Spider Hole is currently approximately 90m long with a vertical range of 55m. The entrance leads directly to the head of a large open North-South trending fracture, well in excess of 50m deep. The fracture is continuous but is best described in two distinct sections. The upper portion starts off as 2m wide x 10m high with a floor littered with large angular, shattered blocks. Some of these have jammed part way down the rift to form a short 4m pitch, below which the rift continues steeply down to the south, narrowing gradually down to 1m wide at the head of a second, much deeper pitch. This drops 20m into a lower section of the fracture, which is considerably larger at 4-5m wide and perhaps 25m high.
The floor here contains a small lenticular gravel deposit which features some rather unusual ‘gravelmites’. There was some discussion that the gravel may derive from washin, either from the road or the old quarry above, and while possible, the fact that two sections of the deposit appear firmly cemented suggests otherwise. This point represents the furthest extent of the cave to the south and lies approximately 18m SSW and 50m below the entrance.
Below here, the trend reverses and the floor slopes steeply away to the north to reach a solid choke composed of further angular, shattered rocks, which have cascaded steeply down the slope from the fracture above. Slickensides, particularly in the lower section, show clear movement along the fracture to the south with the eastern wall being displaced very slightly (approx 0.5m) below the western wall. Evidence for this is clearly visible in the displacement of several well-exposed chert bands and bedding-planes. The fracture is tectonic in origin and there appears to be little solutional activity in evidence bar some fluting on the eastern wall from water trickling down.
The new Spider Hole rift is a particularly fine underground example of one of the major North-South fractures which dominate the south bank of the gorge. There is an apparent (if somewhat tenuous) correlation of alignment with Bone Hole and the little known Vurley Swallet which suggests that it may be a significant local structural feature. This alignment also appears to influence two notable bends in the upper part of the gorge which suggests that the Spider Hole joint may be a major controlling feature, influencing not just the caves but the line of the gorge itself. It will be interesting to note if there is any sign of a continuation of this fracture within Charterhouse Cave.
Future Potential
To speculate on the potential for further discoveries in Spider Hole it is first important to understand and discuss the processes which went into creating the existing known cave systems and the gorge itself.
Overall, the catchment area has probably changed little since its inception following the upheavals of the Permian period. Initially drainage of the plateau would have developed underground, exiting at a long-eroded point high above the current level of the rising. Undoubtedly there have been numerous levels of outlet in the past but for the purposes of this article only the three major ones are examined. These relate to three distinct levels of development clearly visible today in the caves at the foot of the gorge.
The formation of these levels and of the gorge itself is due entirely to three major ice advances, which happened 500,000 years ago, 250,000 years ago and as recently as 125,000-10,000 years ago. That the ice sheet itself did not reach as far south as the Mendips is irrelevant as the resultant tundra environment would have been more than enough to freeze the ground solid, effectively sealing access to any subterranean drainage channels. With the old conduits inaccessible, the waters of the plateau would have been forced to run along the surface, excavating deep river channels at an accelerated rate and exposing levels of limestone well below that of the previous outlet levels. As the climate warmed, normal underground drainage would then have resumed much as before, only this time to an outlet at the new lower level as water is inevitably attracted to the lowest available outlet point. This process would have de-watered much of the previously submerged cave passage, leaving it abandoned high and dry well above the base level of the new water table. Significant cave development would then have proceeded at this new base level, until the next ice advance came along to repeat the process. Eventually we were left with what we see today, namely a very deeply-incised river valley (the gorge) and three distinct levels of water table – the present one at 23m A.O.D and two old fossil ones at approximately 60m and 100m A.O.D. which are represented today by the fossil passages in Gough’s Cave and Great Oone’s Hole respectively.
As all three of these water table levels operated for considerable periods of time, they will all have had welldeveloped cave passages associated with them and their influence would have spread right throughout the catchment area. It is at these levels that significant fossil development in Spider Hole (and any other cave for that matter) is likely to be found. Possible evidence to support this can be found in a number of nearby caves and we are fortunate to have access to two major caves, Gough’s Cave and Reservoir Hole, where significant studies have already been undertaken. Using these as a comparison we can at least make an educated prediction as to the possible nature of any cave passages.
On this basis, the first fossil development, relating to the top of the phreatic loop in the Great Oone’s level, is likely to be encountered around 110m A.O.D, which is approximately 75m below the entrance. The depth of this first predicted fossil development (20m below the current deepest point), may also explain the lack of solutional activity seen in Spider Hole so far.
So does this mean that the dig at the base of the pitch in Spider Hole is likely to be 20m deep? Well yes it might be. After all, there’s an awful lot of material fallen down the rift that just has to have gone somewhere. Furthermore, if the intersection point happens to coincide with the low point of the phreatic loop rather than the high point, then it’s possible that it could be quite a few metres deeper still.
However, this does not mean that the choke is by any means completely solid. The similar choke below Golgotha Rift in Reservoir contained several large voids within its matrix, presumably protected beneath the cover of overhanging walls and some very large boulders. As there are many great boulders already in evidence, there seems no reason to suppose that Spider Hole will be any different and hopefully similar voids will be encountered to act as ‘shortcuts’ down through the choke. In any case who’s to say that the dig won’t emerge in a further open continuation of the rift well before the 110m A.O.D point? After all it’s already happened once. There’s also the possibility that a passage may be found connecting through to other nearby fault-aligned rifts such as the one represented by Topless Aven in Reservoir Hole. Any of these scenarios could potentially bypass much of the choke in one fell swoop.
In summary then, it would seem that there is a reasonable chance that Spider Hole could encounter at least two distinct levels of fossil development, one located at 110m A.O.D and a more extensive one at 55-75m A.O.D.
Using that information, is it possible to predict the nature of any such passages? Evidence from Gough’s Cave and its’ satellite risings reveals few signs of prolonged vadose action. Most passages are phreatic in nature, formed mostly in bedding-planes along the strike and frequently crossed by important N-S joints, smaller versions of the larger ones encountered in Reservoir Hole and Spider Hole. There seems no reason to suppose that Spider Hole will be any different, so there has to be a good chance that the dig will intersect at least some remnant of east-west fossil development, at either or both of the two levels described above. Such passages, (particularly those related to the old Gough’s Cave water table level) are likely to be reasonably well-developed and given the amount of time they’ve been out of the water, probably quite well-decorated, although the devastating nature of the 1968 flood seems certain to have left some impact.
Below this final fossil level, I would anticipate a series of increasingly muddy passages descending steeply down towards the present day water table and while it is possible that these may reach an active open streamway, the correlation between the anticipated depth of this level and the known level of the water table as seen in Charterhouse Cave suggests that this level is likely to be accessible only to cave divers, albeit with considerable potential.
Obviously the hypothesis of Spider Hole intersecting major fossil development at these levels depends entirely upon its proximity to any ancient main drainage channels feeding towards the abandoned outlets at Great Oone’s and Gough’s. In truth it is impossible to accurately plot the path of today’s subterranean river with any certainty, let alone any long abandoned fossil passages. However cave development is not a random affair as passage alignment is largely dictated by local geological structures, with passages developing along the faults, joints and bedding planes. As previously stated, evidence from Gough’s Cave shows the major fossil passages are formed mostly in bedding-planes along the strike (an approximate East-West alignment) that are frequently crossed by well-developed N-S joints.
Editor’s note: Tav’s 2012 longer article has been précised here to summarise the salient points. No attempt has been made to review or update the article following the Frozen Deep discoveries.
