R.R.Glover, June 1973, from LUSS Journal Volume 1 Number 3
From M.A. thesis Speleogenesis of Gaping Ghyll and the Ingleborough Cave System.Over 10km of passage are presently known in the Gaping Ghyll system. Much of this development consists of a network of low arched tunnels, with rocky or sandy floors, which occur at a depth of around 100m. below the level of the moor. Other distinctive features include the absence of major active streamways, and the extensive vertical development, consisting of shafts and avens, which intersect as if by accident the "ground floor" network. Above all, the most striking feature is the presence of at least six very large chambers containing vast quantities of sand, clay and boulders which is slowly being cleared away. The Main Chamber is of course the best example of this.
All these features make Gaping Ghyll quite unlike most of the other large cave systems in Craven, the majority of which consist of active vadose shafts connected by meandering, T-section vadose stream passages.
This difference is particularly apparent when Gaping Ghyll is compared with the many deep shafts, ending in chokes or sumps, at or near resurgence level, which are found on the adjacent areas of Newby Moss and the Allotment.
Following the discovery of several major extensions in the last few years, a pattern of sorts begins to emerge. The component parts of the Gaping Ghyll system appear to fall into one of five categories:
a) The position of Gaping Ghyll at the inner edge of a plateau corresponding to the top of the Great Scar Limestone, below a south-facing cwm, appears to be important. This cwm, Clapham Bents forms a large, high level, catchment which has allowed the drainage to be integrated into a single large stream before reaching the Great Scar Limestone. The east shoulder of Simon Fell has protected much of the plateau around Gaping Ghyll from the direct erosion of ice moving south down Ribblesdale, at least in the later stages of the Dales glacial history. This resulted in the deposition of drift blankets and mounds which has aided the drainage integration.
b) The structure of the limestone has provided important geological controls. The Gaping Ghyll plateau is well jointed, with the "master" joints oriented approximately NW/SE (142 degrees / 332 degrees True). These penetrate both the D1 and S2 limestone to considerable depth and appear to have been initially widened under phreatic conditions and have suffered subsequent vadose invasion and modification. Thus the initial phase of development of the system probably consisted of a group of 300ft deep vadose shafts (similar to the Newby Moss and Allotment types) in close proximity. Unlike these other areas, however, the early Fell Beck encountered these joints almost at right angles. Thus minor diversions of the surface stream route would allow multiple entry and re-entry. In addition to the joints, the plateau is crossed by at least three major faults which trend more nearly east-west than the major joints. These faults penetrate the full depth of the limestone and the points of low-angle intersection with major joints must have allowed particularly easy access to surface water. As with the joint controlled passages, much of the early development in the fault planes was phreatic, at various levels, and subsequent vadose invasion and modification has only partly destroyed these sections. Much the most spectacular fault is that which runs through the Main Chamber, Old East Passage, Mud Hall and Far East Passage to Brothers Junction. The plane of this fault hades south at an angle of approximately 55 degrees to the horizontal and is responsible for the hanging root feature of the main chamber. It originates at the westend of the main chamber and is an oblique hinge fault, down-throwing to the south. Thus the Porcellaneous Bed, visible in both walls some 6m above the floor of the Main Chamber suffers an increasingly large displacement in an easterly direction. At the east end of the Main Chamber the displacement is of the order of 4m, but in Mud Hall it is probably at least 20m. The South East Passage fault, on the other hand, appears to have no vertical throw, but evidence of dextral slide action, in the form of displaced portions of solution tubes in the roof near Bar Pot Aven, and slickensides in Stream Chamber point to recent movement along this line. The Hurnel Moss Fault has not been examined underground, in Mountain Hall and Clay Cavern in the Far Country, but on the surface, to the west of the upper end of Trow Gill, it appears to downthrow to the north by 3 to 4m.
c) The lithology of the limestone in the Gaping Ghyll area has also provided important geological controls. The existence of at least four horizons, at which extensive horizontol development occurs, implies the occurrence, at these horizons, of relatively impermeable beds which have limited the down-joint percolation of surface water. The Porcellaneous Bed is much the most important of these horizons, and much of the maze of low rock floored sub-horizontal phreatic tunnels which link the fault controlled passages and chambers appear to be developed upon it. It is a dense blue white biomicrite, some 50cm thick, and lacks joints by comparison with the coarser grained micrites and sparites above and below it. It appears to have acted as an impervious layer in a similar manner to shale bands, but without the mechanical incompetence of shale. Wherever this bed has been penetrated by fault action, or where it feathers out to the south east, vertical development is facilitated. Good examples of this effect can be found in the Main Chamber, Mud Hall, Sand Cavern, Stream Chamber, Henslers Master Cave and the Far Country. Some 6m above the Porcellaneous Bed, a thin shale band marks a bedding plane of almost equal importance. The roof in Sand Cavern, Stream Chamber, parts of Mud Hall and beyond occur at this level.
d) Finally, the relief of the Lower Palaeozoic rocks upon which the Great Scar Limestone was unconformably deposited appears to have controlled overall direction of movement of water at depth. The presence of at least one, possibly two, buried ridges has resulted in the development of a deep undrained phreatic system under Gaping Ghyll, and has been responsible for the development of Ingleborough Cave as the main resurgence channel at a height of 825ft O.D. where there is the possibility of a further 70m of limestone below this level.
R.R. Glover, June 1973.
This article is Copyright LUSS.