The normal model of cave development in horizontally bedded limestones is that many horizontal passages are guided by bedding planes and many vertical shafts are formed in joints or faults. (Experts in speleogenesis please bear with this oversimplification - yes I'm fully aware of Deej's inception horizon hypothesis etc. but for the purposes of my question, the above is OK.)

So how do sloping ramps form? The best examples I can think of are in Sleets Gill Cave but there are others (the Treasury area in Peak Cavern for example). I'd prefer not to let on why I'm interested in anyone's answers just yet,  as I don't want to influence those answers beforehand. (It's to do with another impressive ramp elsewhere which very few cavers have seen and which we're trying to understand the development of.) I'd really appreciate any suggestions. Thanks.

What do you mean by a ramp? Does this feature cut across the bedding?

Speleology can be so, so simple if you just think out of the box!

Obviously, the Morlocks have wheels.. 

At the risk of setting off a QI style klaxon- is not Sleets Gill ramp a phreatic lift?

I think ramps could form by phreatic stoping in a manner similar to magmatic stoping.  I know it's a bit of a radical view but, here in Cornwall, the Cornubian Batholith which underlies the local rocks and forms Dartmoor, Bodmin Moor, etc, has an underground profile similar to ramps in caves.  It's risen under pressure and dissolved the rocks in all directions, but mainly in an "uphill" direction, usually following a pre-existing weakness to start with but then exceeding the weakness as it rises.  I'll have to give this one a bit more thought as it's only just occurred to me.

Thanks for the contributions so far. What I'm really trying to get at is what "weakness(es)" within the limestone is the water following? Why does a cave SLOPE upwards in horizontal bedding rather than going horizontally or vertically as is more usual?

Gus - I'd like to know what the "pre-existing weakness" you referred to in your analogy is, in a karst situation.

Graham - your 2nd question is germaine but actually that's precisely what I'm trying to sort out myself.

Andy - yes, the Sleets Gill ramps are uphill sections of formerly phreatic passages. (The ramp I'm thinking of is presently under phreratic conditions in a cave system inm the Dales which you've had quite a lot to do with recently.)

Nice photo Joel - reminds me of a certain uphill dig in Speedwell which used to do that to us!

is it possible that in fairly level bedding where phreatic water is pushed upwards under pressure, initially the proto-passage could have a stepped form, but over time the sharp edges of the 'steps' get worn away and smoothed over, to give the impression that the water followed some kind of weakness ? a bedding plane, for example ? that itself was sloping upwards? Could this give rise to a 'typical' oval/rounded phreatic shape?

Pitlamp

Are you quite certain that the feature that you are interested in hasn't followed an oblique joint?

Not arguing, just expressing my own ignorance... but I thought in sedimentary rocks oblique joints were developed at an angle to both dip and strike but tended to be largely perpendicular to the bedding?

I wonder how steep the ramp is and how long? Obviously some ramps (Bar pot?) are large fallen blocks.

Hi Pitlamp,
Also note the orientation of the riser compared to the major joint pattern.
Even allowing for conjugate joint sets it is not easy to envisage what process could initiate the formation of the riser which cuts across both beddings and joint patterns.
A good one to ponder over. With further observations and shared details maybe someone will hit on a good theory?

I just rememberd (and it may not be relevant to this case) but when doing compression tests on rock, they fail at an angle (the angle has meaning but it is more than a couple of decades since I did this at collage so I can't remember what). The point being that there are stress modes that will induce sloping cracks.




and this is the result of a triaxial test

Pitlamp said:

Gus - I'd like to know what the "pre-existing weakness" you referred to in your analogy is, in a karst situation.

Click to expand...

Could be a fault or a greater density of joints in a localised area.  Another possible factor in the formation of ramps could be the nature of insoluble sediments and the way they are deposited on the floor of a passage as these will impede further solution in that direction, thereby encouraging an uphill flow by the same stoping process.

I'm going to try to upload a diagram illustrating the notion I outlined above, with water flowing along horizontal beddings and up vertical joints (black lines), being slowly modified over time (blue lines and red lines).

Bugger; I'll try again:

Here you go Fulk:

Thank you, Rob

Hello again all - just back in from work. Thanks for further contributions above. I suppose I ought to come clean; the ramp in question is in the massive sump between the dry part of Witches Cave (now accessed via the new Shuttleworth Pot entrance - see most recent Descent) and the resurgences at Leck Beck Head. The ramp is a very big submerged passage which slopes upwards at something like 30 degrees between -30 m and -9 m Unfortunately only one person has ever seen it (self). Both ends of this ramp are protected by desperately dangerous chokes which I only passed (literally holding my breath) on three occasions. The most recent was some 14 years ago. So we're very short on direct observations I'm afraid since, on each of those three occasions, survival was the priority.

However it looks almost identical to the entrance ramp in Sleets Gill Cave (albeit that in Sleets Gill isn't as big a passage as the one I'm interested in). So if someone can confidently describe how the Sleets Gill ramp formed there seems a good chance that the Witches Cave / Leck Beck Head ramp may have a similar mode of formation.

(Graham - I'm sorry but I can't rule out the possibility of a deep sloping joint - but I don't remember any direct evidence for one.)

By the way, one of the examples I gave above was inadvertently a bit of a red herring; the ramp in Treasury Chamber in the Peak Cavern system is a bit unusual in being close to a significant pipe vein; I'm 99.999% certain that this does not apply in the case of the Witches Cave / Leck Beck Head  ramp. I suspect one of the postings above is close to the mark but let's see if anyone has studied the ramp in Sleets Gill and, if so, see what we can usefully learn from that and maybe apply to the Witches / Leck Beck Head ramp's possible method of formation.

Following on from Faulks post might it be possible that the passige formed during a period of gradual base level lowering where the passige could switch between vadose and phreatic charecteristics so that you could have a series of steps form under phreatic conditions then lowering of the water table and flow reversal down cutting into the steps under vadose conditions then reinundation and disoloution to form a ramp with aparently phreatic features?

which should give a passige with a ramped floor but stepped roof rather than a symetrical passige.