How long have caves been there in their current form ?

[MPC-Robin]

As we were sat at the bottom of the main shaft at Alum yesterday waiting for one of the party to prussik up, we were talking about how long it took to form Alum and how long had it been there in it's 'current' form.
Are we talking hundreds, thousands of years or much longer ?

 

[pwhole]

I would guess, without any evidence, that it started forming about a million years ago, but would have accelerated as roof and wall collapse began along the fault zone as the water-table lowered and the 'support' of the water inside the cave drained. I've only been in Alum Pot once (prussicking up it), but it seemed like that's the way it would have formed. There is a description of the cave's formation in Volume 2 of Caves and Karst of the Yorkshire Dales, but not the age. Others better acquainted with the area than me will know more.

 

[cavingbiker]

A difficult one to answer as caves are constantly changing due to erosion, also define 'current form'? Do you mean an open shaft of some description or a passage permitting the downward progression of water which could be by a number of drops or a single drop below surface level. Most if not all of the evidence will have been eroded away by now.
The dating of speleothems in the Dales gives ages up to around 400 ka, so for a speleothem to form the passage must be older.

 

[langcliffe]

As cavingbiker implies, there is no simple answer. To quote Alf Latham and Derek Ford:

"The accumulated record of speleothem dates confirm the long-held notion that the Dales caves developed during multiple cycles of glaciation and valley deepening through the Quarternary". (Caves and Karst of the Yorkshire Dales, page 177)

Valley deepening means that some caves become fossil, such the main passage in Witches II, and their on-going development is mainly limited to rockfall. Other caves, like Pool Sink, obviously have a multi-phased development which is very much on-going. Alum Pot fits into the later category.

 

[Pitlamp]

It's well out on the bench from the current shale / limestone margin. Conventional logic suggests that this points to considerable age.

 

[langcliffe]

It's well out on the bench from the current shale / limestone margin. Conventional logic suggests that this points to considerable age.

It's origin, yes - but how much of its current morphology has developed since?

 

[mikem]

Most were changed significantly by the last ice age (& not as much since), except those bits that have formed more recently...

 

[Pitlamp]

It's origin, yes - but how much of its current morphology has developed since?

A very good question.

There is a magnificent image of Alum Pot which was on the wall during the recent display of the Horner collection at the Folly at Settle. It was blown up to huge scale and is incredibly detailed. Such an image might be useful in helping to shed light this question.

 

[langcliffe]

Most were changed significantly by the last ice age (& not as much since), except those bits that have formed more recently...

The ice ages are pretty quiescent as far as cave development are concerned. It's the start of the interstadials when cave formation would have been most active. "Phreatic tubes of up to 2 metres in diameter are considered to have developed within only a few thousand years of flooded conditions beneath ice-dammed lakes in the Scandinavian karst, and it is reasonable to expect that similar conditions could have pertained in the Dales during the Quaternary deglaciation." C&C of the YD page 124.

So for any major cave, the question is, the end of which glaciation was mainly responsible? Speleothem dating then becomes useful to provide a clue for a "before" date. Samples from Victoria Cave date back to 600 ka.

The major caves in Chartreuse in the French pre-Alps have been dated by cosmogenic nuclide dating of cave deposits, to about 4.5 Ma. The landscape has changed dramatically since their formation, which is why large phreatic passages end high up cliffs overlooking deep valleys.

 

[Brains]

I think the limestone pavement common in the dales is the result of glacial removal of soils. This is backed up by the presence of erratic boulders. Such scraping would cause the overlying beds to be stripped away, leaving formerly closed avens as open shafts.
Given the other geomorphology the current appearance can be no younger than the end of the last glaciation (10,000 years?). Together with the gentler climate, I would suggest little significant development has occurred since.
The quoted ages of stals and sediments imply a much older pre glacial origin to the major caves. With the known numerous glacial advances I would imagine the development to have been spasmodic linked to each retreat phase.
Still leaves us with the problem of how to date a hole, being the absence of evidence!

 

[langcliffe]

Such scraping would cause the overlying beds to be stripped away, leaving formerly closed avens as open shafts.

Interesting. Have you a reference for glacial ice stripping away surface beds in the Dales? I would have thought that collapse caused by the sheer weight of a couple of hundred metres of ice (and its subsequence removal) would be a far more convincing hypothesis for the opening up of avens to the surface.

 

[Brains]

Interesting. Have you a reference for glacial ice stripping away surface beds in the Dales? I would have thought that collapse caused by the sheer weight of a couple of hundred metres of ice (and its subsequence removal) would be a far more convincing hypothesis for the opening up of avens to the surface.

Not to hand I'm afraid, but without it there would be no limestone pavement or erratics. Overlying pressure of ice could well collapse a cave roof, and this has to be a mechanism in some cases. In either case I would expect a lot of debris to enter the open cave, unless the void was ice filled during glaciation?

 

[Pitlamp]

Are we assuming Alum Pot Main Shaft was an underground chamber which was then breached from the surface by whatever mechanism? I find this hard to accept as there's no evidence of resulting debris, other than the Bridge jammed across the shaft, which could easily have just peeled off the walls (freeze / thaw action?).

I'd have thought Alum Main Shaft is an (almost) abandoned ancient open sink on the N / S fault, with the present Alum Pot Beck being an underfit stream (due to the main stream having been captured by subsequent underflow via the Long Churn system and associated caves).

 

[langcliffe]

Are we assuming Alum Pot Main Shaft was an underground chamber which was then breached from the surface by whatever mechanism? I find this hard to accept as there's no evidence of resulting debris, other than the Bridge jammed across the shaft, which could easily have just peeled off the walls (freeze / thaw action?).

I'd have thought Alum Main Shaft is an (almost) abandoned ancient open sink on the N / S fault, with the present Alum Pot Beck being an underfit stream (due to the main stream having been captured by subsequent underflow via the Long Churn system and associated caves).

I agree 100%.

 

[langcliffe]

Not to hand I'm afraid, but without it there would be no limestone pavement or erratics.

In your previous post you said that "I think the limestone pavement common in the dales is the result of glacial removal of soils.", which is contradictory!

My understanding is that you are right that glacial ice did remove weaker beds to expose the harder beds, but I find it hard to accept that was responsible for the existence of the surface shafts we see today such as Alum Pot (any more than collapse from the weight of ice!).

 

[Pitlamp]

Langcliffe said: "It's the start of the interstadials when cave formation would have been most active."

I agree 100% with that too; the deglaciation phase is when loads of pent up water is released, with all it's massive erosion potential. Many abandoned caves actually have "negative growth" during interstadials. They're no longer experiencing dissolution and are instead being refilled with speleothems, as the dominant flow is autogenic. Sometimes they heal up completely.

This was the origin of the late Chester's Dad-style joke when he used to work at Clapham Cave. Whilst standing next to the Mushroom Bed flowstone he used to say it was called that because, if it keeps on growing, soon there won't be much room to get past . . . .

(Sorry - a lousy joke but with an origin in truth.)

 

[Brains]

In your previous post you said that "I think the limestone pavement common in the dales is the result of glacial removal of soils.", which is contradictory!

My understanding is that you are right that glacial ice did remove weaker beds to expose the harder beds, but I find it hard to accept that was responsible for the existence of the surface shafts we see today such as Alum Pot (any more than collapse from the weight of ice!).

Confusion reigns...
The "it" I refer to in the second quote is glacial stripping, so I don't see the contradictory nature?

 

[Brains]

If Alum was a pre glacial sink, as it may well be, where is all the basal moraine that should be filling it? Can it all have flushed through? Was it filled with an ice plug over which the ice flowed without blocking it with debris?

 

[Pitlamp]

Maybe the glacial till which fell down the shaft was transported away by the massive meltwater flow entering from the more recently formed Long Churn system (and the even more recently formed Diccan Pot)?

 

[richardg]

It is always enjoyable to have a discussion on our local cave geomorphology, as it is such a relevant aspect of our explorations.