Author: Andy Farrant

Those of you lucky and privileged enough to have joined the UBSS caving elite and actually gone caving, or at least spent some time in the Red Lion, may have heard rumours, myths and legends about the infamous GB Dig. Much of wha t you may have heard may have not been too complimentary - ‘its wet, muddy and horrible…’ ‘why..?’ and ‘digging is for the beardy-wierdy old lags’. So, why have members of the club spent the last 25 odd years toiling away (albeit intermittently) in the muddy grovel at the end of Bat Passage? Good question. There are several reasons.

1. You are a masochistic mud-loving social leper with a perverted interest in small dark shitty holes, and you can’t find anything better to do on a Saturday night. (I think Hugo probably fits into this category as I can’t think why els e he goes digging - it may also explain his penchant for Bos Swallet!).

2. You were told it’s really fun and jolly exciting, that fame and fortune await you, or else you were press-ganged/drunk when you agreed to help.

3. You want to find lots of nice big caverns measureless to man, and can’t afford lots of expensive expeditions to foreign places. And anyway, finding a few hundred metres of passage on Mendip is far more satisfying than a few kilometre s of huge passage that anyone can walk into in some far off tropical rainforest.

A combination of all three is probably nearest the truth, but there are good reasons to think there is a good chance of entering some large well-decorated cave beyond the current end of Bat Passage. This dig has probably the best potent ial on Mendip, as I’ll explain. First, you need to understand a little about the cave and how it functioned in the past (sorry - this is one of my geo-rambles again).

Nearly all the caves on the southern edge of Blackdown occur where streams flowing off the sandstone disappear underground upon reaching the limestone. The water descends steeply down through open vadose (above the water table) passages until it hits the water table and then flows to the resurgence through predominantly phreatic (sub-water-table) undulating passages i.e. sumps. The sump level in Longwood is at 40m O.D., only around 20m above resurgence level. Work by Pete Smart and othe rs suggest only c. 9% of the passage between here and Cheddar is open vadose passage.

At present, the water resurges at the foot of Cheddar Gorge. However, the Gorge has had a complex history and has undergone several phases of active incision by a surface river during cold glacial periods. During each subsequent warm in terglacial phase, a new cave would develop, graded to the lower valley floor level. Thus, above the current active conduit are several tiers of fossil phreatic passages. Detailed work in the swallet caves by several generations of UBSS and other cavers, i ncluding Pete Smart, Tim Atkinson, Dave Drew and Derek Ford, have identified at least 4 of these fossil phreatic levels; at 238, 138, 120 and 90m O.D. In GB, only the upper two have been identified with certainty; at 238m (Devil’s Elbow route) and at 138m , the Ladder Dig - Bat Passage conduit. The lower levels almost certainly exist, but as yet, GB has not been pushed deep enough to find them.

Thus, the Ladder Dig - Bat Passage route is a fossil phreatic passage at the 138m level that probably flowed to a former resurgence in Cheddar. So, there is good evidence for a significant amount of fossil passage beyond the dig face. T here is also a fair amount of depth potential before the main streamway reaches sump level. At present, the streamway terminates at the massive choke beneath Great Chamber. However, this choke has already been by-passed at high-level via Bat Passage. Almo st certainly the continuation of Bat Passage beyond the dig will intersect vadose inlets draining into the continuation of the GB streamway, in much the same way as the Devils Elbow stream is a recent vadose inlet connecting the old phreatic passage with the much younger Boulder Chamber. Blue Pencil passage in Swildon’s Hole is another example.

So, not only is there potential for old high-level phreatic passages, but the prospect of a link back to the main stream continuation. This may also provide a link into the lower fossil phreatic levels and an associated series of vadose inlets (similar to White Passage and Rhumba Alley). One only has to look at the immensity of the Gorge and Great Chamber to get an idea of the potential size of passage still to be found. In addition, it is possible that the downstream continuation of Ch arterhouse Cave may connect into GB, either at or downstream of the Great Chamber collapse. The diagram below is my prediction of the sort of passage we may find (and not a dowsing rod in sight…).

What about the actual dig? Known to some as the GBH dig, (for causing grievous bodily harm to its diggers), it was started back in the 1960’s shortly after the discovery of Bat Passage. A cross section and plan is shown below. At the en d, the large Bat Passage breaks down into a series of two or three parallel paragenetic rifts. Paragenetic cave development occurs when sediment deposition on the floor causes a passage to erode upwards, rather than incise the floor. As a result, these ri fts are almost completely sediment filled. Only the upper rift remained free of sediment. This high level rift leads into a small chamber with two possible leads. However, this was deemed un-diggable due to the abundant pristine formations in the passage immediately before the dig. So, to bypass this, the lower wetter dig was started.

Over the intervening years, many teams have been involved. I began digging in 1989, and eventually broke through into the original end chamber reached in 1966. Although we were hoping to bypass the end chamber, it certainly is not a was te of effort. We can now start digging where the original explorers couldn’t without ruining some fantastic formations. At the end, we have two options; either we continue at low level and hope to break up into the open upper level continuation, or we pur sue the upper chamber leads, which may require banging. Hopefully, once we get beyond this sediment filled phreatic loop, the passage will open up again to a passage similar in form to Bat Passage

At present, the dig face will need a few trips to clear the accumulated spoil and to make the entry up into the end chamber easier. The pump may need an overhaul as well. Once that is done, we’ll have plenty of digging and stacking spac e, plus no bad air problems. It’ll still be a bit squalid getting there, but all the horrible work hopefully has been done.

So, get digging; fame, glory and caverns measureless to man await you…!