Diminishing dimensions downstream...?

Penguin

Penguin

New member
A bit of a puzzle and wondering if anyone has a physical/geological explanation for it (or interpretation of it)...

In typical surface rivers the river channel increases from source to mouth (downstream).  A couple of caves i have been in recently seem to observe the reverse - larger passages upstream than downstream. 

One river has large passages (bit enough to comfortably walk in and room to spare), passes through one pot and the dimensions remain similar, then passes into another.  From this pot it sinks into a narrow bedding plane, which would be just big enough to crawl into though it is now choked (a distance of 75 ft in has been recorded).  The river rises after another 100 m or so, but now the bedding plane is about 6 inches high (although the bed could be dug out somewhat). 

The second river also has large passage upstream, with cross-sections of several square metres, but its rising (or what is believed to be the rising) is through a phreatic tube at most 0.25 sq m in section over the final few metres. 

Why does this happen?  Surely you would expect the passage dimensions to at least remain roughly the same over the length of the cave, or where tributaries join in, to increase in size?  Why does the reverse - at least in these instances - happen? 

Water loss through joints and bedding planes?  Possible unknown passage, possibly now dry?  :confused:

Cheers, Penguin
 
D

dunc

New member
Possible unknown passage, possibly now dry?
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I can't say I have any knowledge of geological matters but that is one possible explanation; a stroll down a stream passage and you can sometimes find the stream getting lower or passage dimensions smaller - look around and you might find an oxbow - when this rejoins the stream it regains previous proportions. So it is possible a choked (hidden) passage exists.. But then again it could be something entirely different like some geological influences restricting passage size etc many possible reasons and theories etc..

There was a question asked a while back about a similar scenario in Pant Mawr Pot:
http://ukcaving.com/board/index.php/topic,6751.0.html
 
Elaine

Elaine

Active member
My two theories are;

1. As the water gets further into the cave it loses its acidity as it dissolves more limestone. Therefore is unable to create as big a passage as it did in the beginning.

2. As the water travels further underground its options to divert increase. Perhaps the passageways further into the cave are not the original ones. This does not necessarily mean there are other passageways to find as they may have been abandoned when still very small.
 
ian mckenzie

ian mckenzie

New member
It seems to me that streams underground naturally collect rather than braid, so the diversion or splitting of a stream must be due to a change of conditions of some sort.  It may be that when bedrock conditions encourage restriction, the stream finds another way, thus braiding the exit.  Or maybe the naturally-downward gradient fills with sediment at the bottom (where else would it?), either creating the appearance of smaller passage, or causing an actual blockage and thence braiding.  Or maybe the accumulation of sediment at the bottom end of a conduit armors the bottom from further solution and...etc. 
 
Les W

Les W

Active member
Generally speaking the effects of solution are minimal in stream caves once conduits get larger than around 50mm or so in diameter. After this size the dominant erosion is by mechanical erosion by stream borne sediment. This would suggest that the more dominant controls on passage formation are by suspended sediment and volume of water rather than the aggressiveness of the water. This doesn't directly answer your question though but maybe passage gradient (and therefore flow rate) may be the main control. Less sediment will be in suspension in slower flow areas and consequently less abrasion and erosion will occur.

Perhaps...  :unsure:
 
Penguin

Penguin

New member
Hmm, lots to think about, cheers!  :)

Les W said:
Generally speaking the effects of solution are minimal in stream caves once conduits get larger than around 50mm or so in diameter. After this size the dominant erosion is by mechanical erosion by stream borne sediment. This would suggest that the more dominant controls on passage formation are by suspended sediment and volume of water rather than the aggressiveness of the water. This doesn't directly answer your question though but maybe passage gradient (and therefore flow rate) may be the main control. Less sediment will be in suspension in slower flow areas and consequently less abrasion and erosion will occur.

Perhaps...  :unsure:
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But...if the volume of water remains the same, the sections of passage with smaller cross-sections will have greater flow rate, be able to hold more sediment, and thus the water here will have greater erosive capacity, so it would enlarge the passage until a natural balance was reached...?  :confused:

"Curiouser and curiouser."  :-\

 
Les W

Les W

Active member
You might just have spotted a problem that has dogged karst scientists for years, actually there are 101* different mechanisms that might affect the size and shape of a cave and lots of different factors that will exert a local control.

The truth is that nobody really knows, as the whole thing is very complex. Solution can still play a part if the water is not saturated with carbonate and sediment can block a passage and reduce the flow. Phreatic and vadose passage formation also have different flow rates and erosion. It takes a lot of intensive study to get the big picture and even then years of arguing over factors such as "which way the water flowed here" etc. to determine how a cave evolved. Even the well understood caves are not that well understood. :-\

I think that what I'm trying to say is that there are lots of causes, lots of answers and only some intensive study can possibly supply some of the answers. :-\

You could fill a whole encyclopaedia (or more) with this stuff. Oh somebody already has.  ;)

http://www.amazon.co.uk/Encyclopedia-Caves-Karst-Science-John/dp/1579583997/ref=sr_1_1?ie=UTF8&s=books&qid=1230594150&sr=1-1
http://www.amazon.co.uk/Encyclopedia-Caves-David-C-Culver/dp/0121986519/ref=sr_1_7?ie=UTF8&s=books&qid=1230593989&sr=8-7

This is the best one  (y)
http://www.cavebooks.com/book_review0.html#geology



*Well lots anyway ;)
 
Penguin

Penguin

New member
Les W said:
I think that what I'm trying to say is that there are lots of causes, lots of answers and only some intensive study can possibly supply some of the answers.  :-\
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So, a good excuse to go caving and diving...  ;)

Cheers!
 
Pitlamp

Pitlamp

Well-known member
I suspect the most likely explanation for passages getting smaller downstream (other than localised geological factors) is abandoned outlets. Cave streams differ from surface streams in one major way; upwards development is not prevented by gravity in phreatic conditions.

The all time classic example is the Speedwell Cavern streamway in Derbyshire, which frequently reduces in size throughout its known length as one goes downstream. Near the upstream end it's a stomping great "master cave" but by the time it's emerged from Russet Well it is miniscule. The reason is the existence of various distributaries along its length, either relict ones, flood only overflows or permanently active ones. (Examples of these, respectively, are the Boulder Piles route up into Far Sump Extension, Treasury Sump providing flood flow into Peak Cavern's Upper Gallery and the split which must occur in the downstream sump such that water flows both to Russet Well and Slop Moll risings.

Surface rivers can't flow uphill, hence the normal dendritic model of converging and enlarging channels eventually debouching as a single outlet into a lake or the sea. (I'm ignoring the localised distributaries associated with deltas here, for the purposes of argument.) Cave streams CAN flow "uphill", so (in phreatic conditions at least) many distributaries draining to different risings are possible - hence a reduction in the size of individual passages.

So Penguin's suggestion of "Possible unknown passage, possibly now dry?" may well have answered the question!
 
ian mckenzie

ian mckenzie

New member
Penguin said:
But...if the volume of water remains the same, the sections of passage with smaller cross-sections will have greater flow rate, be able to hold more sediment, and thus the water here will have greater erosive capacity, so it would enlarge the passage until a natural balance was reached...?  ]
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Unless of course the restriction caused a sump or other point where the grainier sediments settled out.

Maybe the entire paleo flowpath was upgradient, which then neatly explains everything. ;)
 
Cave_Troll

Cave_Troll

Active member
in the given example, where a passage passes down a pot, remains the same size, passes down another pot and gets dramatically smaller. I'd be inclined to blame different properties of the layers of rock. in simplistic terms ;
a softer layer to create the pot
a normal layer
another softer layer on the second pot
a harder layer where the passage is smaller.
 
Pitlamp

Pitlamp

Well-known member
. . . but surely CT that idea ignores Deej Lowe's cave inception hypothesis?

I remember Clive Westlake giving a lecture many years ago about how, in north Derbyshire swallets, the passage size is greatly influenced by whether it is guided by a joint or a bedding plane. He showed photographs of many places such as Giants Hole, where a large rift (joint guided) entered a low crawl (bedding plane guided) to emerge in another large rift - followed by a low crawl - etc, etc. Caves are fundamentally influenced by structures within the limestone.

In the example of the Speedwell streamway there is a large number of distributaries (well into double figures) and the streamway progressively gets smaller all the way downstream (except where size is masked by floor sediments, such as immediately upstream of the Bung Hole).
 
Cave_Troll

Cave_Troll

Active member
all well and good, but you also have to remember that caves will do what they want. I think i've given up trying to work out what caves are likely to do "when we get round the corner" in the dig.
 
Cookie

Cookie

New member
Pitlamp said:
I remember Clive Westlake giving a lecture many years ago about how, in north Derbyshire swallets, the passage size is greatly influenced by whether it is guided by a joint or a bedding plane. He showed photographs of many places such as Giants Hole, where a large rift (joint guided) entered a low crawl (bedding plane guided) to emerge in another large rift - followed by a low crawl - etc, etc. Caves are fundamentally influenced by structures within the limestone.
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What a coincidence that perfectly describes where I was yesterday. The entrance series to Ogof Rhyd Sych is a succession of rifts (woo-hoo stood up!) and flat-out bedding crawls (bugger I can't turn my head again!).

The other coincidence was that Clive was with us.
 
Maisie Syntax

Maisie Syntax

Active member
Elaine said:
This does not necessarily mean there are other passageways to find as they may have been abandoned when still very small.


I seem to remember hearing about a charitable organisation whose mission was to prevent this sort of thing from happening. I believe it was the Society for the Prevention of the Abandonment of Cave & Karst and that it had a series of fund-raising events going by the name of SPACK-AID?
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Pitlamp

Pitlamp

Well-known member
Nice one Cookie!

CT - if you remember the late and great "Mr.P.B.Smith" you might also remember him on many occasions sagely declaring that "Caves are where you find 'em". In many ways he was right of course.

("Naah den").
 
graham

graham

New member
Pitlamp

Do caves flow uphill or is it just that when they pass through deep pools, you cannot always see the surface?

Les

I would say that quite a few caves are well understood. What is difficult is predicting in detail the bits we haven't yet seen, given that the constraints on cave development are both multifarious and very dependant on local factors.

Mind, I once won a pint off Trat by correctly predicting the direction of the final survey leg of an Irish cave extension just before the survey was done.
 
Penguin

Penguin

New member
Les W said:
You could fill a whole encyclopaedia (or more) with this stuff. Oh somebody already has.  ;)
Click to expand...

*sigh* they cost money...i'm saving up...  :(

Pitlamp said:
So Penguin's suggestion of "Possible unknown passage, possibly now dry?" may well have answered the question!
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So, any low tech solutions for finding such things? 

I suspect it will have water in it.  We found one sump but it seems to be a tributary, which was a little unexpected.  :confused:  The large passage to this sump is mostly all canal, there are some small relict tubes leading off it that are now dry, small and higher level than the current stream.  Thinking about it now, these are of similar dimensions to the believed final rising of the river...

When you say caves can flow uphill, what does this mean?  How? 

Also, is it possible for flow direction to reverse in a cave system? 

Thanks everyone!  :) 
 
Les W

Les W

Active member
Penguin said:
Les W said:
You could fill a whole encyclopaedia (or more) with this stuff. Oh somebody already has.  ;)
Click to expand...

*sigh* they cost money...i'm saving up...  :(
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If you're saving up then the best one to get is Art Palmer's "Cave Geology". This is the definitive work on the subject and has recieved glowing reviews. Art Palmer is one of the most respected Karst scientists in the world, his writing is very easy to follow and well explained for the layman with no real geological background. It's highly recommended.  (y)
You can get it here for $38 (plus postage) but you may find it cheaper if you shop around on the web, etc.
https://nssbookstore.org/index.php?mode=store&submode=showitem&itemnumber=01-0609


Penguin said:
Pitlamp said:
So Penguin's suggestion of "Possible unknown passage, possibly now dry?" may well have answered the question!
Click to expand...

So, any low tech solutions for finding such things? 
Click to expand...

Dig.  ;)

remember abandoned passages can become infilled completely, covered by sediment and even completely blocked/covered by flowstone.


Penguin said:
Also, is it possible for flow direction to reverse in a cave system? 
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Yes.
Capture of stream flow by lower passages is one of the methods of formation for the more complex cave systems, OFD, Draenen, Lancaster-Easegill, etc. Even Swildons shows multiple levels with many stream captures. Passage water flow can reverse when a stream is captured and there is evidence of just this in parts of the Swildons Short Round Trip.
 
Pitlamp

Pitlamp

Well-known member
Gents - just pointing out that I didn't write "caves can flow uphill" - but a stream within a section of a cave system certainly can.

Obviously the overall route from sink to resurgence will not be uphill (discounting, for simplicity, the sea mills of Argostli, or brief intervals when a rising might be higher than the input in the case of a siphoning action). My point was that a stream within in a cave can flow uphill under phreatic conditions. Examples: Sleets Gill entrance (in flood), the big ramp in Witches Cave (draining to Leck Beck Head), the deep rising in the upstream sump in Gavel Pot, Main Rising in Speedwell, the Devil's Staircase in Peak Cavern (in flood) - and many more. Fast forward a few hundred thousand years to when these are drained (and of course possibly blocked by accumulations of sediment) and you have the equivalent of many of our well known dry cave systems which we currently enjoy doing trips in.

A stream in a section of a cave system which shows phreatic features should always be considered in terms of three dimensions - unlike the way a surface river (and tributaries) flows downhill in all parts of its course. Therefore cave development under phreatic conditions should always be considered in three dimensions. As a result relict distributaries may exist - which can allow a major trunk passage to become smaller as one goes downstream.

One problem is that many people who study cave development have no choice but to base most observations on abandoned phreatic passages. However cave divers have the opportunity to observe still active phreatic passages and often spot glaringly obvious things which non diving researchers understandably miss. (An example of this is the common phenomenon of water layering, which has a profound effect on passage development.) Serious students of speleogenesis should spend time browsing CDG literature, because it contains many useful observations which can help their understanding of how phreatic caves really develop.

Enough - I'm going caving.
 
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