Author Topic: Photos of the lake in Pridhamsleigh  (Read 7353 times)

Offline Neil

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Photos of the lake in Pridhamsleigh
« on: August 01, 2011, 07:30:47 pm »
Hi

We are looking for a good (well lit) photo of the Pridhamsleigh lake for an article my partner is writing about caving for 'Active Dartmoor' magazine. Your photo would of course be credited to you. We need it this week to meet the deadline!

Thanks
Neil

Offline Burt

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Re: Photos of the lake in Pridhamsleigh
« Reply #1 on: August 02, 2011, 11:27:53 am »
Hi Neil

PM me your email and I'll send you a couple.

Burt
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Offline The Old Ruminator

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Re: Photos of the lake in Pridhamsleigh
« Reply #2 on: August 02, 2011, 07:50:32 pm »
Any help.



Do what you like with it. My photo.

Offline gus horsley

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Re: Photos of the lake in Pridhamsleigh
« Reply #3 on: August 03, 2011, 11:36:18 am »
How deep is the lake?  Someone told me it's 100ft+.  I've done a lot of trips into Prid but I've never seen any real fluctuation in water level so I'm assuming it's an inactive phreatic feature.

Offline Les W

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Re: Photos of the lake in Pridhamsleigh
« Reply #4 on: August 03, 2011, 12:28:31 pm »
There is a survey on the wall at Crest Hill (DSS Hut). I thought it was deeper than 100ft though. I seem to remember it being nearer to 200 ft but of course I could be wrong.  :-\
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Offline Neil

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Re: Photos of the lake in Pridhamsleigh
« Reply #5 on: August 03, 2011, 12:35:18 pm »
Thanks for the photos Bert and Old Ruminator. Still very happy to receive more though :)

Offline mike barnes

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Re: Photos of the lake in Pridhamsleigh
« Reply #6 on: August 03, 2011, 12:42:37 pm »
I did many dives in there in the 90's. Some narrow rifts were found around the edge of the submerged chamber which, according to the old type of depth gauge I was wearing, showed a max depth of 36m. However, after getting a computer gauge, this always read about 2.5m deeper than the analogue gauge. The computer was confirmed as accurate by being used alongside several digital gauges over the years. So the deepest part I ever dived was probably 38.5m.
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Offline gus horsley

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Re: Photos of the lake in Pridhamsleigh
« Reply #7 on: August 03, 2011, 01:31:21 pm »
Thanks Mike, so have you any theories as to the formation of the lake?  I'm interested in geomorphology.

Offline graham

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Re: Photos of the lake in Pridhamsleigh
« Reply #8 on: August 03, 2011, 02:16:16 pm »
Thanks Mike, so have you any theories as to the formation of the lake?  I'm interested in geomorphology.

At a guess it's a solution void caused by slow moving phreatic water.
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Offline The Old Ruminator

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Re: Photos of the lake in Pridhamsleigh
« Reply #9 on: August 03, 2011, 06:12:50 pm »
Intrepid cave divers ( Not ! ) at The Lake, Pridhamsleigh.


Offline The Old Ruminator

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Re: Photos of the lake in Pridhamsleigh
« Reply #10 on: August 03, 2011, 06:24:30 pm »
Of course as Peter Glanvill will tell you the lake itself is only part of the feature. The line in the photo takes you through an archway part way down the vertical pitch that is The Lake. This then rises to an airspace in a huge flooded chamber. Measurements have been taken with a sonar device to ascertain the size of this flooded chamber.No doubt Peter has details and I expect its written up somewhere.

Offline The Old Ruminator

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Re: Photos of the lake in Pridhamsleigh
« Reply #11 on: August 03, 2011, 06:26:54 pm »
Good old Wiki.

Pridhamsleigh Cavern is a cave on the outskirts of Ashburton, Devon, England. It is approximately 1.1 kilometres in length with a total depth of just over 50 m including Prid II.

Pridhamsleigh is a good site for novice cavers although it is quite muddy. It has a large variety of passages which lends itself to longer explorations. Due to the nature of the connecting and non connecting overlapping passages in the cave, surveys (maps) of the cave are hard to interpret. The cave contains 'The Lake', a large elongated pool, which has been dived to discover a second chamber, with no passages leading off it. This chamber is the biggest in Devon and is named Gerry's Chamber after its discoverer, the late Gerry Pritchard.

An accurate, hand-drawn, plan of the cave is held in the reference section of Plymouth Library. Although not requiring any great skill, the cave is quite complex, there being three distinct routes from 'Bishop's Chamber' to the lake. First-timers should note their route carefully as it is possible to get lost.

The cave is the type locality for the cave shrimp endemic to the south-west of England, Niphargus glenniei.

Vertical range: 53m

The cave is a complex multilevel maze. The entrance is large and there are series of passages on either side of the main route in - a low stoop. The cave is extremely muddy but caver erosion over the last half century has resulted in many low crawls becoming of walking size as well as a cave diving site, The Deep Well, now becoming choked with mud.

At the far end of the system is the Lake. Water levels here fluctuate considerably - more so since the construction of the A38 dual carriageway outside the cave. The Lake is over 30 metres deep and in the early 1970s divers from the Devon Spelaeological Society passed an underwater arch to enter the void of Pridhamsleigh Two. Pridhamsleigh Two is a partially water filled chamber approximately 40 metres long 20 metres wide with a total height of 45 metres. Above water there is a high level ledge and the chamber is well decorated with a wide range of speleothems. A stout series of shot lines enable dives to be made safely between Pridhamsleigh One and Two.

A full description and survey were published in Cave Science in the 1960s by the late John Hooper.

Offline The Old Ruminator

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Re: Photos of the lake in Pridhamsleigh
« Reply #12 on: August 03, 2011, 06:31:02 pm »

Offline mrodoc

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Re: Photos of the lake in Pridhamsleigh
« Reply #13 on: August 03, 2011, 07:44:05 pm »
I have sent Neil a PM. Remember that depth gauges are usually set for seawater which is denser than freshwater so Prid lake is best measured with a string or sonar! Of course they are fine to dive with because it is pressure a diver is concerned with not depth.

Offline mike barnes

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Re: Photos of the lake in Pridhamsleigh
« Reply #14 on: August 04, 2011, 11:40:18 am »
Hi Pete,

It's been such a long time since diving in the sea, I'd forgotten about that. If I remember, a diver is 2.5 to 3% deeper in fresh water than the gauge shows, unless calibrated for fresh water? So the above mentioned dive is about 1m deeper, 39.5m. Sooo glad I didn't know that at the time. I'd seen a fairly snug rift leading off from the bottom and descended in absolute zero vis before it felt like it pinched in, so turned back. Once in better vis, everything seemed a lot darker. On the deco, realised a double dive torch had come off my helmet in the low passage.

 The following week, I went back. Just before the mud rolled in near the dropped reel, saw the torches, and even better, a larger passage leading off at the bottom. Being young and foolish, I picked up the reel and rushed into it without putting on a belay. Within 7/8 m, it closed down, but above me was a big, crystal clear void. I started to swim up into this, with the intention of rejoining the line around the base of the chamber. But I quickly realised I might not be able to find the line, and would have to return along the low passage with even less air. Dropping the reel in an absolute blackout, within a few m's the line went into a 3 cm slot, and it felt tight all around. Thinking 'sod this', I went back to the reel intending to exit via the big clear space, hopefully. However, the blackness was now all prevailing and all I could find was solid rock. At this point, I felt close to panic, and my breathing rate went right up. Decided it would be better to find the way out, so returned to the 3 cm slot to try and find that passage. I have never felt such a surge of relief as when I put my hand on the last lead block belay. And despite mild hypothermia from 30mins of deco due to tables, and a wetsuit with too many holes, I just couldn't stop smiling.

I've included the above story to dispel the myth that Prid is a 'safe' dive. Staying on the mainline, with a max depth of 24m and clean overhead rock it's fairly straightforward. It was afterall my first cave dive. But if you leave the rope line and venture more into the chamber, the silt rains down on you from above, the floor is deep incredibly fine silt, and the water is deep enough to narc you. I almost panicked when I re picked up the reel and could only find rock. Luckily, I was able to focus on what had to be done, and survived. So, a great dive, yes, often sensational vis. But a 'safe' dive, I think not.

Gus, I've often wondered about the chamber and how it formed. There appear to be very few boulders on the floor. Though the silt could be concealing much, like the big river passage that leads out to the sea at Berry Head!
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Offline mrodoc

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Re: Photos of the lake in Pridhamsleigh
« Reply #15 on: August 04, 2011, 01:18:18 pm »
Yes, it's a lonely place on the bottom of the lake! I have had my own embarassing time in there. On one occasion I had this cunning plan to lay a line right round the wall held in place by fishing weights. My buddy Jim was to swim behind me looping on the weights as I swam round the wall. Most of the way round I turned to see Jim behind me and a line running out across the chamber - we had dragged the line and weights with us. Having run out of air we had to return to base and another group had to clear up the line. Andy Stewart has now done a much better job I believe although it has resulted in a number of line junctions in Prid 2 now. I cannot repeat Mike's advice enough - never leave the line cos that 30m vis will go to 0m as soon as you touch the walls or floor.

Offline gus horsley

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Re: Photos of the lake in Pridhamsleigh
« Reply #16 on: August 04, 2011, 02:56:09 pm »
There appear to be very few boulders on the floor. Though the silt could be concealing much, like the big river passage that leads out to the sea at Berry Head!

Surely that's just wishful thinking!  Now I'm wondering if the cave shows multiple phases of phreatic development or if there's only one phase which extends over a large vertical range.  Is it possible it once connected to the caves on the other side of the valley and that they comprised one system which has been bisected and shared a common resurgence at one time.  Maybe I'm off in fantasy land now.

Offline mrodoc

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Re: Photos of the lake in Pridhamsleigh
« Reply #17 on: August 07, 2011, 07:44:34 am »
I have heard it suggested it might have a hydrothermal origin.

Offline graham

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Re: Photos of the lake in Pridhamsleigh
« Reply #18 on: August 07, 2011, 04:22:57 pm »
I have heard it suggested it might have a hydrothermal origin.
Intrigued. Where did you hear that?
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Offline gus horsley

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Re: Photos of the lake in Pridhamsleigh
« Reply #19 on: August 08, 2011, 01:01:08 pm »
I have heard it suggested it might have a hydrothermal origin.
Intrigued. Where did you hear that?

Hydrothermal?  I find that hard to believe but I'm also intrigued.  Are we talking about late-stage Variscan Orogeny (cross-course) hydrothermals?  If so that would make Prid about 270 million years old.

Offline mrodoc

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Re: Photos of the lake in Pridhamsleigh
« Reply #20 on: August 08, 2011, 02:46:58 pm »
Gus' post is way over my head. I think Chris Proctor considered it once.

Offline Roger W

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Re: Photos of the lake in Pridhamsleigh
« Reply #21 on: August 08, 2011, 06:01:22 pm »
'Twas probably dug out by some Mendip guys on holiday...
"That, of course, is the dangerous part about caves:  you don't know how far they go back, sometimes... or what is waiting for you inside."   JRR Tolkein: "The Hobbit"

Offline graham

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Re: Photos of the lake in Pridhamsleigh
« Reply #22 on: August 08, 2011, 08:08:25 pm »
Gus

We have a hydrothermal cave here in Bristol, at Pen Park Hole. It is thought to have been active as a hydrothermal vent in the Rhaetic and is thus about 200 million years old.
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Offline gus horsley

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Re: Photos of the lake in Pridhamsleigh
« Reply #23 on: August 09, 2011, 08:31:07 am »
Graham, this is turning into a very interesting post.  Do you know what evidence there is for a hydrothermal origin for Pen Park Hole?  The UK has many examples of post-mineralisation caverns, which could have formed at virtually any time in the past 200 million years, but we're talking about something different here aren't we, which are caverns forming at the same time as late-stage hydrothermal mineralisation due to uplift following the closure of the Rheic Ocean in late Carboniferous - early Permian times.

Offline graham

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Re: Photos of the lake in Pridhamsleigh
« Reply #24 on: August 09, 2011, 09:36:14 am »
Gus

Here's the paper.

Essentially, the cave has hydrothermal features additional to the mineralisation & the overlying Rhaetic clays contain disseminated lead that came out of a hydrothermal vent.
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Offline gus horsley

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Re: Photos of the lake in Pridhamsleigh
« Reply #25 on: August 09, 2011, 01:32:09 pm »
Thanks Graham, I'll have a good look at the article later but I can see from the survey that there's a resemblance to Prid.  Of course that doesn't mean they have the same type of origin.

Offline Roger W

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Re: Photos of the lake in Pridhamsleigh
« Reply #26 on: August 09, 2011, 05:01:06 pm »
'Tis a very interesting bit of stuff, that, Graham.

How common are caves produced by the limestone being dissolved by hot volcanic water from below rather than by rainwater/streams from above?

I would think that some really hot and possibly acidic volcanic water could be quite agressive.
"That, of course, is the dangerous part about caves:  you don't know how far they go back, sometimes... or what is waiting for you inside."   JRR Tolkein: "The Hobbit"

Offline Peter Burgess

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Re: Photos of the lake in Pridhamsleigh
« Reply #27 on: August 09, 2011, 05:08:13 pm »
There are quite a few in Hungary.

Offline paul

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Re: Photos of the lake in Pridhamsleigh
« Reply #28 on: August 09, 2011, 05:41:28 pm »
There are quite a few in Hungary.

Plus Carlsbad Caverns and Lechuguilla in the US which were largely formed from water containing amounts of sulphuric acid relating to petroleum deposits.
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Offline graham

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Re: Photos of the lake in Pridhamsleigh
« Reply #29 on: August 09, 2011, 06:19:17 pm »
Then there's the things with the gert big crystals. There are quite a few hydrothermal caves around.

But only one is a mile down the road from me.  8)
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Offline whitelackington

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Re: Photos of the lake in Pridhamsleigh
« Reply #30 on: August 10, 2011, 08:56:38 am »
Gus

We have a hydrothermal cave here in Bristol, at Pen Park Hole. It is thought to have been active as a hydrothermal vent in the Rhaetic and is thus about 200 million years old.
Canada Combe Cave,
near Hutton, Mendip is also thought by some to be hydrothermal.

Offline gus horsley

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Re: Photos of the lake in Pridhamsleigh
« Reply #31 on: August 10, 2011, 12:34:53 pm »
A possible scenario could be that caves such as Pen Park Hole could have originally formed at depth by ascending thermal waters charged with CO2; these theoretically would dissolve along lines of weakness but create only small cavities initially.  Indications of these would be calcite-filled vughs and evidence of altered rock.  At a later time uplift would have raised the caves where normal phreatic solution could take place, enlarging the existing voids.

How does that sound?

Offline graham

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Re: Photos of the lake in Pridhamsleigh
« Reply #32 on: August 10, 2011, 01:48:43 pm »
A possible scenario could be that caves such as Pen Park Hole could have originally formed at depth by ascending thermal waters charged with CO2; these theoretically would dissolve along lines of weakness but create only small cavities initially.  Indications of these would be calcite-filled vughs and evidence of altered rock.  At a later time uplift would have raised the caves where normal phreatic solution could take place, enlarging the existing voids.

How does that sound?

In part it depends on whether the enlargement is pre- or post-mineralisation.

IIRC a number of Derbyshire lead mines are 'normal' caves formed by meteoric water which have undergone subsequent mineralisation at depth.

PPH not only has all the features of a hydrothermal cave, but is quite close to currently operating hydrothermal features, there are hot springs only a few miles away in the Avon Gorge, as well as, famously, in Bath, just down the road.
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