Diminishing dimensions downstream...?

[tony from suffolk]

An associated phenomenon - cave passages decrease in size over time. Deposition of calcite formation can partially explain it, but doesn't account for the breathtaking speed at which previously comfortable squeezes become a life-or-death struggle after a few years.

 

[Les W]

A question not obvious from the original post - is the first bit of cave to the bottom of the pitches vadose and the last bit phreatic (or only recently so.

Vadose cave has downcut and will be larger than the stream that formed it whereas phreatic cave will form a conduit necessary for the volume of water that needs to flow along it. This would easily explain the size differences. Pitches (waterfalls?) also erode much larger than the stream passage.

 

[Pitlamp]

Les - phreatic caves may be far bigger than the minimum size necessary to convey the greatest volume of water which must flow through them. The size of a phreatic passage is more a function of its maturity rather than simply depending on the discharge it conveys.

For example the downstream sump of Lancaster Hole has a typical cross sectional area of maybe 40 square metres. Even in the highest discharge I don't think the river downstream of the Leck Beck Head / Easegill confluence has a cross sectional area of this magnitude - and by then it's picked up all the Leck Fell water as well. If it did have this sort of cross sectional area it might not fit under the road at Cowan Bridge!

Another good example is Duke Street in Ireby Fell Cavern - its size is far in excess of what would be expected from measuring the discharge of the Ireby stream. Conversely, sometimes very small phreatic passages can carry relatively big discharges - for example the immature distributaries which convey all of the Malham master cave water out from Aire River Passage to the foot of Malham Cove, leaving the downstream portion of this large main conduit abandoned (and truncated under talus to the west of the base of the present risings).

I bet you think I'm being a bit nit picky - in which case you have my apologies! But I guess this is a useful discussion to have.

 

[susie]

Another good example is Duke Street in Ireby Fell Cavern - its size is far in excess of what would be expected from measuring the discharge of the Ireby stream.

A fine example of a misfit stream... But then, what has the formation of Duke Street to do with the Ireby Fell Cavern stream? If I were to hazard a guess, I would say very little.

 

[Les W]

I bet you think I'm being a bit nit picky

Yes. 

in which case you have my apologies!

Thanks. 

To be honest, I know that phreatic passages will increase in size with maturity as you quite rightly point out, but I was really looking to answer the original post and was looking for reasons why the lower passage might be smaller (so I conveniently ignored the possibilities detailed in your post above  )

The point made later by Penguin...

Thinking about it now, these are of similar dimensions to the believed final rising of the river...

...suggested to me that the final section of the cave may be small as it might be phreatic whereas the majority of the cave might be vadose, and this would bear no direct relationship to the volume of water that formed it.

But I guess this is a useful discussion to have.

It is certainly one of the better discussions on here for a while and I'm sure that people can get some useful insight from it, so I agree. 

 

[graham]

I hate the use of the word "maturity" to explain the size of a given piece of cave passage. It is completely nonsensical. I can think of some pieces of cave passage that are probably very old indeed but are still bloody small, that is simply 'cos the stream that formed them was always bloody small.

All cave passages* will increase in size over time as water with the power to dissolve or otherwise corrode the bedrock continues to run through them. That increase in size may of course be balanced by a parallel decrease in size due to sediment or mineralogical deposition. Thus, to give a classic example a paragenetic phreatic tube may appear to maintain its size over time but to move upwards through the bedrock as solution of its roof is neatly balanced by sediment deposition on its floor.

Cave passage size is purely a function of two things:

1 The amount of energy that has been applied to the passage to remove rock from it.

2 The susceptibility of the relevant piece of bedrock to that application of energy.

Both of the above vary widely which is why the prediction of where the next piece of cave to be discovered will be and where it will be heading is so damned difficult.






*Point for pedants I am talking about limestone karst here, not lava tubes or other stuff.

 

[Pitlamp]

Er - is it not also a function of a third factor - time? (i.e. how long your other two variables have been at work?).

I think the adjective "mature" is quite useful - it's certainly in widespread use amongst cavers. Mature need not necessarily relate to the age of a passage; it refers to how well developed it is. How long it has taken to get to that stage of development isn't necessarily relevant in this discussion. So yes, it's perfectly possible to have an immature passage which is extremely old.

Susie - you took my bait; thank you!  (I included that Ireby example on purpose to make my point.)

 

[graham]

Thanks, Pitlamp for falling exactly into my trap. No, it is not necessarily a function of time. You cannot simply by measuring its size say how old a passage is. A given size is reached after a given quantity of rock has been removed. That may take a very long time or a much shorter time depending (mainly) on the stream size. That is why I don't like the word "mature" it is really unhelpful as it implies age in an unjustifiable way.

 

[Les W]

HA HA!!

You've all fallen into my trap.  :tease:

Not really, just felt the need to say it.

Maturity is a geographical term and is used to describe stages of River erosion, where it does not denote time but a phase in the development of a river. A river can be youthful, mature, and in old age, all at the same time, just in different locations. Whilst time might be implied, what is actually denoting the "phase " of river development is geographical features.

River erosion terms are readily (though maybe not correctly) applied to cave morphology, and however unhelpful they are, we are probably stuck with them.
It is good that we can debate them though so that more people understand the limitations of using such terms to describe cave passage and processes.

Maybe if enough people agree then we can dispense with inappropriate geographical terms and have our own set of jargon. 

 

[Penguin]

Art Palmer's "Cave Geology". This is the definitive work on the subject and has recieved glowing reviews. Art Palmer is one of the most respected Karst scientists in the world, his writing is very easy to follow and well explained for the layman with no real geological background. It's highly recommended.

Got my eye on it!

Gents - just pointing out that I didn't write "caves can flow uphill" - but a stream within a section of a cave system certainly can.

Got you.

(An example of this is the common phenomenon of water layering, which has a profound effect on passage development.)

Interesting. The sump that we did find at the end of one of the caves was distinctly layered, with a thick layer of silt (raised by me) and a very clear layer above of a few inches, which would silt out but clear again fairly rapidly. I thought that this was due to water flowing into the cave (not out as we assumed it would) from this sump - observation of the flow out of the sump pool seems to support this.

A question not obvious from the original post - is the first bit of cave to the bottom of the pitches vadose and the last bit phreatic (or only recently so).

That is possible. What would distinguish vadose from phreatic passage?

Here is a rough section of one of the caves (the easier to sketch and photograph!) - it also appears to be the simplest of the two...

This is the passage in the larger section just downstream from the first pot the river passes through (looking back to the pot)...

Further downstream of the first pot...

And the start of the 2 ft high bedding plane...

I think i have confused matters a bit by introducing two different and unconnected caves into the discussion!

The one above is Tiragarvan and has the distinctly narrowing river passage.

The other (Aphuca/Fin Mc Cool cave) appears to be much more complex, where the cave the stream sinks in is a much larger cross-section - which continues for some distance very evenly - but the presumed rising has a much smaller section.

Cheers

 

[graham]

penguin

It would help to see the geology marked on that section, especially lithology changes, dip, faulting, jointing and frequency of bedding planes.

 

[Penguin]

penguin

It would help to see the geology marked on that section, especially lithology changes, dip, faulting, jointing and frequency of bedding planes.

Errrrr...

Ok, i know what you mean but we've not surveyed to that amount of detail yet!!! 

 

[Les W]

Hi Penguin,

The first two photos show a vadose streamway, you can see that the passage originally started at roof height and would have been an eliptical tube formed along the weakness of a bedding plane. It would have been phreatic at this point, (i.e completely full of water). At some point the stream would have started down cutting due to external influences, perhaps erosion in the river outside the resurgence lowered the base level, and the cave stream started cutting the floor away more than the walls.
An air space would now be present above the water and all erosion would be on the stream bottom (and lower walls), as the stream down cuts the passage above is left high and dry. This forms a "vadose canyon" with the stream now just taking up a small part of the cross section of the passage.
The bottom of the passage in your photos is quite wide and much wider than the higher canyon, I would guess this is either because the rock here is much weaker (at a bedding plane joint perhaps) and more easily eroded, or that the external factors that caused the down cutting have been negated somewhat (water has reached a new base level).
The water now appears to have eroded the sides of the passage a lot and stopped down cutting. The sediment on the floor supports this theory.

There are two possibilities for the lower passage. One is that the passage is as large as the rest but buried in the sediment on the floor and the other is that the lower passage remained below the base level of the resurgence so there was no vadose down cutting there.

Where are the caves? This might help with this remote diagnostic of speleogenesis. 

 

[Elaine]

In my slightly euphoric state of having taken advantage of men who are only willing to kiss me at the stroke of midnight on New Years Eve I just want to ask/confirm that vadose type passage forms when the water table drops yes? Actually I think you more or less said that in your post. Sorry ignore me.



PS Ooooh look. I have the honour of the first post of 2009!

 

[Les W]

Base level is the local control on any water table that might exist, but water table is a concept that is not really applicable in non porous rocks such as Carboniferous Limestone. 

 

[graham]

Base level is the local control on any water table that might exist, but water table is a concept that is not really applicable in non porous rocks such as Carboniferous Limestone. 

So we talk about piezometric surfaces instead 

 

[graham]

In my slightly euphoric state of having taken advantage of men who are only willing to kiss me at the stroke of midnight on New Years Eve I just want to ask/confirm that vadose type passage forms when the water table drops yes? Actually I think you more or less said that in your post. Sorry ignore me.



PS Ooooh look. I have the honour of the first post of 2009!

Could be worse, could be

 

[cap n chris]

 

[Pitlamp]

Don't agree Graham - time is an important variable since if your other two factors apply constantly (which I realise is only a theoretical situation) then it will take a certain amount of time for a passage to reach what can be considered maturity (see below).

Les - your point about the word "mature" being borrowed from the geographers and applied to cave development is a good one. A "mature" passage need not be old and vice versa. In cave development the word "mature" has a different meaning from "age". So I agree with you.

Anyway, apart from all this - happy new year to all of you!

 

[graham]

OK, one last try.

The problem with "maturity" as the term is used by cavers, if not geographers, is that it has anthropocentric connotations. I am thinking here of a particular cave passage in Co. Clare. The passage is huge, in the context of the landscape and known karst development it is pretty bloody ancient; trouble is that it is 90 - 95% full of fluvial sediment and you spend all your time in it crawling flat-out. Consequently there ain't a caver in the world who would call it "mature".

I agree with Les, that caves need their own jargon. Apart from the obvious vadose and phreatic, I would include active, fossil, relict, paragenetic. These terms have specific cave usages that don't really apply to surface landforms in the same way.