Author Topic: Solution Caves in Chalk  (Read 6910 times)

Offline Peter Burgess

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Solution Caves in Chalk
« on: June 01, 2006, 12:32:05 pm »
Given that in chemical terms, chalk is very much the same as the massive limestones of our classic karst regions, why is it that there are virtually no known solution caves in the chalk karst regions of Britain?

Chalk downland does exhibit a number of typical karst features which might encourage one to look for caves. These include dry valley systems, innumerable clay-filled solution pockets, and, I believe the occasional closed basin. What we don't find are open shafts, resurgence caves, and, apart from a few rare exceptions, natural collapse features.

With the chalk being the most important exploited aquifer in southern England, you would think we would know all there is to know about it.

One possibly important difference between the chalk landscape and the classic upland karst regions is that very few significant water-courses flow off surrounding impervious strata onto the chalk, and where they do it is usually effectively at sea level (the principal rivers of Kent and Sussex).

One exception is the River Mole. The river crosses the chalk with a steep (for the SE) gradient, and lots of it disappears into swallow holes in the river bed and bank. These are choked with flints and dead leaves etc. The water rises again at Leatherhead in the river bed. There have been a few reported collapse features in the area.

There are one or two other sites where smaller streams flow off impervious beds onto the chalk and disappear. There is a well-known one near Potters Bar, and another one in Farnham Park. The one in Hertfordshire was being dug by a caver some 20 or so years ago, and a short length of stream passage was seen, but nothing came of it. There are other similar locations, but not many.

The caves at Beachy Head and St Margaret's Bay are fossil cave passages.

So, one reason for there being no caves might be that unlike limestone areas, the chalk downs tend to be the highest features of the landscape, with no significant water flows crossing it at a point where the vertical range to a potential resurgence might encourage caves to form.

Another reason may be the structural strength of chalk. Could chalk simply not be strong enough to support a permanent cave passage system? The fact that a few caves do exist suggests that this is not a valid reason.

Perhaps it's the way water flows through the chalk. Perhaps water finds its way through innumerable fissures rather than migrating through a few discrete joints and fissures in the rock. By its flow not being concentrated, the solutional effect is dispersed and no large passages result, with a few exceptions. But the evidence from wells sunk into the chalk might suggest this to be a false premise also. When 19th century engineers were digging deep wells to tap the large reserves of water held in the chalk aquifer, they often had disappointing results. Having reached the saturation level, the flow of water into the new well was often insufficient. So the engineers resorted to driving a number of levels from the base of the well, below saturation level, (which indicates how poor the water flow was), until they intersected fissures from which substantial amounts of water then flowed. The well then often filled up to a level well above that of the horizontal tunnels. This suggests that the water flow is contained within discrete fissures.

Much of the rain that falls on the chalk downs finds its way underground. Much of it does not sink at discrete locations, but generally through the soil and into the bedrock. It all comes out somewhere else - the River Wandle, for example. So who can tell me why cave systems, however modest, have by and large not been discovered?

Offline Peter Burgess

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Solution Caves in Chalk
« Reply #1 on: June 01, 2006, 01:27:03 pm »
Oh, I forgot to mention. Seeing as how were having a regional 'discussion' elsewhere, I'd just like to remind you that Yorkshire has it's fair share of chalk landscape, as well as Northern Ireland.

Offline Peter Burgess

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Solution Caves in Chalk
« Reply #2 on: June 01, 2006, 01:32:18 pm »
http://www.wcms.org.uk/cgi-bin/wcmsgallery.pl?imgno=566

Here's a photo of a natural solution feature in the side of a chalk pit in the Arun valley, West Sussex.

Makes you wonder what there might be down there.....

Deeply Mendippy

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Solution Caves in Chalk
« Reply #3 on: June 01, 2006, 01:57:51 pm »
I know very little about geology, but i'll put my two'pennoth worth in anyway.

By guess would be that water just goes through the millions of tiny fissures in the chalk.  Any rain that falls on the downs seems to drain into the soil pretty rapidly. If you walk at the base of the chalk cliffs then every 40 or 50 yards there is a little 'spring' where there is a trickle of water that looks to be coming out of patches of the chalk itself.

Is it possible that the water travels through the chalk in channels that are made up of more fissured rock. In other words each channel is just a line of rock that water has eroded (or chemically reacted with some inclusion in the chalk) millions of tiny fissures into.

Out of curiosity has anyone done dye-tracing to see how long it takes water to travel through the chalk?

Offline Peter Burgess

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Solution Caves in Chalk
« Reply #4 on: June 01, 2006, 02:10:03 pm »
Quote
just a line of rock that water has eroded (or chemically reacted with some inclusion in the chalk) millions of tiny fissures into.


Do you mean that there are no singular large fissures, just concentration clusters of micro-fissures in discrete lines? I suppose this is possible, but I don't really know.

Chalk is a peculiar rock. A very significant proportion of it is empty space - hence it has a very low density compared to carboniferous limestone, for example. I believe it is one of Britain's lightest rocks. The microcavities that make up the empty space are so small that a lump of chalk is effectively impervious. All the water that passes through chalk does so in the fissures and not through the rock, which would be the case in a sandstone, for example.

This is why many wells sunk failed to find water initially since the well-sinkers did not intercept any significant fissures.

Perhaps a visit to Birling Gap is in order to look for solutional features, but as I am no geologist, I don't think I am going to find anything that better qualified scientists may have missed.

I am unaware of any dye-tracing, though I have a feeling that the Hertfordshire swallets may have been tested at some time.

Offline AndyF

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Solution Caves in Chalk
« Reply #5 on: June 01, 2006, 02:29:48 pm »
There are lots of chalk caves in Northern France. Some several KM long.

Geologically, this is the same beds as SE England.
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Offline Peter Burgess

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Solution Caves in Chalk
« Reply #6 on: June 01, 2006, 02:38:29 pm »
Quote
There are lots of chalk caves in Northern France. Some several KM long.

Geologically, this is the same beds as SE England.


Absolutely. I neglected to mention these. Has anyone been down any of them? I believe a few of them were discovered by well-diggers.

Deeply Mendippy

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Solution Caves in Chalk
« Reply #7 on: June 01, 2006, 02:48:48 pm »
Quote from: "Peter Burgess"
Quote
just a line of rock that water has eroded (or chemically reacted with some inclusion in the chalk) millions of tiny fissures into.


Do you mean that there are no singular large fissures, just concentration clusters of micro-fissures in discrete lines? I suppose this is possible, but I don't really know. .


Exactly that, you have expressed my thoughts far better than I did.

Perhaps these clusters of channels occur as a result of some sort of capillary action. Or maybe the alkaline nature that the water eventually takes on from prolonged contact with the chalk reacts with veins of acidic material that are deposited within the chalk. In that way places where water pools will start to erode these small fissures so that in time the as the pool of water ‘sinks' through the chalk and creates a permeable channel that is not discernable by the naked eye.

I have no real idea what I am talking about. Pete, perhaps you could also summarise this post in a way so that it makes sense and sounds intelligent! :lol:

Whatever the cause it doesn't look like there are many big chalk caves out there to be found.

Offline Peter Burgess

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Solution Caves in Chalk
« Reply #8 on: June 01, 2006, 03:08:42 pm »
http://www.kurg.org.uk/sites/natural_caves.htm

This link (again) sums up what there is pretty well, including a reference to the Hertfordshire site being dye-traced.

[edit - added comment about dye-tracing]

Offline Peter Burgess

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Solution Caves in Chalk
« Reply #9 on: June 01, 2006, 03:41:07 pm »
I've tried googling for those French caves but can't find anything. I am sure I've read someting on the 'net about them.

Offline AndyF

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Solution Caves in Chalk
« Reply #10 on: June 01, 2006, 04:19:36 pm »
I have a reference at home, I'll look it up....
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Offline AndyF

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Solution Caves in Chalk
« Reply #11 on: June 01, 2006, 10:13:09 pm »
"Life's a pitch, then you fall down one..."

Offline Peter Burgess

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Solution Caves in Chalk
« Reply #12 on: June 02, 2006, 11:29:56 am »
These are very interesting! I am sure I have read accounts of caves discovered when wells were sunk - I wasn't aware of these coastal caves.

Online Huge

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Solution Caves in Chalk
« Reply #13 on: June 02, 2006, 10:18:19 pm »
Have a look at this from the latest edition of Oxford Uni.C.C.'s newsletter Depth Through Thought.

http://www.oucc.org.uk/dtt/vol16/dtt16_07.htm

The relevant article is at the bottom of the page.

Offline mrodoc

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Re: Solution Caves in Chalk
« Reply #14 on: December 08, 2006, 08:19:20 pm »
Just to put a comment in. I have explored a 12 metre chalk cave in the landslip between Seaton and Lyme Regis. This is definitely solutional and ends in a 1 metre deep pot. In the fields inland collapses into chalk 'pipes' occur - I examined one a friend's tractor nearly descended  a few years ago. There is also a big rising called the Gusher at Charton Bay also on the Landslip and coming out at the base of a slipped chalk cliff.

Offline gus horsley

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Re: Solution Caves in Chalk
« Reply #15 on: December 13, 2006, 08:46:12 am »
I think that local geological conditions dictate whether caves exist or not, regardless of the type of limestone.  On Anglesey there are sizeable areas of Carboniferous Limestone but very little cave development.  On the other hand, one of the longest caves in the world is developed in gypsum, which is a much softer material than chalk.  Perhaps it's time to reappraise the potential for cave development in chalk. 

Offline SimonP

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Re: Solution Caves in Chalk
« Reply #16 on: August 04, 2018, 10:45:49 pm »
My understanding is that chalk is porous whereas carboniferous limestone is only permeable.  The difference being that water falling on chalk can soak through it as though it were a sponge whereas water can only penetrate limestone along joints and bedding planes. It is this restricted route for the water that allows passages to form. Impermeable layers in chalk might concentrate the water into specific channels and in theory, that would allow a passage to form but that requires very specific geological conditions to allow it to happen. The sea caves at Flamborough are truly spectacular and do demonstrate that chalk is at least structurally capable of forming large cave passages.

Offline aricooperdavis

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Re: Solution Caves in Chalk
« Reply #17 on: August 04, 2018, 11:07:56 pm »
I was looking at this recently too after the excellent BCRA Cave Science weekend with Andy Farrant. I believe you do get channelisation in chalk, and chalk aquifers do usually contain some conduits. This complicates pollution control, as rather than chalk being a sponge-like uniform porous rock that pollutants diffuse through, the channels can accelerate pollution spread. A relevant journal article about that here.

Offline chriscastle46

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Re: Solution Caves in Chalk
« Reply #18 on: August 05, 2018, 08:30:00 am »
No one has mentioned the caves at Beachy Head. I've never seen them myself but I know one of them was a fair-sized affair, a long phreatic passage. I've a vague memory of an article in Descent, probably from the 80s, perhaps someone with the index could look it up. I think the exploration was carried out by the Chelsea SS.

Offline Les W

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Re: Solution Caves in Chalk
« Reply #19 on: August 05, 2018, 09:37:55 am »
No one has mentioned the caves at Beachy Head. I've never seen them myself but I know one of them was a fair-sized affair, a long phreatic passage.

Size is of course relative. The tube is now split into two by a cliff fall but the longest part still goes some distance, perhaps a hundred metres or two, and at one or two places you can actual sit up...  :-\
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