Author Topic: Porcellanous limestone in Chapel-le-Dale  (Read 1450 times)

Offline Pitlamp

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Porcellanous limestone in Chapel-le-Dale
« on: December 19, 2018, 02:06:40 pm »
Anyone with no special interest in the fine details of the Great Scar Limestone depositional sequence in the Yorkshire Dales - stop reading now!
However, for those few who are interested . . . .

Three or four years ago there was a BCRA field day, based on the Porcellanous Bed of limestone, famously seen in Gaping Gill. We met at Ingleton in the Community Centre for a morning of presentations, then split into two groups to view the PB in Crummackdale and Kingsdale. I was involved in planning for that day and we specifically scheduled to meet up again in the afternoon so that arrangements could be made for anyone interested to do follow up work and map the PB elsewhere in the Dales and / or look for it underground.

Unfortunately this all went pear shaped, because (with no prior warning and despite our booking having been made well in advance) we were asked not to reassemble at the community centre in the late afternoon, due to an event. This is a great shame because that was really the most important part of the whole initiative to bring people together. Everyone drifted away and, apart from a small number of individuals who are still quietly working on mapping the PB, we still have no co-ordinated collective effort to arrive at a better understanding of how the PB influences cave development and whether its outcrop might lead us to discover more caves. This was a great opportunity missed.

Since then, two further things have happened which should make possible the resumption of this work. Firstly, Tony Waltham and Deej Lowe produced their fabulous Part 2 of their "Caves & Karst of the Yorkshire Dales", book which has many references to the PB at the boundary between the Cove and Gordale limestones.  Secondly, the owners of this website have provided this area for BCRA to use as a forum (thank you!).

Thus we can now communicate easily and so we perhaps have the opportunity to get this project off the back burner again.

What's prompted me to post this is that I found a very thick bed of porcellanous limestone in Jingle Pot last week. (I'll ask Badlad or Pegasus if they'll post an image of a specimen for me; I still find posting photos a bit of a challenge I'm afraid  :-[) This is from just above water level at the base of Jingle Pot's entrance shaft.

The 64,000 dollar question is whether it's one of the other porcellanous beds known to exist in the Great Scar limestone, or the main Porcellanous Bed (of GG fame, at the top of the Cove limestone - but absent at Malham, note). This latter bed outcrops outside the entrance of Midge Hole, further down valley from Jingle Pot. But the dip is up-dale and steeper than the normal regional dip, so the GG PB "should" be at least 40 m to 50 m below water level at Jingle Pot. (A glance at the useful elevation on page 381 in Part 2 of the Caves & Karst book reveals this - bear in mind though that this was the best possible interpretation at the time of publication and may need to be revised in light of future observations.)

The possible existence of the main PB in Jingle Pot (as opposed to a different pb) can't be ruled out with the knowledge currently available - between Midge Hole and Jingle Pot there are numerous fairly major faults crossing the dale, which collectively could account for significant displacement. This would be unusual but the possibility has to be borne in mind. All we know as I type is that there is a thick PB outside Midge and a very similar thick pb (or "PB?") some 15 m below the riverbed at Jingle Pot.

One test of this might be whether the Jingle Pot pb can be found at surface exposure in the dry riverbed down valley from Jingle Pot. So if anyone happens to be in that area, do please keep your eyes open for it.

As mentioned above, certain noteworthy individuals continue to search for and find the PB elsewhere in the Dales. Please could you consider adding notes of your findings here? This would then develop a single point of reference, which could grow into a valuable resource.
Thanks.

Offline alastairgott

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #1 on: December 19, 2018, 05:23:25 pm »
I've spotted this guys website, perhaps one of your contributors? http://www.mudinmyhair.co.uk/PB.html
No doubt you'll know his website/or him already.


I'm off to Easegill at new year, now i've forgotten... can I call that the Dales?  :tease:



Anyway, I don't know much about this Porcellanous band, is the one he talks about in the section on his website likely to be the same Great Porcellanous band. And if so, will it dip or rise as it gets further North nearer to Easegill? https://goo.gl/maps/G8QebxaZ2yJ2


Edit: I've found more info hidden in the PDF at the bottom of his page, for Easegill:
Quote from: Mudinmyhair.co.uk @ "locations PB.pdf" accessed 19/12/2018
Easegill Reported by Dick Glover at lip of fall Easegill Kirk. Visits by SEW and DLM on April 30, 2015, and by SEW and RG on 7-5-15 found the PB as described by Glover. Porcellanous bed approx. 0.3m thick, forming ledges at each side of stream channel at head of fall of Easegill Kirk (lower), NGR 66133 80010 Alt. 236 - 244. At least one higher bed is prominent upstream from head of fall as a small barrier across stream bed. It is also prominent at an even higher level too but all is somewhat obscured by faulting and folding due to proximity to the Dent Fault. The total extent of these porcellanous beds is yet to be determined. From this point however, the PB seems to be forming the bed of Easegill for some considerable but as yet indeterminate distance upstream although it is very much obscured by great quantities of superficial deposits, rock-fall and stream debris. From altitudes of Lost Johns Cave at 360m where GB seen at the entrance and Shuttleworth Pot at 279m, and the assumption that GB and PB separated by some 100m, then the PB should be at 260m in Lost Johns and a little lower in Shuttleworth, allowing for an estimated regional dip of 1-2 degrees. This leaves a discrepancy for the altitude of the PB between Shuttleworth and Easegill Kirk that can be accounted for by the observation that Shuttleworth appears to be very close to a subsidiary branch of the Dent fault. The displacement is estimated to be in the order of some 15 – 25m. Further detailed studies are needed to clarify this issue.
« Last Edit: December 19, 2018, 05:36:10 pm by alastairgott »

Offline Pitlamp

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #2 on: December 19, 2018, 05:58:04 pm »
Thanks Alastair,

Yes, the caver behind the excellent mudinmyhair website is a personal friend and is one of those quietly working on this independently.

Yes, Ease Gill (note, two words) is part of the Dales but you're either in Lancashire or Cumbria, depending on which side of the beck you're standing.

If the PB is there in Ease Gill, it's probably in the region of the Kirks, not too far above Witches Cave. Andywebman (on this forum) has looked for it around there but I don't know if he found it. I'm sure he'll post something here if he did.
I found a boulder made of PB in the Lower Kirk but have yet to spot the PB in situ.

In case it helps - and this is greatly simplified The Great Scar Limestone of the Dales can be thought of as having three layers, named Kilnsey, Cove and Gordale limestone (from bottom to top). For our purposes you can consider that the PB is at the top of the Cove limestone and at the start (i.e. bottom) of the Gordale limestone. Thus most of GG Main Shaft descends through the Gordale limestone but the last few metres (below the Porcellanous Bed) is in the very top of the Cove limestone. If you want the detail I strongly recommend the two recent BCRA Cave & Kerst books by Deej Lowe & Tony Waltham

I've used "PB" and "pb" to refer to the "Porcellanous Bed" as described above and any other "porcellanous bed" elsewhere in the Great Scar Limestone respectively.

The porcellanous limestones are incredibly fine grained and look totally different on fresh fracture from normal "sparkly" limestone. Hopefully Badlad or Pegasus will post a photo from me when they get my email today. Porcellanous limestones also weather to a fairly white colour, which is why the GG PB stands out so well in the walls of Main Chamber. They also tend to break with a "conchoidal" fracture - shaped like a sea shell.

If you can find it in Ease Gill (or anywhere else for that matter) - great!

Offline Benfool

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #3 on: December 20, 2018, 09:50:01 am »
Image from Pitlamp.

B

Offline Pitlamp

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #4 on: December 20, 2018, 10:10:52 am »
Thanks!  :thumbsup:

Offline Ian Ball

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #5 on: December 20, 2018, 02:32:48 pm »
That's a beautiful piece of stone.

I should go home and read the books for myself which I just bought, but is the PB visible in the roof tunnel of Valley entrance in Kingsdale.  I always assumed it was and the same one as in GG.

Offline Pitlamp

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #6 on: December 20, 2018, 02:40:20 pm »
I think the KMC Roof Tunnel is formed several metres below the PB, which is believed to outcrop on the surface, just up that gully from the Valley Entrance. (But check in the Caves & Karst book because Tony and Deej included a lot of such information.) Some of the folk on that BCRA study day referred to above went looking at that location in the afternoon; I was there - I remember some very fine grained limestone being found but it wasn't a perfect match with the PB seen in GG.

The PB outcrops in most places across the Dales. It's missing at Malham but further east it's seen in the roof of Scoska Cave in Littondale and it's also there at Foss Gill Cave in Wharfedale.

Offline Pitlamp

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #7 on: December 21, 2018, 08:54:16 am »
As an aside, I'm encouraged by the >400 views already; I didn't realise there was quite so much interest in the fine details of limestone geology in the caving community!

Offline andywebman

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #8 on: December 21, 2018, 08:10:16 pm »
After discussions with Deej Lowe and John Cordingley I have been out looking for the PB on Leck Fell. Deej suggested I look it in the dry valley below the gate near where the old shooting huts used to be before the carpark for Notts 2. It occurs at SD 6638 7815 bed dips at about 20 degrees to north. Alt:289m. This exposure is close to the North Craven Fault area so not a good indicator of where else it might occur on the Leck Fell as the beds here are faulted and dip steeply.
I have also found it in the stream bed at the top of Upper Ease Gill Kirk. This, I think the exposure Dick Glover showed me many years ago. It is fractured and faulted in this region. I have a photo somewhere. I will have a look. Need to check underground in Kirk Pot.
RRCPC:-)

Offline Pitlamp

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #9 on: December 24, 2018, 09:07:44 am »
Thanks Andy.

I went back to Chapel Beck in Chapel-le-Dale yesterday and looked more closely. There are two porcellanous beds exposed at surface. The one outside Midge Hole (across much of the area of the riverbed as far as the 1 metre step down, just down valley) is the Porcellanous Bed of Gaping Gill fame. The second is exposed in a tiny hollow in the dry riverbed about 60 m upstream, i.e. not far downstream from the large depression which is over the end of the Mosquito Plane in Midge Hole. It's an estimated 6 m higher in the sequence than the main GG PB seen outside Midge.

I'm pretty sure that the thick porcellanous bed identified recently in Jingle Pot is the main GG Porcellanous Bed after all.

Offline AR

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #10 on: December 24, 2018, 08:56:25 pm »
Going off at a bit of a tangent, I wonder how well this stuff polishes up? Just musing on what a chessboard made from it and Ashford Black Marble could be like....
Dirty old mines need love too....

Offline braveduck

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #11 on: December 26, 2018, 03:54:49 pm »
You may need planning permission for that . ;D

Offline Pitlamp

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #12 on: January 16, 2019, 11:50:37 am »
From two further trips recently, It's become apparent that some information in my last post above may be wrong.

The upper of the two porcellanous beds in Chapel-le-Dale, mentioned in my post of 24-12-18 above, is there in the walls of the entrance shaft of Jingle Pot. It's several metres thick (although  as I type I've yet to measure this accurately).

"Bargh's Entrance" is a nearby cave, a fine discovery by Bradford Pothole Club members several years ago. It gives direct access to a deep water chamber (in the main water-filled sub valley conduit) called Frog Hall. A similarly thick bed of porcellanous limestone is found in the walls of Frog Hall, just above water level. This is at least 2 metres thick but disappears below water; it's true thickness will have to be ascertained by divers.

I think the Frog Hall porcellanous bed is the upper bed as seen in the walls of Jingle Pot's entrance shaft (and also seen in Chapel Beck about 50 m to 60 m up valley from Midge Hole's entrance).

Clearly there are two quite thick porcellanous beds in Chapel-le-Dale, the upper one being significantly thicker than the lower one. I've never come across any other porcellanous beds anywhere near the thickness of the upper one in Chapel-le-Dale, anywhere else in the Dales.

In Gaping Gill, the "Porcellanous Bed" (note upper case here) is the upper and thicker of the two. This makes me think that the porcellanous bed found directly outside Midge Hole (the lower of the two) is not the "Porcellanous Bed" of Gaping Gill fame after all. It may be that the one 50 m to 60 m up valley from Midge Hole is the real Gaping Gill "Porcellanous Bed".

Work continues . . .

Offline langcliffe

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #13 on: January 16, 2019, 12:12:22 pm »
Do you reckon that the large boulder by the Braida Garth gate is PB? Not exactly in-situ bedrock, but it certainly discourages inconsiderate parking?

Offline andys

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #14 on: January 16, 2019, 12:52:11 pm »
Just out of curiosity, I've just had a quick look at the Geological Survey map of the area - http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1003658 - and spotted that it shows a fault running right through the Jingle Pot / Hurtle Pot vicinity. Could it be that the two PBs seen in that area are in fact the same PB but displaced by the fault?

Either way, this area is to the north of the "Sulber Nick" fault - which appears to traverse fully under the Ingleborough summit to reappear on Lead Mines Moss - whereas GG Main Chamber is to the south of it, so the relative altitudes of the PBs in in upper Chapel-le-Dale and above Clapham could differ as a result of any offset there, in addition to anything as a result of the general dip of the area.

Just curious, but I'm no geologist!
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Offline Pitlamp

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #15 on: January 16, 2019, 03:15:24 pm »
Langliffe - dunno - but next time you're up there maybe quietly chip a tint bit off and see what the fresh fracture looks like? (With your geological knowledge I'm sure you don't need further explanation.)

Andys - re your second paragraph - no there are definitely two porcellanous beds crossing Chapel Beck and two porcellenaous beds both in Frog Hall and in Jingle Pot itself. It's not a repeat sequence due to faulting. The beds are also significantly (and reliably) different thicknesses.

Re your first paragraph - thanks for posting the link to that map. I suspect the fault they show is the one passing through Jungle Pot's entrance shaft and also the obvious valley to the north west of Jingle Pot . But that's only one of umpteen faults which cross Chapel Beck. They're obvious in underground exposures but some can only be viewed by cave divers. In Frog Hall there are two separate parallel faults only 5 m apart - the chamber being formed in the brecciated zone between them. I suspect Weathercote entrance is on a fault and there is a whole complex of faults at Hurtle Pot's entrance and just downstream (in Greenback's Grotto and Stiletto Aven).  That map isn't really large enough scale to show all these faults, several of which couldn't have been appreciated by the mappers.

Offline langcliffe

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #16 on: January 16, 2019, 03:36:42 pm »
Langliffe - dunno - but next time you're up there maybe quietly chip a tint bit off and see what the fresh fracture looks like? (With your geological knowledge I'm sure you don't need further explanation.)

We did that a couple of weeks ago - it certainly looked like it. I was just thinking that if anybody else could confirm it, then it would a good place for those interested to see what it looks like.

Offline Pitlamp

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Re: Porcellanous limestone in Chapel-le-Dale
« Reply #17 on: January 16, 2019, 04:50:05 pm »
An excellent point Langcliffe.

The BCRA meeting spent some time in that gully which goes up the hillside from the Valley Entrance and found porcellanous limestone not that far above VE. It's not difficult to see how a pb boulder could have ended up alongside the road.

Next time I'm passing I'll have a glance.