langliffe happy - as he knows Ingleborough Cave like the back of his hand - I'm talking about the bit of streamway immediately prior to Lake Avernus, thus removing the variable of the Shrimp Pool inlet.)

 

[ian.p]

Right then third time lucky:

Just about anything is possable in terms of cave morphology theres all sorts of bizare things underground but youre not going to make head nor tail of them unless you look at WHY they are the way they are

If you have a length of passage that is subject to exactly the same conditions along its entire length then it will have exactly the same morphology along its entire length and it will either be completly vadose or compleatly phreatic and be exactly the same size.
A passage doesnt suddenly decide f@ck it vadose is getting a bit boring now maybe ill try phreatic for a bit there are underlying reasons for the change and these will be the drivers for deciding wether the passage is bigger or smaller than the precceding passage you wont ever have a hard and fast rule about what to expect from a passage just because youve reached a bit thats flooded every case will be different.

 

[graham]

Right then third time lucky:

Just about anything is possable in terms of cave morphology theres all sorts of bizare things underground but youre not going to make head nor tail of them unless you look at WHY they are the way they are

If you have a length of passage that is subject to exactly the same conditions along its entire length then it will have exactly the same morphology along its entire length and it will either be completly vadose or compleatly phreatic and be exactly the same size.
A passage doesnt suddenly decide f@ck it vadose is getting a bit boring now maybe ill try phreatic for a bit there are underlying reasons for the change and these will be the drivers for deciding wether the passage is bigger or smaller than the precceding passage you wont ever have a hard and fast rule about what to expect from a passage just because youve reached a bit thats flooded every case will be different.

Wholeheartedly agree.

The only reason that I posted the example of Faunarooska was that Rob was not going to accept an argument that wasn't illustrated by actual cases, so I came up with one that countermanded his theory; even though I do not, personally, believe that this phase change was the reason for the size change.