How do ramps form in caves?

[Pitlamp]

Just back in from caving - hello Ian; the only problem with the staged phreatic then vadose idea is that the ramp I really want to try and understand is in a big deep sump, so it's permanently phreatic. It looks exactly like the Sleets Gill ramps, so if we can understand the latter it'd (hopefully) shed light on the former.

Last night I delved into two extremely useful sources for questions like this; Limestones & Caves of N.W.England (Ed. Waltham, 1974) and the Caves & Karst of Great Britain geological conservation review (by Waltham et al, 1997 [from memory]). Neither gives any suggestion as to how the Sleets Gill ramps formed - by which I mean what inception horizon(s) allowed a sloping passage in horizontal limestone. So I'm still in the dark.

Come on Graham - you're good at this sort of thing . . . . .

 

[graham]

OK Pitlamp

If you can rule out my first suggestion of an oblique joint (and from what you said, you can't) then what's the objection to Fulk's proposal? A phreatic riser that is originally stepped up a series of joints and beddings could quite easily end up with the profile shown in his diagram, why not?

I rather suspect that you are reading too much into Deej's 'inception horizon' hypothesis. Whilst it is undeniably true that some horizons are preferentially picked out by karst processes (and this was recognised fifty years ago by Derek Ford) it is also true that you - or anyone reading this - will be unable to find a cave of any length that slavishly follows only one horizon.

 

[Fulk]

This is a bit off-topic, but it?s vaguely related insofar as it concerns the role of beds and joints in speleogenesis.
In Short Gill in Barbondale (and, presumably, the rest of Barbondale), the bedding gets more and more tilted as you go from east to west, owing, I guess, to its proximity to the Dent Fault, until it is well-nigh vertical. This is seen very well at Short Gill Pot, where a more-or-less vertical shale band has been washed out across pretty much its whole ?height? ? now its width!
However, further down the gill ? i.e. further west ? in Short Gill Cave there are perfectly normal-looking phreatic passages developed in what I assume must be near-vertically-bedded rock. So would the ?inception horizons? in this situation be now-flat joints?

 

[Pitlamp]

I guess so Fulk - but the ramp we're trying to understand is in horizontally bedded limestones. (Lovely place, Barbondale, eh?)

Graham - there are passages which follow a single inception horizon for some distance; two examples are the Speedwell streamway (1.2 km, not including another 155 m on the same bedding plane in the downstream sump) and the main passage in Peak Cavern between Far Sump and Halfway House Sump (quite a bit more than 1.2 km). But I agree with you that the useful diagram which Fulk proposed may well be near the mark (for which thanks Fulk).

I've had some useful correspondence with Beardy (offboard) about this.

I wonder if a zone of minor fracture planes (within the brecciated area of a fault zone) is the culprit? Has anyone seen any evidence for this in Sleets Gill? It's been a while since I went down there. There is evidence for it in the Witches / Leck Beck Head ramp. (Actually this idea is compatible with what Fulk suggested anyway, on reflection.)

Beardy and I are both slightly surprised that no-one seems to have asked this question before, as these ramps are a significant passage form.

 

[braveduck]

Pitlamp,don't forget you have two smaller ones in Ingleborough Cave!
Connecting Far Eastern Bedding Plane with the Upper series.

 

[Pitlamp]

That's a very good point!

I just thought of another in the Dales - the slope up from Styx Sump in Magnetometer Pot.

 

[Robert Scott]

and the requirements of the DDA.

 

[potholer]

I seem to remember there being quite a sloping 'bedding-plane-like' passage leading down to the sump in Kirk Pot, though I got the impression from the rest of the cave that the bedding was essentially horizontal.

Is there something odd in that particular area?

 

[gus horsley]

I've seen the ramp in Sleets Gill and got the impression that it led to a fossil resurgence which was active when the valley floor was higher than it is now. So perhaps some of these features are similar to Vauclusian springs which cut across the bedding and relate to preglacial rersurgences.  There are scallops in Bar Pot which indicate an uphill flow too, just to throw that one into the mix.

 

[Fulk]

I've seen the ramp in Sleets Gill and got the impression that it led to a fossil resurgence which was active when the valley floor was higher than it is now. So perhaps some of these features are similar to Vauclusian springs which cut across the bedding and relate to preglacial rersurgences

I'm sure you're absolutely correct in inferring that the ramps (including the entrance passage) in Sleets Gill Cave are old resurgences . . . but it still leaves the question as how exactly they formed.

I guess that caves can only form if there is a pre-existing network of minute (?) cracks and fissures in the limestone for water to get to  work on. 'All bedding planes and joints are equal, but some are more equal than others' in their susceptibility to erosion. If there is no 'direct' route of micro-fissures  to, say, a down dip resurgence at a lower level, but the pre-existing fissures go deep and then come up again, this would explain how Vauclusian resurgences arise.

 

[TheBitterEnd]

There's a good set of Sleets Gill pics on flickr - I've linked one below. It looks a bit like there is a joint/small fault in the roof left of centre that is giving the passage its onion shape.

Two thoughts come to mind
[list type=decimal]
[*]Is it possible that the joint/fault is partially filled with secondary mineralization/fill  (clay, quartz, etc.) which for what ever reason filled the joint at an angle making a less permeable sloping horizon, so the phreatic action was forced to follow the slope
[*]In most caves we see larger fallen blocks, here we see scree, so I guess the rock is heavily fractured. So could it be formed by some kind of "stoping" action where the roof tends to dissolve preferentially to the floor
[/list]

I like the second idea better...

 

[Pitlamp]

Nice picture! You're right about the angular blockfall - possibly frost shattering has played a role here though to some extent, so close to the entrance? However, that "nobbly" roof doesn't look like a traditional bedding plane guided passsage does it? Maybe there's a clue in this image.

 

[TheBitterEnd]

Can't claim credit for the picture, google Sleets Gill cave or search Flickr for Sleets Gill and you will find the set.

 

[The Old Ruminator]

Not sure how you would classify a ramp but according to Willie Stantons theories Golgotha Rift in Mendips Reservoir Hole would be one of the biggest. The top of it is even higher than the entrance.
One tends to forget how high the static water tables actually were in eras past. I seem to remember the highest proposed Mendip water table was around 150m above todays sea level. Here is a sketch survey of RH. Golgotha starts at Grand Gallery probably the source of all the phreatic water under hydrostatic pressure in the cave. Golgotha ends at Herberts Attic where Willie had actually started a downward dig again. The top end of a massive phreatic loop. A fall in base levels meant this was eventually abandoned as the stream found lower routes.

For more information see.

http://www.ubss.org.uk/resources/proceedings/vol17/UBSS_Proc_17_2_121-128.pdf

So here other phreatic ramps or lifts are proposed from Goughs Cave to higher levels of the old system in Great Ooones Cave above.

 

[Pitlamp]

I once got chance to go on a trip in Reservoir and I came back up north newly inspired to crack on with several old tired digs. RH is an amazing place in so many ways. (Many local cavers at the time told me we'd no chance of getting permission but I wrote to Willie Stanton and he couldn't have been more helpful.)

From memory though, Golgothat rift is formed on a vertical fracture plane, whereas the sort of ramp I was asking about doesn't seem to have a single obvious vertical structural feature guiding it. The more I think about it the more I reckon Fulk's earlier diagram is the best explanation for the Sleets Gill and Leck Beck Head ramps.

 

[Fulk]

This is an attempt to show a plausible method of formation of a ramp such as that in Sleets Gill Cave. It follows on from the comments above about caves needing some sort of pre-existing openings for their formation. If limestone is conceived of as solid blocks with hair-line fractures, then I don?t see how it is possible for caves to form; there must be a way for the water to get underground first, and hair-line fractures would not do the business. I remember reading ages ago about experiments done to discover the locus of cave formation, using chunks of soluble substance (sugar lumps??) to simulate limestone; but this assumed that ?all cracks and fissures are equal? ? as already mentioned, ?yes they are, but some are more equal than others?.
In Figure 1, all the joints and beddings shown in black are very tight and impermeable to water, whereas those in red are open to a certain (limited?) extent. Thus water flowing in a stream on the surface can only get underground along this route, shown in blue in Fig. 2, which takes a convoluted path from sink (A) to two resurgences (B and C), instead of the most direct route shown in green in Fig. 1. At first the ?proto-passages? are tiny and can only swallow up a small fraction of the stream, which continues flowing along the surface (this is, of course, a two-dimensional, cross-sectional notional view, and does not purport to show that the surface stream and the underground one arrive at the same place, B). What will eventually become a ramp has a stepped profile R.  At this point there is little to choose between the two route, but that to point B has a greater head of water behind it and therefore might be expected to undergo erosion more quickly, an effect that will be self-sustaining ? the bigger it gets, the more water it can take, and hence more erosion, and the bigger it gets, etc.
As time goes by the passages get bigger and bigger until eventually a point is reached at which the sinkhole is capable of swallowing up all the water in the surface steam (under normal flow conditions) so the surface stream downstream of the sink dries up. The sharp corners in the proto-ramp have started to get eroded away (Figure 3).
At this point, any further increase in passage size means that the water from the stream can no longer fill the passages down to level X (Fig. 3). This allows for the development of an airspace, so there is an automatic change from phreatic to mainly vadose conditions in the upper passages of the system ? though the lower section is still phreatic, with a Vauclusian resurgence ? without the need to invoke any cataclysmic event such as an ice age (Figure 4). Hence the abundance of passages that have a typical phreatic/vadose morphology ? Lost Johns?, Ireby, Sunset Hole, Long Churn, County Pot, etc. etc. S (Fig. 4) illustrates a perched sump ? e.g. the Duck in Simpson?s Pot, the sump from Sink Chamber to Battleaxe Pitch in Lost Johns?.
If the surface valley is still being lowered by a conventional stream, then there could be concomitant lowering of the outlet for the water, and over a period of time the resurgent stream may cut its own little valley and thereby drain the phreas; however, if all (or most of) the water in the tributaries to the (surface) stream has found its way underground, there will be little downcutting of the valley taking place, and the phreas will remain as such for a long time. Figure 5 is simply a development of Fig. 4, with more erosion having taken place and the resurgence becoming more ramp-like.
Let us now suppose that the area undergoes a period of glaciation, which affects the valley where resurgence C is sited much more than that where resurgence B is situated (Figure 6); this chops off the end of that bit of cave, thereby enabling the water to take the ?new? route out to the surface at point D (once the ice has melted). Eventually this will become capable of taking all the water  (at least under normal flow conditions) and the original route dries out ? and there we have it, a ramp (Fig. 7).
Well, this is all very speculative, but I?d welcome feedback on this issue.

 

[Fulk]

Hmmm,
I'm not very good at this, am I? Figures 1 to 6 seem to ahve disappeared, so hear we go again;

 

[Fulk]

Oh b****r, could some kind soul rescue my diagrams from the limbo where they've been exiled to?

 

[paul]

Oh b****r, could some kind soul rescue my diagrams from the limbo where they've been exiled to?

[gmod]You only have a link to a single image in your post.

Put the URL for the image between appropriate 'img' tags:

[IMG][/IMG]

Always press "Preview" before "Post" to check benefit from the various buttons, including "Insert Image", which will stick the tags in for you, and also to check for for cock-ups.  [/gmod] 

 

[barrabus]

Ramps Figs 1?6 by john_forder, on Flickr

This is an attempt to show a plausible method of formation of a ramp such as that in Sleets Gill Cave. It follows on from the comments above about caves needing some sort of pre-existing openings for their formation. If limestone is conceived of as solid blocks with hair-line fractures, then I don?t see how it is possible for caves to form; there must be a way for the water to get underground first, and hair-line fractures would not do the business. I remember reading ages ago about experiments done to discover the locus of cave formation, using chunks of soluble substance (sugar lumps??) to simulate limestone; but this assumed that ?all cracks and fissures are equal? ? as already mentioned, ?yes they are, but some are more equal than others?.
In Figure 1, all the joints and beddings shown in black are very tight and impermeable to water, whereas those in red are open to a certain (limited?) extent. Thus water flowing in a stream on the surface can only get underground along this route, shown in blue in Fig. 2, which takes a convoluted path from sink (A) to two resurgences (B and C), instead of the most direct route shown in green in Fig. 1. At first the ?proto-passages? are tiny and can only swallow up a small fraction of the stream, which continues flowing along the surface (this is, of course, a two-dimensional, cross-sectional notional view, and does not purport to show that the surface stream and the underground one arrive at the same place, B). What will eventually become a ramp has a stepped profile R.  At this point there is little to choose between the two route, but that to point B has a greater head of water behind it and therefore might be expected to undergo erosion more quickly, an effect that will be self-sustaining ? the bigger it gets, the more water it can take, and hence more erosion, and the bigger it gets, etc.
As time goes by the passages get bigger and bigger until eventually a point is reached at which the sinkhole is capable of swallowing up all the water in the surface steam (under normal flow conditions) so the surface stream downstream of the sink dries up. The sharp corners in the proto-ramp have started to get eroded away (Figure 3).
At this point, any further increase in passage size means that the water from the stream can no longer fill the passages down to level X (Fig. 3). This allows for the development of an airspace, so there is an automatic change from phreatic to mainly vadose conditions in the upper passages of the system ? though the lower section is still phreatic, with a Vauclusian resurgence ? without the need to invoke any cataclysmic event such as an ice age (Figure 4). Hence the abundance of passages that have a typical phreatic/vadose morphology ? Lost Johns?, Ireby, Sunset Hole, Long Churn, County Pot, etc. etc. S (Fig. 4) illustrates a perched sump ? e.g. the Duck in Simpson?s Pot, the sump from Sink Chamber to Battleaxe Pitch in Lost Johns?.
If the surface valley is still being lowered by a conventional stream, then there could be concomitant lowering of the outlet for the water, and over a period of time the resurgent stream may cut its own little valley and thereby drain the phreas; however, if all (or most of) the water in the tributaries to the (surface) stream has found its way underground, there will be little downcutting of the valley taking place, and the phreas will remain as such for a long time. Figure 5 is simply a development of Fig. 4, with more erosion having taken place and the resurgence becoming more ramp-like.
Let us now suppose that the area undergoes a period of glaciation, which affects the valley where resurgence C is sited much more than that where resurgence B is situated (Figure 6); this chops off the end of that bit of cave, thereby enabling the water to take the ?new? route out to the surface at point D (once the ice has melted). Eventually this will become capable of taking all the water  (at least under normal flow conditions) and the original route dries out ? and there we have it, a ramp (Fig. 7).
Well, this is all very speculative, but I?d welcome feedback on this issue.

Oh b****r, could some kind soul rescue my diagrams from the limbo where they've been exiled to?

There you go.