Brimble Pit/Locke’s Hole

1st April 2012. A busmans holiday to look at someone elses digsites. Willie Stanton originally dug at these sites but permission has recently been given to Stu Lindsay, Trever Hughes, et. al. (BEC) to continue the excavation.

Interesting closed basin development seems in places to follow washed out mineral vein. Colourful rocks comprising greys, red, white, yellow, orange (see below) and calcite with gravel and clay infill.

Spent a while in conversation with Stu Lindsay who suggested that there is some historical confusion about which site is which. Barrington and Stanton (1977) descriptions are as follows:

“Brimble Pit Swallet ST 5081 5075 alternative names Westbury Hill Swallet, Frog Hole.

50 yards east of Priddy-Westbury road at Brimble Pit Pool, in a depression by a wall taking the overflow from a small covered reservoir in the next field. Tight vertical rift dug by MNRC 1957-58, blocked by a boulder 50 feet down. Entrance collapsed about 1960. MNRC flood-tested the stream to Rodney Stoke rising in 1956. The Brimble Pit closed basin and overflow channel, and those of Cross Swallet nearby are scheduled as an SSSI; taken together they show all the geomorphic features of Mendip closed basins.

Locke’s Hole ST 5088 5072 alternative name Westbury Hill Swallet.

100 yards east of Priddy-Westbury road at Brimble Pit Pool, in a depression taking the natural overflow of the pool. Entrance shaft dug open by MNRC, 1955-56, entered sloping passage ending in sand-choked chamber. Digging stoppedwhen entrance shaft collapsed. A cow is said to have died in the entrance shaft about 1964, when the farmer half-filled the depression with rubble.”

Reference: Barrington, N. and Stanton, W. 1977 (third revised edition). Mendip: The Complete Caves and a view of the hills. Barton Productions in conjunction with Cheddar Valley Press.

31st March 2012

Rather surprised to find the gate chained and locked at the farm so over to School Hill and a longer walk up to the cave. Spent a while prodding and scratching away at the sediments above the rift, still doesn’t appear to be anything solid above – need to proceed with the utmost caution. The roots appear to have grown nearly 300mm since the last visit. Alex spent some time removing the lumps of calcite flow that impede the progress of the skip up Witches Cauldron. Crawled along to ‘Merlin’s’ for a reassessment of the area – there is a possible route along the right hand side that would avoid damage to the formations, but would require a good deal of effort initially.

After digging took a stroll up the valley to the east of Hallowe’en Rift to take another look at the rock exposure and to explore the head of the gully. Came across this interesting little spot. A number of holes were noted in an arc that showed some signs of previous excavation, there is a large badger site on the opposite side of the gully or this is the site that Trevor Hughes tentatively dug in the early 1980s.

17th March 2012

We continued work at the top of the rift/pot. What appeared solid is not, and there are a lot of boulders, cobbles in a matrix of finer sediment above the opening. Managed to bring quite a bit of this down and now have a small chamber about 1m x 1m x 1m although not quite big enough to turn around in safely. The downside of this work is that a good quantity of debris has fallen into the rift/pot so there is a considerable amount of clearing to be done. We are puzzled by this passage development we did expect to see another wall to the rift – are we heading into a chamber? Only digging will reveal what lies ahead.

10th March 2012

After a prolonged break from digging at Hallowe’en Rift due to other commitments we got back to work. Due to the conditions I decided to wear a plastic suit (rather tatty Petzl) slipped easily along the tube like a bar of soap. With my shiny new bar started to prod at the dig face, ‘that sounds hollow’ I thought. Removed some cobbles and a root to reveal a tiny gap, interesting, removed some more cobbles the gap got larger, getting very interesting, pulled out some larger cobbles and small boulders to reveal open space!The image above shows the hole in the dig face, unfortunately the photos ain’t very good! Passage height is about 0.30m and width about 1.00m.

After some more work removed a quantity of loose debris to reveal a clean washed rift/pot with abundant root growth. The top of the rift is too tight to allow entry and there is a lot of very loose boulders and sediment in the rift above – some serious gardening is required. The rift/pot appears to be about 1.0m deep to a maximum of 0.5m wide and a length 2.0 – 2.5m in length. It is possible that there is a continuation of the passage almost straight ahead, this will only be confirmed when the rift/pot is accessed. It looks very tantalising and exciting times are ahead – there are some seriously promising leads in this cave.

Looking down into the rift/pot. The main roots are about 12mm in diameter.

It’s days like this that make digging so exhilarating – it’s just such a buzz, I love it!

Of course we called in to our favourite hostelry for a little celebratory beverage.

28th January 2012

A fine winter’s day, bit of a chill in the air and clear skies. We are interested in determining some of the rock strata characteristics in the area that surrounds Hallowe’en Rift so decided to venture up and take a few readings of the rock exposure east of the cave. The lower section (pictured above) has a very general alignment east/west 065/245 [all figures are degrees] and the upper section (pictured below) also has a east/west 070/250 alignment. We also took some preliminary readings of discontinuities recording strike and dip: upper section – strike 140/320, dip 20 southwest (sw); mid section – strike 140/320, dip 16 sw; and lower section – 125/305, dip 16 sw. We discussed the possibility of a fault(?) between the mid and lower sections [further investigation required]. As a comparison we took some readings in a small stone quarry just above Hallowe’en Rift: strike 140/320, dip 7 sw and strike 120/300, dip 10 sw.

After this brief detour we returned to the main task in hand, digging Hallowe’en Rift. Bit of a clearing session in the ‘poo’ mine, it seems to have become even more wet and muddy, and, after a while it’s rather cold. I went to the base of Witches Cauldron to fill the skips and Alex did the hauling and dumping [at the base of the entrance]. All clear after about two hours, plenty of room for digging spoil next session. Refreshments at the Hunter’s.

21st January 2012

First digging session of the year, 2012 will be 30 years of exploration in Hallowe’en Rift, hopefully we have something good to celebrate. We were surprised to note that the field from the farm on the walk up to Hallowe’en Rift had numerous daisies, buttercups and dandelions in flower, evidence of the extremely mild winter so far. In between a lot of chatting we set about clearing the last bit of spoil dumped at the base of Witches Cauldron which didn’t take too long. I went to the end of the tubes to continue to dig into the rift, Alex in the meantime settled down to clear more out from the lower part of Witches Cauldron. The rift is beginning to look very interesting and we have pulled out some quite large lumps of flowstone (about 250mm x 250mm in area and 50-60mm in thickness) there is not much evidence for this material to have been transported any great distance. Possibly this flowstone originates from a capping layer on the surface of the sediments. After 2.5 hours of digging and soaking up water and a rather cold draught today began to get cold and we had just about run out of stacking space so called it a day and returned to the surface. Excellent session and can’t wait to get back down to continue the work. Popped into the Hunter’s for quick refreshments.

30th December 2011

One of those typical Mendip winter days – grey, damp and the forecast is for it to be very damp. Following a conversation I’d had with the farmer on christmas eve and a previous visit with the kids, before the digging session we ventured up to have a look at an interesting exposure of Triassic rocks – mostly of Dolomitic Conglomerate.

There are a number of interesting features that are worth looking at and a couple are depicted in the photographs above and below that might be evidence of some initial cave formation processes within Dolomitic Conglomerate. We will return to this area to carry out a field survey and comparison of strike/dip angles with those at Hallowe’en Rift area.

After our detour we decided to return to the prime objective – digging in Hallowe’en Rift, especially as the weather was closing in. Cleared out Witches Cauldron of last sessions digging spoil and that was enough, very claggy. I returned to the surface to emerge in some very low cloud, waited for quite some time for Alex to return. Alex had a bit of an ‘epic’ – at the bottom of Witches Cauldron where it is rather restricted for space Alex’s foot had slipped into a narrow rift filled with ‘gloop’ and while we had been working away, his foot became stuck fast and had required a considerable effort and some contortion to extract it. He arrived at the surface rather relieved not to have sacrificed his wellington boot!

17th December 2011

My turn to be the ‘mud monster’ and soak up all the water, get’s rather cool after a couple of hours. This kit was clean at the start of the session, note the ‘natty’ caving hat.

After a chinwag with the farmer and his son we wandered up to Hallowe’en Rift in bright sunshine but a rather chill wind. The spoil was all cleared last weekend so straight into digging mode, I went to the end of the tube, Alex settled in at the base of Witches Cauldron. Very moist along the tube so decided to load spoil directly into the skip, nice drip into the ear. Continued to work under the rift and pulled out a number of small boulders which is interesting, these boulders appear to be mostly conglomerate although there is some calcite present. Need a slightly bigger bar that will be more effective in working the compacted sediments. Meanwhile, back at the Witches Cauldron, Alex was busying himself by digging out the base of the pot after hearing a trickle of water. Digging was ended when all the stacking space was filled, another clearing session next trip. Retired to the Hunter’s Lodge Inn for a pint and a cheese and pickle roll, in my opinion the greatest hostelry in the land – marvellous!

10th December 2011

Alex emerges from Hallowe’en Rift after soaking up the puddles in his nice cotton ‘blotting paper’ overalls, damp and very muddy. I, on the other hand, was rather more comfortable in fleece suit and cordura caving oversuit.

Spoil clearance session with Alex. After getting changed at the farm and ready to go, we had to return to BEC HQ, The Belfry, to get the spanner that we had forgotten again! Eventually got up the hill to Hallowe’en Rift and set about putting up the tripod ready for hauling. Alex went down to the bottom of Witches Cauldron and cleared the spoil from there and I hauled and dragged the skips to the base of the entrance. It’s become pretty mucky along the passage, but at least the skip is suitably lubricated and slids easily. We then set about clearing the spoil from the base of the entrance back to the surface, I did the hauling up the rift while Alex was at the bottom loading the skips. Lot’s of room for dumping the digging spoil now.

26th November 2011

With Alex. Well, the original plan was to clear out all the dumped spoil back to surface, but someone forgot a spanner to erect the tripod – so continue digging was our only option. As usual we cleared the base of Witches Cauldron before I headed off for the tube. It seemed particularly sticky today like trying to cave through ‘velcro’, there is not enough space to free your oversuit from the clag and in the more snug spots it can be a little ‘hard going’. Continued clearing the sediment at the end of the tube and did manage to get a better look at the rift but am still a little undecided about the best way to proceed – don’t want to leave myself in a position where collapse of the sediment overhead might pose an inconvenience. Air movement was not as obvious during today’s session.

breccia and conglomerate

A noticeable feature when describing either breccia and/or conglomerate is clast angularity, generally breccia is more angular while conglomerate is more rounded, of course it must be emphasised that the boundaries between the rock types is blurred and there may be a high degree of variability. The angularity or roundness of the clasts within breccia/conglomerate might be used as evidence of the amount of transportation that has occurred, i.e. more roundness might indicate greater transport/higher energy. Clast size might also be a factor, smaller size would require less energy to be transported a further distance.

Dolomitic Conglomerate

Boulder of Dolomitic Conglomerate removed from Hallowe’en Rift, the purple-red colour is the result of high iron content and evidence of haematitization (see below).

On the Mendip Hills, the Dolomitic Conglomerate mostly comprises clasts of Carboniferous limestone cemented into a matrix of sandy marl or fine grained limestone debris, locally derived material from the Old Red Sandstone and Quartzitic Sandstone Group is also present. The rock clasts are angular to rounded and range in size from varying sized gravels to very large boulders. The Dolomitic Conglomerate forms bold crags, it might be eroded into gorges, and it can support underground drainage systems including caves and swallets and is the locus for extensive lead/zinc mineralization. The formation represents Triassic scree and outwash fans adjacent the ancient hills of Paleozoic rocks, and it fills ‘fossil’ wadis or gorges that had been cut into the hills. Recent erosion has partially re-excavated some of these gorges, as at Burrington Combe.

Exposure of surface weathered Dolomitic Conglomerate [slightly silicified] in Harptree Combe.

The conglomerate found on the Mendip Hills has, in many cases undergone considerable secondary changes, in particular silicification, haematitization and dolomitization. Haematitization is the conversion of the conglomerate into an ‘earthy’ iron ore known as ‘red ochre’. The presence of all gradations from unaltered conglomerate to impure haematite rock show that the formation of the latter is the result of metasomatic replacement of calcium carbonate by haematite.

The most widespread form of alteration is dolomitization, hence the name ‘Dolomitic Conglomerate’. All stages of dolomitization are present, both matrix and clasts showing varying degrees of alteration. Dolomitization is usually accompanied by hydration or partial hydration of the originally disseminated haematite to limonite so that macroscopically the colour has changed from red to yellow and yellow-brown. Dolomitization affects more than one level in the Dolomitic Conglomerate though it is most marked in the upper parts of the succession.

metasomatism is a metamorphic process [where rocks are changed by heat, pressure and fluids] in which the chemical composition of the rock is changed significantly, usually as a result of fluid flow.

References:

Green, G.W. et. al. 1965. Geology of the country around Wells and Cheddar. HMSO, London.

Keary, P. 2001. The New Penguin Dictionary of Geology, 2nd Edition. Penguin.

20th November 2011

The bar to the left of centre in the image is about 300mm in length.

Solo trip. Just a quick Sunday morning jaunt to take some photographs of the end dig in order to update the online gallery before retiring to the Hunter’s Lodge Inn.

19th November 2011

The image above shows Alex working his way along the approach tube from the base of Witches Cauldron to the dig site – a fine spot!

With Alex. Continuing the quest for passages measureless to mice! First job was to clear the spoil dumped at the base of the Witches Cauldron before returning to digging along the left-hand tube. The pot/rift edge noticed last session can be seen (take a look at the photo taken on 20th and shown on the post for that day) but some more forward progress still required before a ‘good old prodding’ is possible – might be a good idea to wear some goggles as crap falls into your face when lying on your back and waggling a bar around in an effort to dislodge a blockage above your head. There is certainly a change occurring bur more work still required. The dig has become a little squalid over the past week or so – the result of a strong drip, or lots of drips, obviously the surface above ground has become saturated and the water is permeating into the cave, emerged rather muddy, slightly knackered, but rather satisfied after a steady 2.5 hour session.