Deep Well - Pridhamsleigh

[fat pat]

pridhamsleigh cave dive
link for Pit lamp taken last year, I've been back though and cant see any underwater stals..We will be going back March/April and will have a better look

 

[Pitlamp]

Many thanks. I didn't see any stals either but sometimes they're not always so easy to spot; they may not look like normal due to partial resolution. Some of those we've found recently elsewhere have dissolved away so much that only small patches remain, Even a small fragment can be dated though - so, yes please, do have a look for us!

 

[Cartwright26]

Could someone let Pitlamp know I've just been reading through some old documents and you may like to know Edgar Reed and Lt J D Graham dived the lake on the 5th of March 1952 and reported stals at appx 40 feet!!!

 

[Pitlamp]

That's extremely good of you to take the trouble to pass on that information; thanks. In fact Christine (who often posts on this forum) recently supplied me with the actual reference (in a DSS publication).

I understand that there is some question about the reliability of that report, in that the divers weren't actually cavers so they may have been less skilled in recognising stal than most of us. But there are two regular users of this forum who have (separately) offered to look specifically next time they get a chance to dive in the Lake.

If those stals really are there at -12 m then I suspect Devon would have the deepest recorded stals in the country  (beating those in Kingsdale which are at -7.2 m max). It also been pointed out to me that those stals observed in 1952 would be in the first shaft directly below the Prid Lake rather than in Prid 2, as the latter wasn't discovered till much later (early 70s). I really hope they're there because it'd really get our cave scientists scratching their heads as to how they came to exist at such depth!

 

[richardg]

Interesting stuff  ! 

Richard

 

[graham]

If those stals really are there at -12 m then I suspect Devon would have the deepest recorded stals in the country  (beating those in Kingsdale which are at -7.2 m max). It also been pointed out to me that those stals observed in 1952 would be in the first shaft directly below the Prid Lake rather than in Prid 2, as the latter wasn't discovered till much later (early 70s). I really hope they're there because it'd really get our cave scientists scratching their heads as to how they came to exist at such depth!

Of course the simple answer is that they were not formed at that depth but that base level has risen by that amount since their formation.

I am not sufficiently au fait with Devon geomorphology to take a stab at when that might have been, but would like to hear Don McFarlane's opinion on dating them.

 

[Pitlamp]

Exactly Graham - what mechanism could account for a water level rise of (at least) 12 m?

In Kingsdale the explanation is closely related to the valley's glacial history (see two articles in Caves & Karst Science in recent years) but I suspect that Prid is in a completely different topographical situation, as well as having been less influenced by the Pleistocene ice advances.

Before anyone devotes too much time to this we ought to get confirmation (or otherwise) of the existence of these deep stals - and preferably get a sample to help see where this fits into the wider chronological picture.

 

[graham]

Exactly Graham - what mechanism could account for a water level rise of (at least) 12 m?

Global Warming  8)

More seriously, I do not know the controls on base level in this area, but sea level around the south coast has risen by far more than 12 m since the Last Glacial Maximum about 20,000 years ago.

 

[Les W]

Base level controls here would be the River Dart, however, as somebody already alluded to here, the construction of the bypass may have changed local drainage routes also.
I doubt the bed of the Dart has risen significantly, much more likely to have down cut, which wouldn't account for higher water levels though in the cave.

The bypass construction is looking like the most likely culprit, assuming there are stals at depth in the cave...

 

[graham]

Base level controls here would be the River Dart, however, as somebody already alluded to here, the construction of the bypass may have changed local drainage routes also.
I doubt the bed of the Dart has risen significantly, much more likely to have down cut, which wouldn't account for higher water levels though in the cave.

The bypass construction is looking like the most likely culprit, assuming there are stals at depth in the cave...

You sure about that, Les. As I say, I don't know the history of the area at all well, but looking at a map, the lower reaches of the Dart, the Teign, Kingsbridge Estuary & the Avon look a lot like they could be rias to me.

edit:

And the ever reliable* wikipedia agrees with me:

The south coast of England is a submergent coastline which contains many rias, including Portsmouth Harbour, Langstone Harbour, Chichester Harbour, Pagham Harbour, Southampton Water, Poole Harbour, the estuaries of the Exe, Teign and Dart, the Kingsbridge Estuary, and Plymouth Sound in Devon, and the estuaries of the River Fowey and River Fal in Cornwall.

*Well it is when it backs me up.  8)

 

[Pitlamp]

Exactly Graham - what mechanism could account for a water level rise of (at least) 12 m?

Global Warming  8)

More seriously, I do not know the controls on base level in this area, but sea level around the south coast has risen by far more than 12 m since the Last Glacial Maximum about 20,000 years ago.

H'm - I understood that Prid is sufficiently far above (present) sea level for fluctuations not to be a likely factor in this investigation. But I've never had the opportunity to go caving in Devon and I'm just not familiar with its Pleistocene history. We need a sample to get a date on ideally (if these stals are there). Even if the -12 m stals can't be found, there is a more recent reference to stals at -5 m under the Prid 2 air surface in the CDG Newsletter. I've never seen a reference to the Lake surface falling as much as that, so it does seem that the water level is higher than it must have been formerly.

Was there a pitch of at least 5 m down into the Lake before they built that road? If not, should the finger of blame be pointed at some much less recent event?

Wish I lived a bit nearer as I'd do several dives there myself to go and look, as I find this very interesting.

 

[Les W]

Base level controls here would be the River Dart, however, as somebody already alluded to here, the construction of the bypass may have changed local drainage routes also.
I doubt the bed of the Dart has risen significantly, much more likely to have down cut, which wouldn't account for higher water levels though in the cave.

The bypass construction is looking like the most likely culprit, assuming there are stals at depth in the cave...

You sure about that, Les. As I say, I don't know the history of the area at all well, but looking at a map, the lower reaches of the Dart, the Teign, Kingsbridge Estuary & the Avon look a lot like they could be rias to me.

You are correct, the estuaries are rias, but the River Dart, at Buckfastleigh is about 30 metres above current sea level. The river contains several weirs, both in the area, and downstream as well, and is quite fast flowing. It is somewhere between a youthful and a mature stage river at this location so erosion ought to be the predominant process here rather than deposition. Although it can't be ruled out it is unlikely that the river has built up its bed and banks, especially in such confining valleys.
I guess the clincher would be to see if the river had bedrock as a bed, rather than alluvium, but I can't see that from here. I will check next time I am there though. There is a particularly good viewpoint to study the river, from the terrace of the Abbey Inn, with Tribute on tap as an added bonus. 

 

[graham]

I can think of rivers in Portugal that have aggraded quite significantly since the last glacial maximum. You are quite right that hunting for bedrock will help come to a conclusion on the matter, but I certainly wouldn't rule out base level rise in the area until sufficient evidence was in.

 

[mrodoc]

A few comments. The Navy divers weren't as you say cavers and the best account is reproduced in the Cave Science paper on Pridhamsleigh written by John Hooper where the letter is verbatim. I am sure there are copies in the BCRA library! Pitlamps remarks on re-solution are relevant. The walls of Pridhamsleigh lake at depth are quite extraordinary in that they have  a brittle crystalline surface such that one crunches into it when one hits it - does not do much for visibility. I haven't encountered this material in caves anywhere else. The Dart has a limestone bedrock, certainly outside Dart River cave only a mile or so from Pridhamsleigh. Finally the husband of my cousin, who is a civil engineer, told me some years ago that when they were building the bridge pilings for the overpasses to the dual carriageway outside Prid they had to pump large amounts of concrete into the cavities they found. Othere cavities were noted elsewhere on the dual carriageway construction site by Fish when they were working on it. Hope this helps.

 

[Les W]

The Dart has a limestone bedrock, certainly outside Dart River cave only a mile or so from Pridhamsleigh.

Dart River Cave is right by the Abbey Inn (where they keep an exceptional pint of Tribute...  ). In fact I believe it is accessed from their terrace...

I think some summer study of the river bed there is in order... 

 

[bograt]

Dart River Cave is right by the Abbey Inn (where they keep an exceptional pint of Tribute...  ). In fact I believe it is accessed from their terrace...

I think some summer study of the river bed there is in order... 

Are you on commission?

Seriously, I think we may be looking at some inter-glacial developement here, question is Which Glaciatian(?) the only way on is to find it, get a sample and date it.

P.S. Messages crossed

 

[Les W]

Dart River Cave is right by the Abbey Inn (where they keep an exceptional pint of Tribute...  ). In fact I believe it is accessed from their terrace...

I think some summer study of the river bed there is in order... 

Are you on commission?

No, but I am partial to a decent pint of premium Cornish Ale... 

 

[graham]

Pete

The nature of the Lake walls is interesting. Maybe you need to get a proper geomorphologist (Joyce?) to dive in there and take a look at them. If there is stal & re-solution then it can only be explained by base-level rise. Someone must surely have done a study of the Pleistocene history of that part of Devon.

But are you absolutely certain that the stream bed is bedrock & not sediment? If the downstream end of the Dart is a ria, as it seems, then it is highly unlikely, in my view that this stretch of the river will be actively downcutting today.

Maybe I need to organise a pint with Les in July.

 

[Les W]

Maybe I need to organise a pint with Les in July.

 

[graham]

Seriously, I think we may be looking at some inter-glacial developement here, question is Which Glaciatian(?) the only way on is to find it, get a sample and date it.

bograt, what you northerners forget is that this part of the world was not not glaciated during the Devensian, so we ain't looking at inter glacial development here, as you cannot be inter two events that didn't happen - I don't remember* whether ice got this far south during MIS6 but I don't think it did.


*I've drunk too much whisky tonight, but sometimes too much is just right.