The Porcellanous Band, or bed, (PB) is a thin bed of
very fine-grained micritic limestone or mudstone, otherwise known as
'china-stone', which usually weathers very much whiter than the surrounding
limestones. It occasionally occurs as twin bands, referred to as the upper and
lower bands respectively. It is located about a hundred metres below the top of
the Great Scar Limestone - the grey, white-weathering limestone forming the
limestone scars throughout the lower dales: Wharfedale, Littondale,
Ribblesdale, Chapel-le-Dale and Kingsdale. It is thought to act as a barrier to
cave development. It is not really well recorded in the Casterton and Leck
Fells area above or below ground and I hope to remedy this.
The Porcellanous Bed can be seen here on
the left hand wall of Gaping Gill main chamber. Photo by Eli Simpson
Background to the project.
BCRA FIELD MEETING In July 2013
I attended the above field meeting which
took place in the Methodist Church in the centre of Ingleton Followed by a
field trip to Kingsdale In the morning there will be a series of talks
explaining what the Porcellanous limestone bed is, where it occurs and why it
is important to cave formation in the Yorkshire Dales. After lunch we split
into two groups to go and look at the porcellanous bed in Kingsdale and in
Crummackdale. Some of the talks given are summarised below The
Dales provide a prime example of geological influence, or even control, over
the patterns of cave development. Though there are some classic exceptions, the
morphology and even the origins of almost every cave passage in the Dales can
be related to features of the limestone geology. The numerous shale beds are
obvious influences, but their role is not always as simple as might first
appear. There are also other influences within the stratigraphy, including
those known as inception horizons and also that slightly mysterious bed of
porcellanous affinity (PB). And then there are the joints and faults, and yet
more to debate. We know more about the geological factors than we did fifty
years ago, but there is still much to understand
Simplified view of the Carboniferous
Limestone Succession in the Yorkshire Dales
Caves are the explorable segments of an integrated
underground drainage network that underpinned the development of the more
visible surface karst landscape of the Yorkshire Dales. Caves and tributary
conduits occur within and through some 180m of largely massive limestone beds
now known as the Great Scar Limestone Group. Close to the middle of the Great
Scar Limestone succession is the Porcellanous Bed, a widespread but thin
(commonly 0.6 to 0.9m) marker horizon that can be traced at the surface, but is
also recognisable in cave passages shafts in parts of the district. Views about
its origin, broader geological context, lateral ramifications and speleogenetic
significance are still evolving. Controversy relating to its supposed local control
of cave development has been overtaken by its subtly different interpretation
as an inception horizon, guiding the earliest speleogenesis and acting more
generally as one of several geological signposts that might point towards
likely locations of elements of a primitive, potentially Dales-wide, imprint of
bedding-related dissolutional voids.
Saturday
afternoon field-trip
I went on the follow up
field trip in the afternoon to Kingsdale, where the Porcellanous band is to be
clearly seen at a number of locations along the western valley side between
Valley Entrance and Ravenray Bridge. There are several springs and clear
exposures if you know where to look. If you want more detail of where it is see
link to Steve Warren's article below. I also have several rock samples of the
PB if anyone wants to see what it looks like.
Other
work on the Porcellanous Band
Steve Warren from Craven
Pothole Club has done a considerable about of work on the PB locations across
the Dales and I am grateful to the information and photos he has supplied via
his website. See link below.
I contacted Chris Thomas at the British
Geological Survey, who was recently mapping the geology on Leck and Casterton.
His report is below:
Scoska Cave has an imposing entrance but
the most significant feature of this scene is the presence of a band of
porcellanous limestone (PB) at roof level. (photo credit: Steve Warren)
The
Porcellanous Band is one of those iconic lithological horizons that eluded me
when I was mapping in the area, though I know it has been considered a key
horizon by many of those studying the Carboniferous in the area and those
studying speleogenesis. My key horizon was the Girvanella band near the base of
the Alston Formation. Because our mapping was focussed at Formation scale and
partly because of time resources, we were less concerned with the deeper detail
of the stratigraphy in the Great Scar Limestone, so, while I am aware of the
Porcellanous Band, I can't say that I ever recognised it unequivocally in the
field, and with sufficient confidence to map it out as an horizon in the way
that I could with the Girvanella Band. So apologies for this apparent
ignorance! However, from what I understand of its position in the stratigraphy,
I would expect to find it in the lower parts of Ease Gill, not far above the
resurgence of Leck Beck. I did look for it in that ground, but, as I say, could
never convince myself that I'd found it.
Dick Glover (now
deceased) reported to others that he had found the PB on Casterton Fell in a
couple of locations. One was near Leck Beck Head and the other was at the lip
of the fall in Easegill Kirk. These have not been confirmed.
My
Recent work
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 SD6638 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. The whole dry valley above
and below the road crossing needs checking.. It might be quite difficult to
locate the Porcellanous Band across the rest of Leck Fell as the location I
told you about is close to the North Craven fault and the bed dips
about 20 degrees to the North and there not many exposures across the fell
until you reach Ease Gill.
On Casterton Fell area
John C claims to have found a boulder of PB in the Lower Easegill Kirk just
upstream of Witches Cave so I intend to look for this and he also thinks it
might appear in Kirk Pot at base of the pitch.
Follow
up work on Casterton and Leck Fells (help needed)
Casterton Fell: Identify
surface areas where the bed is not yet known or in dispute and try to arrange
excursion in Easegill Kirk and Leck Beck head area to look for PB. Nice
summer's day walk.Leck Fell: NO PB seen in dry valley above road.
Possible sighting, from road, in dry valley well down from road crossing almost
last rock outcrop in the lower dry valley to be checked out. Once we have
confirmed its location on the surface we can use this information and the
Easegill cave survey to work out possible locations for it underground and then
go on trips look for it. We need to identify areas underground where it has not
been recorded, but ought to be there.
We could then write this
up in an article. (Might even get it published in Cave Science!)
I am quite keen to pursue
this project over the next few months but I need some help from members. If you
are interested please contact me.
References
D. Checkley. The Porcellanous Bed and Cave
Formation in the Yorkshire Dales. BCRA Speleology 19 (Dec 2013)
D.
Brook. The Porcellanous Band (The Philosopher’s Stone?) BCRA Bulletin 13 (Aug
1976) R.
Glover. The Porcellanous Band in Yorkshire - Part 1BCRA Bulletin 12 (May 1976)
Up to date website about
Porcellanous Band by Steve Warren of CPC, very useful
http://www.mudinmyhair.co.uk/PB.html
Dick Glover wrote
extensively about the Gaping Gill Main Chamber PB in two articles (CPC Journals
1973 and 1974).
Dick's chapter in
Limestones & Caves of N W England (Ed: Waltham, 1974)
Photo of Porcellanous Band exposure in
dry valley on Leck Fell, near old shooting huts (now demolished)
Andy Hall