Diminishing dimensions downstream...?

[Pitlamp]

Hey Graham - I'd probably call that big old sediment filled passage "mature"; the passage is still well developed even if you have to crawl along it because of infill.

Incidentally I have a question; your posting above refers to "fossil" as well as "relict". I once got told off by John Gunn (or was it Deej?) for using the word "fossil" - apparently it should have been "relict". What's the difference?

 

[graham]

Oh hell, I'll probably bring down the wrath of the professional geomorphologists with this one, but anyway.

I'd use relict to describe a piece of cave passage that no longer has any connection with an active karst drainage system, such as a bit of truncated passage found high up on a gorge wall somewhere such as Cheddar.

Fossil on the other hand I'd use to describe a piece of a drainage network that is no longer used, although its connections with the current drainage network are clear and extant, such as, for example, Bat Passage in G.B.

<Sits back to await sneers from academia.>

 

[Pitlamp]

Well, whatever the academics think - I reckon that's a pretty useful distinction.

 

[Penguin]

Lots of excellent info, thanks! 

Hi Penguin,

The first two photos show a vadose streamway, you can see that the passage originally started at roof height and would have been an eliptical tube formed along the weakness of a bedding plane. It would have been phreatic at this point, (i.e completely full of water). At some point the stream would have started down cutting due to external influences, perhaps erosion in the river outside the resurgence lowered the base level, and the cave stream started cutting the floor away more than the walls.
An air space would now be present above the water and all erosion would be on the stream bottom (and lower walls), as the stream down cuts the passage above is left high and dry. This forms a "vadose canyon" with the stream now just taking up a small part of the cross section of the passage.
The bottom of the passage in your photos is quite wide and much wider than the higher canyon, I would guess this is either because the rock here is much weaker (at a bedding plane joint perhaps) and more easily eroded, or that the external factors that caused the down cutting have been negated somewhat (water has reached a new base level).
The water now appears to have eroded the sides of the passage a lot and stopped down cutting. The sediment on the floor supports this theory.

There are two possibilities for the lower passage. One is that the passage is as large as the rest but buried in the sediment on the floor and the other is that the lower passage remained below the base level of the resurgence so there was no vadose down cutting there.

Where are the caves? This might help with this remote diagnostic of speleogenesis.  

Co. Monaghan, near Carrickmacross.  The photographs and section are of the Tiragarvan river cave. 

 

[Roger W]

Hi, folks!  Back online at last...

An associated phenomenon - cave passages decrease in size over time. Deposition of calcite formation can partially explain it, but doesn't account for the breathtaking speed at which previously comfortable squeezes become a life-or-death struggle after a few years.

Tony,

Do you find that wetsuits and the waistbands of your caving overalls tend to get smaller with the passing of time?


Phreatic passages, dimensions of...

On a more serious note - in my considerable ignorance, I thought that the diameter of a phreatic passage would be somewhat related to the flow of water that formed it, in that it would be totally full of water while it was being formed.  If the water flow then decreases, you get a small stream flowing in the bottom of a round tube.  Then, as the solution/erosion/whatever is limited to the bottom bit of the passage, you get a vadose 'canyon' formed, the bottom bit of which may be wider or narrower than the original tube depending on the 999 different geological factors involved.  The question is, how long can you expect the 'small stream in a round tube' situation to go on for before a groove starts to appear in the floor?  Or am I totally konfuzed?

By the by - I thought a relict was an old out-of-date caver whose advanced years and rheumatics didn't let him go underground any more... 

 

[Les W]

On a more serious note - in my considerable ignorance, I thought that the diameter of a phreatic passage would be somewhat related to the flow of water that formed it, in that it would be totally full of water while it was being formed.  If the water flow then decreases, you get a small stream flowing in the bottom of a round tube.  Then, as the solution/erosion/whatever is limited to the bottom bit of the passage, you get a vadose 'canyon' formed, the bottom bit of which may be wider or narrower than the original tube depending on the 999 different geological factors involved.  The question is, how long can you expect the 'small stream in a round tube' situation to go on for before a groove starts to appear in the floor?  Or am I totally konfuzed?

Hi Roger,
The main control on whether a passage is phreatic or vadose, is where the local control on the "water table" is.
If a passage is in the phreatic zone then it will remain full of water regardless of flow, equally if the "water table" falls for whatever reason then the passage will start to de-water and  a vadose trench will form. Somewhere such as Swildon's stream way exhibits this quite well, with the stream down to Sump 1 pretty much all vadose but the sump itself is phreatic, then the passage reverts to vadose again until sump 2 and so on. The local control will be the top of the original phreatic loop, as the water level drops on the down stream side of the highest point of the loop then the stream down cuts until it achieves an ideal gradient. The sumps are below this gradient so they stay full of water.

Note: The concept of a Water table is not normally applicable in non porous rocks, controls on water levels tend to be local. Graham would probably refer to a "piezometric surface" as a more accurate term, but I used water table so that most people would know what I meant.

 

[graham]

Les is right to say that piezometric surface is a preferable term. It is important to note that this surface, whatever you call it is not actually level and is affected by the presence and by the size of any nearby voids (cave passages).

 

[Pitlamp]

OK you lot - here's one to make you scratch your heads.

Can a vadose canyon form in permanently phreatic conditions?

 

[Les W]

OK you lot - here's one to make you scratch your heads.

Can a vadose canyon form in permanently phreatic conditions?

No.
Anything that forms under phreatic conditions must, by definition, be phreatic. 
It may be possible for something that looks like a vadose trench to form under phreatic conditions by preferential erosion along a joint or other weakness but it would still be phreatic.
A paragenetic canyon will form under phreatic conditions, in fact it has to be phreatic at the time of paragenesis but that will have its own passage form, although similar to a vadose trench it will normally exhibit some evidence of it's phreatic nature.

ps. I got paragenetic in before Graham  :tease:

 

[graham]

ps. I got paragenetic in before Graham  :tease:

Only 'cos you got up earlier!

But you are right a vadose canyon must, by definition form above the "water table". I can conceive of rising water conditions where one would become flooded later, though.

 

[Pitlamp]

OK then, so could "something that looks like a vadose trench" form in a phreatic cave (where there is NO weakness to allow preferential solution)?

 

[Roger W]

As Les sez, solution along a vertical joint could produce a canyon-like passage.  But in uniform rock, I suppose you ought to get a circular passage..?

Do you have a particular passage in mind?

 

[Peter Burgess]

Not necessarily circular, as even if there are no distinct joints or bedding planes, the water may erode the limestone slightly more (or less?) in the direction of bedding than across the bedding. The Drainpipe in Goatchurch is phreatic, is not circular, and I think is wider in the direction of the inclined bedding I seem to recall. Or is the Drainpipe following a distinct joint? I'm sure there are classic phreatic passages that are oval in section.

 

[graham]

OK then, so could "something that looks like a vadose trench" form in a phreatic cave (where there is NO weakness to allow preferential solution)?

yes, a paragenetic canyon from which the sediment had subsequently been flushed would look very much like a vadose canyon.

Or do you mean a "vadose trench" below an obvious phreatic half-tube? In this case, it's pretty difficult to work out a scenario. An alternative possibility is that it is a genuine vadose canyon that has subsequently been flooded owing to a rise in base level, unusual but by no means unknown.

 

[graham]

I'm sure there are classic phreatic passages that are oval in section.

Indeed there are, the long axis is usually influenced by a fracture of some kind which has allowed preferential solution.

 

[Peter Burgess]

I'm sure there are classic phreatic passages that are oval in section.

Indeed there are, the long axis is usually influenced by a fracture of some kind which has allowed preferential solution.

A fracture? Not just by the differential ease of erosion depending on the bedding direction? By which I mean rock where there is no 'bedding discontinuity', but perhaps some other factor which makes the rock stronger across the bedding, say, than along it. Sorry for not using correct terminology as I am not a geologist.

 

[graham]

Sorry Pete it's the terminology issue. A geomorphologist will say "fracture" instead of "joint" or "fault" 'cos it saves him having to bother as to whether there has been any movement along it or not. In teh same vein I should have said "plane of weakness" to encompass bedding planes as well.

 

[Peter Burgess]

OK then, so could "something that looks like a vadose trench" form in a phreatic cave (where there is NO weakness to allow preferential solution)?

yes, a paragenetic canyon from which the sediment had subsequently been flushed would look very much like a vadose canyon.

Or do you mean a "vadose trench" below an obvious phreatic half-tube? In this case, it's pretty difficult to work out a scenario. An alternative possibility is that it is a genuine vadose canyon that has subsequently been flooded owing to a rise in base level, unusual but by no means unknown.

I suppose a vadose canyon which subsequently found itself in phreatic conditions would then continue to develop as a phreatic feature, but in the longer term carrying some original vadose hallmarks. Is there such a place that anyone knows?

 

[Pitlamp]

You see? You're all discussing semantics whilst not understanding the more important principle itself. This is why observant people who aren't academics but spend a lot of time in active phreatic passages (i.e cave divers) can help. I've lost count of the number of times I've tried to encourage John Gunn to read the CDG Newsletter! (OK, to be fair, his is a very busy bloke.)

Anyway here goes. I believe you can get a phreatic tube with a vadose-style canyon below it in a permanently flooded passage. I've seen this on many occasions where there is no vertical fracture which might guide such a canyon by allowing preferential solution. There's a very good one about 1.2 km into the Marble Steps Branch of Keld Head for example. Further afield there's one in a sump in the Grotte Du Plaisirfontaine in France. I could mention various others but these will suffice for now.

I've never been totally convinced by the paragenesis idea (or at least the great contribution that some people afford to it in developing phreatic passages) but what I'm talking about is a different situation anyway. I'll try to explain how I think these canyons form. The classic model of phreatic development is "equal solution in all directions". This certainly can occur but in a high proportion of cases phreatic "tubes" are actually not circular. Having watched sediments being transported in high discharge whilst diving in underwater caves I believe that physical erosion is far more important than many people appreciate.

We all know from our school geography lessons that one of the main ways rivers erode downwards into bedrock is by swirling pebbles round, which eventually drill their way downwards forming "potholes", or "rockmills" (or "marmites" as the French call them). The often heavy cobbles which form these features are inevitably going to spend most of their time at or near the floor of a sump. Therefore you get exactly the same rockmills in the floors of sumps. As more and more of these develop they start to overlap each other and eventually coalesce to form a canyon. This is effectively the same process as that which occurs under vadose conditions in some caves, which is why arguing the toss over phreatic versus vadose isn't as helpful as might at first be thought. Even if bigger pebbles and cobbles aren't being twirled around enough to form rockmills they still bang into the floor a lot more often than the walls or roof, causing incision of the floor. Really big ones may just get scraped along the floor slowly - but this still abrades the rock in a vadose kind of way. Canyons formed as above may be further widened in a minor way by pseudokarren grooves forming, as sand piled up on ledges cascades down the wall - again this is a process which can actually be observed by cave divers.

Many non diving cavers are fooled by the sight of a relatively placid sump pool at the end of a boisterous streamway - yet underwater passages often have violent currents which certainly can transport big heavy rocks and use them as agents of erosion. And because the larger pebbles and cobbles are profoundly influenced by gravity they attack the floor rather more than than roof and walls.

Gravity is an extremely important factor in the formation of phreatic passages - witness, for example, the effects of hydrostratification where water layers of different densities (and dissolutional aggressiveness) are extremely common. This often leads to chemically aggressive allogenic water being trapped in the roof by denser, more chemically benign autogenic water beneath. The upper layer carries on dissolving limestone at roof level in between flood events when one might expect there not to be much dissolution occurring. It "stopes" upwards, forming high solution domes on cross joints (without comparably sized pits in the floor on the same joints). Evidence for hydrostratification is common in drained phreatic passages (i.e. those accessible to non diving cavers) but often goes unnoticed.

So you see - a canyon in the floor of a drained phreatic passage may NOT have been formed under vadose conditions as has always been assumed. And its mode of formation may have more in common with the processes which occur in vadose streamways than with the classic phreatic model. A canyon in an undrained phreatic tube (i.e. a sump) does not necessarily indicate that the tube has been drained at some former stage - yet it probably does say a lot about the mechanisms responsible for the formation of that passage.

So - what should we call these submerged trenches folks - "pseudovadose canyons" perhaps? "Phreatic canyons"? I dislike the latter because it could cause confusion with truly paragenetic sections of passage - but I'm no expert and I'm willing to be told I'm wrong.

Anyway there you go - a few ideas to mull over for those who don't get chance to visit and directly observe underwater caves.

 

[graham]

This is going to require thought and careful phrasing.

Firstly, Pitlamp, we are not really just discussing semantics as there is undeniably a clear difference between a passage with an open water surface and one wholly filled with water where rock removal by water* can potentially take place on all surfaces.

Secondly, I wholly agree with you that abrasion has a far greater effect in most caves than has sometimes been realised. In fact Charlie Self and I once had a paper in Cave Science (vol. 23.2, 1996) which dealt with that very point. I think, in part this happened because it is far easier to measure carbonate in solution than it is to measure rates of mechanical removal.

Thirdly, I wholly agree that sediment deposits within passages are extremely significant in defining the directions in which a passage will "grow" owing to their ability to armour the bare rock surface. This is so in all passages, whether flooded or not. In vadose passages it will be one reason for variation in passage width as solution and abrasion can only affect the unarmoured passage sides above an armoured floor.

Fourthly, however, it is important to stress that both solution and abrasion will favour points or planes of weakness in the bedrock, fractures, bedding partings, mineral veins, differences in lithology etc. It is this and the presence/absence of sediment that will be the most important factors in defining passage shape.

So, although I agree with you that canyon-like features such as you describe can and do form in wholly phreatic conditions, there are some caveats that need applying. The first is that cavers (and cave scientists) tend to focus on "classic" passage forms, such as the phreatic bore passage in Peak Cavern as they are good examples for teaching purposes, but are relatively rare in such pure form. The second is that the formation of marmites does, indeed, rely in part on the effect of gravity. This is significant in this discussion. In vadose passages there tend to be vertical features just upstream of such potholes as there needs to be reason why the rock removal agent (in this case cobbles) acts at that particular point. So, in a sump where, by definition, there will not be a local gravitational effect within the stream there will need to be some other reason why the cobbles act at that particular point or on that particular horizon. I would guess (I might even predict!) that such features are found at the lower elbow of a sump, where cobbles would - just - settle as the gravitational force to move them up the next piece of passage above the low point. Alternatively - and I suspect that this might be the answer in the specific case that you describe - it may be that cobbles will catch differentially where there is a small precursor groove in the floor of a passage formed by solution on a line of weakness in the bedrock. This might be so small that it would be difficult to recognise in the field after the much larger line of marmites/pseudo canyon has formed.

And that's enough thinking for one day. Time to feed the dogs.


*This is one of the wooliest bits of our vocabulary. For the benefit of this post solution means rock removal by wholly chemical means - dissolution and removal in solution and abrasion means rock removal by wholly mechanical means - impact and removal in suspension, driven by the kinetic energy of rapidly moving water.