The porcellanous band, seen in Gaping Gill

[Pitlamp]

Many cavers will be familiar with that white band of fine grained calcite mudstone known as the porcellanous band, seen a few metres above the floor in the walls of GG main chamber. The late Dick Glover did a lot to raise cavers' awareness of its significance.

I remember Dick mentioning that it can also be seen at outcrop in Crummackdale. My question is - can anybody point me at where it's exposed in that valley?

Thanks.

 

[gus horsley]

Thedre are a number of porcellaneous bands in the Yorkshire limestones, the term refers to microcrystalline calcite beds which are found at several horizons within the main limestone sequence.  I've been looking at some of my old field notes and I believe there is a porcellaneous band exposed in Crummackdale between SD781707 and 767710, resting unconformably on Lower Palaeozoic strata but I'm not sure if it's the same Band as in GG.

 

[Pitlamp]

That's extremely useful Gus - thanks; I'll check it out.

I'm told the 1924 QJGS Garwood & Goodyear reference is good for giving the locations of PBs; there is a certain co-editor of Cave & Karst Science chasing this one up for me at the moment.

Next question; Dick Glover wrote extensively about the Gaping Gill Main Chamber PB in two articles (CPC Journals 1973 and 1974). In these he mentioned that the GG PB can be seen in Ease Gill "near Leck Beck Head". There was a BCRC Field meeting in the mid 1980s where Dick apparently showed this exposure to the people involved.

So - does anyone know where the Ease Gill exposure actually is? (Or does anyone know where the outcomes of that BCRA field meeting might have been documented?)

 

[Jon]

Did you ever get an answer to this Pitlamp?

 

[danthecavingman]

Sounds like a good excuse for a walk up Ease Gill to me.... 

D.

 

[Les W]

There is a new BCRA field meet just announced that may be relevant to this discussion...

Saturday, 25 May, 2013 BCRA Field Trip. Ingleton.
"The Significance of the Porcellanous Bed to Cave Formation in the
Yorkshire Dales. This will involve visits to two surface locations in
the Dales. Contact Dave Checkley.

 

[Pitlamp]

Yep - guess who suggested it?   

The idea is to get various people together who have been working independently and try and make the study of the PB a more co-ordinated thing. It should be quite good - not just science for the sake of science but directly relevant to cavers interested in exploration.

 

[Speleotron]

What's the significance of it? I've often looked at it and wondered what effect it's had.

 

[Pitlamp]

H'm - good question; it's one that may be answered a bit better after the BCRA event.

The late Dick Glover described its influence on cave development in the GG system in the 1970s. The best explanation is probably Dick's chapter in Limestones & Caves of N W England (Ed: Waltham, 1974). In case you don't have access to that, Dick drew attention to the fact that much of the GG system of muddy tunnels is formed on top of the PB (e.g. a lot of the passage between Main Chamber & Bar Pot) and often where other areas of the system form below it are where it has been fractured by faulting (Main Chamber, South East Pot, Mud Hall, Hensler's Master Cave, etc). Dick interpreted this in terms of the PB acting as an acquiclude - an impermeable later which prevents downwards development unless it's breached by faulting.

Dick later went on to extrapolate this to other areas of the Dales, where the evidence was perhaps less good. Certain other noteworthy cavers disagreed at the time. Dick later became ill and never had the chance to follow up his research before he died. But various individuals have maintained an interest in the PB, which is probably traceable from Ease Gill in the west right over to large areas of Wharfedale in the east. It clearly does influence cave development but quite why is not yet understood. The BCRA meeting will hopefully be a step in the right direction to understanding it better.

And if we understand it better - it might help us find more cave.   

 

[gus horsley]

I was looking through some info on oil deposits a few days ago and came across a mention of calcareous beds forming a barrier to the upward migration of fluids.  So, in order to understand why the one in GG has such an effect on cave development, I looked up references to calcareous beds and discovered that the majority of them (PB included) are actually calcareous mudstones in which silt occurs as microscopic laminates "frozen" by the crystallization of calcite.  These therefore would act in a similar way to laminates in clays which align themselves and link together, effectively forming a whole series of impermeable layers within a very thin section, creating an effective non-porous barrier.  Not only that but this type of structure is relatively plastic which means that the PB would not be affected by jointing the same way as the surrounding limestones (which are relatively brittle).  So, unless it is faulted, there would be no movement of water through it.

I realise this is largely conjecture on my part but to me it seems fairly sound.