How do ramps form in caves?

[TheBitterEnd]

Now then, Sirch2, pay attention, please  .

The hypothesis is that water can only get underground along well-defined paths ? where's all this 'acidic deep ground water' coming from?

I'm not sure what you mean by "well defined paths" obviously the vast majority of the water in carboniferous limestone travels along discontinuities. But it could travel up from deeper strata as well as down - how do you think minerals such as lead and copper get into the limestone?

The limestone was formed at the bottom of an ocean, it was compressed, uplifted, faulted, etc. Water did not wait until all this stopped and then ask permission to enter. Most of the discontinuities in the rock probably spend the majority of their time filled with water either because they are below the piezometric surface, because of percolating meteoric water or because of capillary action. The thing that causes cave formation is acidic water, any static water would be neutralised. So the important thing is flow paths, only where the water can be refreshed will we see increasing cavity size.

And surely it's a matter of observation that, by and large, you enter caves at the top and work down to the bottom?

Just because we enter that way and water does that in vadose passages large enough for humans to enter does not mean that it is true in the initial stages of speleogenesis.

there seems to be a lot of opinion that suggests caves form most rapidly at the phreatic/vadose boundary

Don't believe it; I think that cave formation proceeds most rapidly when you've got a major underground stream or river blasting its way through a vadose system.
[/quote]

Ok I meant in the early stages of speleogenesis, which, after all, is probably what matters in the development of ramps. Once a flow path has formed it is going to be preferentially enlarged over dead-ends and slower moving flow paths.

 

[Fulk]

Well, Sirch2, you make some very interesting and valid points, but to take just one of them:

The thing that causes cave formation is acidic water, any static water would be neutralised. So the important thing is flow paths, only where the water can be refreshed will we see increasing cavity size.

Yes, the important thing is flow paths, thus caves will only form when water starts to flow through the limestone. My guess would be that ? in a situation like the Dales ? this could only happen when the limestone had been uplifted, allowing water to flow under gravity downhill (that is to say, downhill overall, notwithstanding that the last bit of passage might be a Vauclusian rising). So I suspect that until limestone is uplifted and the surface burden eroded away, thus revealing at least some limestone open to subaerial erosion, cave formation will be very slow or non-existent (except in the special case of sulphuric acid upwelling to form Lechuguilla etc., which to the bet of my knowledge is quite rare (?)).

 

[TheBitterEnd]

Have a look at the link I posted earlier -

http://books.google.co.uk/books?id=oxsa9nbQxuEC&lpg=PA225&ots=_xLPGnvKP5&dq=phreatic%20loop&pg=PA223#v=onepage&q=phreatic%20loop&f=false

The indication in there is that water can be forced down tens to hundreds of metres before finding it's way back up again, so cave development well below the spring line is quite possible.

 

[Fulk]

You know, I think that to a certain extent we're at cross-purposes here; if you go back to my hypothetical diagrams you will see that I showed water going deep down before coming back up again, so we are, in fact, in agreement.

Perhaps the only point of disagreement is the source of the water ? in the case of the Dales, I'd say that it was purely meteoric.

 

[Pitlamp]

To answer a question from sirch2 several posts above - yes, ramps do form in a downwards direction in the phreatic zone - big ones exist in the downstream Lancaster Hole sump and also in Hurtle Pot. Withough doing a statistical analysis, my impression (from flopping about in sumps for a great number of years) is that ramps formed by upward flow are much more common than ramps formed in a downhill direction.

I think that the two above examples of ramps forming downhill are both aligned along faults; both show great amounts of fault breccia along their lengths with all the mini-fracture planes parallel with the long axis of the passage. This seems to be different from certain uphill ramps, where they seem to have developed perpendicular to the fracture planes in a wide zone of disturbance. (The Witches cave to Leck Beck Head ramp is like this, as is, I suspect, the entrance ramp in Sleets Gill - though I've not had chance to go and inspect it directly since this thread got going.)

 

[TheBitterEnd]

Thanks Pitlamp, like I said earlier, I think doing some mapping would shed some light which I may have been motivated to do but I don't have the time. It seems fairly obvious that given the chance, water under pressure will seek out lower pressure/higher levels (not that that necessarily explains ramps over vertical shafts but possibly explains more uphill ones than downhill).

Fulk, yes I think we largely agree, I just think your model is possibly a bit simplistic - and I like a bit of sport on web forums  . (BTW You were right about 90Sr as well - if I' carry on being nice are you going to come out and play on Wednesday?)

 

[Fulk]

BTW You were right about 90Sr as well

Errrrrr . . . did I post something about strontium-90? Must've been pissed, 'cos I can't remember.

 

[psychocrawler]

Hydraulic gradient?

 

[Fulk]

Hydraulic gradient?

Would you care to expand this a little bit?

 

[Pitlamp]

I think he means where water goes in is usually (but not exclusively) higher than where it comes out. (The Sea Mills of Argostoli provide an unusual example of where the above doesn't strictly apply.)

There is an excellent summary (in the Kingsdale chapter, written by Dave Brook) of the two laws governing the path taken by underground waters in "Limestones & Caves of N.W.England" (Ed. Waltham, 1974) on page 319. If you've not got access to this I could type it here (but am just a bit busy right now).

 

[Fulk]

If you've not got access to this I could type it here (but am just a bit busy right now

That's a very generous offer ? but I have got a copy of Limestones and Caves of NW England.

Thanks

 

[psychocrawler]

I mean Rule 1 in DBs excellent Kingsdale chapter in 'Slimestones and Graves of NW England' about phreatic flow developing along the overall line of least resistance between inlet and outlet (i.e. water at depth in a developing phreatic tunnel has to have an exit if there is a constant input).

In the case of Sleets Gill, see also Alf Latham's article Sleets Gill - A Cave With A Difference in ULSA Review 12 pp36-39. He postulates that The Ramp (after Hydrophobia - much more spectacular and well decorated than the entrance ramp) is a pre-glacial resurgence from depth prior to glacial deepening of Littondale. He does not, however, speculate on the mode of formation of either ramp feature.

 

[Fulk]

I mean Rule 1 in DBs excellent Kingsdale chapter in 'Slimestones and Graves of NW England' about phreatic flow developing along the overall line of least resistance between inlet and outlet

.

I think that this goes to the heart of the matter . . . the big question is, which is the 'line of lease resistance'. If you go back to my original diagram 1, I am at pains to point out that the superficially most direct route ? the green lines ? is not the route followed by  the water, but it takes a convoluted route along the red line(s) . . . the actual line(s) of least resistance, because these are open cracks or fissures in the rock.

 

[psychocrawler]

You might also want to discuss ramp formation with certain well known N Wales cavers. I seem to remember going to a nice 'ramp' feature called Grand Turk Passage some while ago which seemed to be a modern downstream passage but is probably a phreatic lift to an abandoned resurgence following lowering of the local water table by mining activity. It was also fairly close to the pub which was handy for ranting about caves afterwards.

 

[dunc]

Talking of N.Wales, reminds me of a few significant 'ups and downs' (sorry for blinding you with scientific descriptions!) in Llyn Parc, not sure if they would be classed as 'ramps' or not, but a lot of that system was phreatic prior to the above mentioned mining activity (and probably part of the same system as GTP)..

 

[Fulk]

psychocrawler said:

He postulates that The Ramp (after Hydrophobia - much more spectacular and well decorated than the entrance ramp) is a pre-glacial resurgence from depth prior to glacial deepening of Littondale. He does not, however, speculate on the mode of formation of either ramp feature.

Well, he's quite right ? the 'inside ramp' is more spectacular than the entrance one. How did two such features form ? simultaneously, independently, sequentially . . . . .

Incidentally, one of the most spectacular such features I've ever seen is the entrance passage to Grotte Favot in the Vercors, which is a tunnel, pentagonal in shape, with a height of about 6?7 m and a width of maybe 10 m, descending at such a steep angle that a handline is quite useful on a descent.

 

[TheBitterEnd]

This paper

http://institute.speleoukraine.net/libpdf/Klimchouk_2009_Principal%20features%20of%20hypogene%20speleogenesis_HypoConf.pdf

summarises some interesting ideas which may be relevant to this thread (particularly see figure 3).

Thanks to rhychydwr1  for posting in this thread http://ukcaving.com/board/index.php?topic=11786.0 - have a look at the other papers as well.

 

[Pitlamp]

The entrance passage of Lower Foxholes (near Clapham) is one such ramp; I happened to be passing today so I took a look. The limestone in the area of the Foxholes entrance ramp does look a bit stressed and shattered. More & more I think that diagram which suggested a ramp is initiated as a series of small steps which get smoothed out with continuing dissolution seems to be the answer.