How do ramps form in caves?

[Les W]

Why are some deeper joints more open? What mechanism would account for some of the joints to be selectively more open?

I guess some folding (anticline?) might open joints on the outside of the fold and close them on the inside. I can't imagine another mechanism.

 

[barrabus]

..... ? and there we have it, a ramp (Fig. 7).
Well, this is all very speculative, but I?d welcome feedback on this issue.

I've now taken the time to read your thoughts and it seems reasonably plausible to me.

 

[Pitlamp]

I also think you're near the mark. In your diagrams the ramp on the right is just like Sleets Gill's entrance and the new rising on the left is the equivalent of Moss Beck Head.

In the case of Sleets Gill though the new resurgence is still not mature enough to carry all the water in high discharge, so it backs up and the entrance ramp occasionally becomes a resurgence once again. (There's an interesting video of this happening on You Tube.)

 

[Fulk]

Quote from Les w:

Why are some deeper joints more open? What mechanism would account for some of the joints to be selectively more open?

.

I don't know!

But if you look at joints and beddings in surface outcrops of limestone, you'll see that they all appear to be more or less open; this is a result of erosion, of  course. But if you look at the same features underground, you will see that many are tight, hair-line fractures, with no sign of penetration by water. These observations comprise one reason why I believe that you can't have caves unless there is a network of open fractures for water to exploit.

Look at the caves of the Easegill System ? why is the longest (?) cave in Britain situated where it is? The obvious answer is that it is in the angle made by two big faults, the Dent Fault running more or less N?S and the Craven Fault System running SE?NW to meet it. It is in this region of, presumably, broken up rock, that there were 'loads' of open cracks and fissures just ready for the development of a major cave system as soon as it became accessible to meteoric water.

 

[Pitlamp]

I wouldn't disagree with your last point above Fulk but there is a range of factors which account for the amount of known passage in the Easegill system. One is very likely to be the proximity of two major faults as you say - but another is probably the amount of exploratory attention given to this area by cavers compared with eslewhere.

For Les; is one possible factor in certain joints being less "tight" than others bedding plane slip during folding?

 

[graham]

But if you look at joints and beddings in surface outcrops of limestone, you'll see that they all appear to be more or less open; this is a result of erosion, of  course. But if you look at the same features underground, you will see that many are tight, hair-line fractures, with no sign of penetration by water.

Joints at the surface tend to be more open in large part because they are not being kept tightly shut by the weight of lareg thicknesses of rock above them. On the whole, the deeper you go, the tighter the joints.

These observations comprise one reason why I believe that you can't have caves unless there is a network of open fractures for water to exploit.

Assuming that you are talking about karst caves, of course.    Indeed so, but joints and bedding plane partings will be opened by a number of factors, most notably the tectonic movements mentioned above., but also such things as uneven prior erosion on palaeokarst surfaces.

 

[TheBitterEnd]

And Mrs Fulk thought you were working hard for a deadline yesterday  :spank:

I still think a vertical discontinuity (fault) cutting through the bedding may well have something to do with it (or why don't we see more ramps - generally we see cascades rather than ramps). What I guess would be really handy is some detailed geological mapping of Sleets Gill.

Whilst looking for something elses I came across this, which has some interesting stuff on phreatic loops
http://books.google.co.uk/books?id=oxsa9nbQxuEC&pg=PA225&lpg=PA225&dq=phreatic+loop&source=bl&ots=_xLPGnvKP5&sig=AsjIQJO30hSeoCVeul1DP0TcsUI&hl=en&ei=irmLTZSDFsiAhQeIz4igCw&sa=X&oi=book_result&ct=result&resnum=6&ved=0CDkQ6AEwBQ#v=onepage&q=phreatic%20loop&f=false

 

[Fulk]

I still think a vertical discontinuity (fault) cutting through the bedding may well have something to do with it (or why don't we see more ramps - generally we see cascades rather than ramps).

Well, I guess that most caves start life relatively high up, in mountainous and hilly areas, and work their way down to valley level, so you'd expect a lot of cascades and pitches. On the other hand, ramps are formed by water flowing upwards under pressure at resurgences. There are fewer resurgences than sinks, and not all will be Vauclusian ? so this is a rarer event than water flowing down under gravity, hence there are more cascades than ramps?

 

[graham]

To take Fulk's last point -and I know this appeal to Pitlamp - *we* see more cascades than ramps 'cos *we* spend more time in vadose passage than we do in sumps.

 

[gus horsley]

I've had another thought as to how ramps could form.  If a valley floor is raised, say with glacial debris, and a resurgence is blocked, the water would theoretically form a temporary phreatic zone which would be much more extensive than the previous one because there would be nowhere for the water to escape.  Over a period of time the water would preferentially dissolve joints towards the new valley level and these would coalesce towards the most convenient point for the water to escape, forming a phreatic  ramp.  When subsequent erosion lowered the valley floor again it's possible the old resurgence would become active again, leaving the ramp as a fossil feature.

 

[Pitlamp]

What Graham? - you mean some people spend more time in dry caves?! 

Fulk said "ramps are formed by water flowing upwards under pressure at resurgences". Just to clarify a point for people who apparently spend more time in the vadose zone  - ramps can form anywhere in a phreatic passage, sometimes a long way back from a resurgence. (The two ramps braveduck mentioned in Ingleborough Cave are good examples.)

 

[Fulk]

Pitlamp said:

'Just to clarify a point for people who apparently spend more time in the vadose zone  - ramps can form anywhere in a phreatic passage, sometimes a long way back from a resurgence.';

fair point, and I stand corrected, but does it alter the fact that water spends more time going downhill in caves than uphill?

 

[Fulk]

gus horsley says:

I've had another thought as to how ramps could form.  If a valley floor is raised, say with glacial debris, and a resurgence is blocked, the water would theoretically form a temporary phreatic zone which would be much more extensive than the previous one because there would be nowhere for the water to escape.  Over a period of time the water would preferentially dissolve joints towards the new valley level and these would coalesce towards the most convenient point for the water to escape, forming a phreatic  ramp.  When subsequent erosion lowered the valley floor again it's possible the old resurgence would become active again, leaving the ramp as a fossil feature.

That does, in deed, sound plausible, but one problem I can see with the idea is that you'd need a helluva lot of glacial debris to , e.g. raise the level of Littondale valley floor to the level of Sleets Gill entrance, debris that has had a relatively short period of time since the disappearance of the glaciers in which to get washed away again.

 

[graham]

... but does it alter the fact that water spends more time going downhill in caves than uphill?

Water only ever goes uphill in the short term. In the long term it always goes downhill. But that doesn't mean that a given cave cannot have a longer uphill section (in plan) than downhill. Consider the simple system that goes Big pitch - Long sump - Rising.

 

[Fulk]

Quote from Graham:

Water only ever goes uphill in the short term. In the long term it always goes downhill. But that doesn't mean that a given cave cannot have a longer uphill section (in plan) than downhill. Consider the simple system that goes Big pitch - Long sump - Rising.

Agreed; what I meant was that in section, water will cover a greater distance downhill than uphill (even if, in the case of a short vadose section leading to a deep phreas, most of the downhill section is actually under water).

By the way, what happened to water tables; personally I don't believe that they exist in limestone (except, perhaps, over a very small, localized area where you might get a quasi-water table) (though maybe you get them in chalk)?

 

[Les W]

By the way, what happened to water tables; personally I don't believe that they exist in limestone (except, perhaps, over a very small, localized area where you might get a quasi-water table) (though maybe you get them in chalk)?

http://ukcaving.com/board/index.php?topic=11498.msg148189#msg148189

 

[TheBitterEnd]

...
On the other hand, ramps are formed by water flowing upwards under pressure at resurgences.
...

Do you have any evidence that it is always water flowing up hill? Could down-hill phreatic flow create ramps?

Also I'm not sure about caves starting high up in mountains, there seems to be a lot of opinion that suggests caves form most rapidly at the phreatic/vadose boundary. The limestone is fractured throughout, meteroic water and acidic deep ground water can enter at all levels.

I suspect that flow rate needs to be considered. The faster the flow, presumably the faster the rate of solution. Suppose that two bedding planes are intersected by a fault and it just happens that the path of least resistance is diagonally up the fault from one bedding plane to another, you get a ramp.

Do we have any information on typical ramp angles? are they often similar or do the vary a lot?

 

[Fulk]

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?

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

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.

 

[gus horsley]

That does, in deed, sound plausible, but one problem I can see with the idea is that you'd need a helluva lot of glacial debris to , e.g. raise the level of Littondale valley floor to the level of Sleets Gill entrance, debris that has had a relatively short period of time since the disappearance of the glaciers in which to get washed away again.

I wasn't thinking of Sleets Gill in particular but if the valley floor is quite wide a lateral moraine  could remain in situ for a considerable period of time.  I'm coming to the conclusion that any of the models (or a combination) so far described could apply to ramps and it's an interesting debate which doesn't tend to find its way into geomorphology literature.

 

[graham]

Moving slightly off topic here, but

'Acidic deep groundwater' was responsible for Lechuiguilla and Carlsbad.

The balance between dissolution and mechanical erosion in forming caves is an interesting and complex one, but certainly the faster moving the water, the greater the energy supplied and the more cave will be formed.