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Porcellanous limestone in Chapel-le-Dale

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Pitlamp:
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.

alastairgott:
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.

--- End quote ---

Pitlamp:
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!

Benfool:
Image from Pitlamp.

B

Pitlamp:
Thanks!  :thumbsup:

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