IN A GEOLOGICAL CONTEXT
Abridged and amended version of a paper read before the Torquay Natural History Society on 27th March, 1951, by E.T. Vachell, M.A., F.G.S., F.INST.PET., F.N.I.
"The geological structure in the neighbourhood of Kents Cavern is complex. All the ramifications of the cave-system occur in an outcrop of Torquay Limestone. The area of this shown on the accompanying map includes both the thin-bedded limestone of Eifelian age and the overlying massive, and thick-bedded, limestone of Givetian age (these two limestones are always difficult to distinguish in the field and have consequently been mapped together as one group). The limestone, as mapped, therefore represents the upper part of the Middle Devonian and the lower part of the Upper Devonian; and it is in the upper, thick-bedded portion of this limestone group that the caves occur.
In the neighbouring exposures at Babbacombe the Torquay Limestone is directly underlain by dark Norden Slates (Eifelian), of Middle Devonian age; and it is probable that they just outcrop in the base of the scarp below the present entrance to the caves. However, exposures in this region are much obscured by scree from the cliff, by debris thrown out of the caves during excavations and by cultivation. Shale-like material was observed below the entrance to the Caves when the accompanying map was drawn up in 1951, but it was not possible to ensure that it was actually 'in situ'.
Westward and northward from the Caves the limestone outcrop can be traced without a break into Wellswood village and Asheldon Copse. Drainage excavations near the bottom end of Higher Warberry Road, and an outcrop at the lower end of Harrington Road, showed that in this direction the limestone overlies buff-weathering beds referable to the Norden Slates which normally underlie it, and which extend westwards up the flank of Warberry Hill.
Thus, in the Wellswood/Asheldon Copse area the succession and structure seems perfectly normal, the limestone forming a relatively thin sheath on the eastern flank of the Warberry uplift, which, further west, brings Lower Devonian rocks to the surface.
Opposite the caves, on the eastern side of the Ilsham Valley, the limestone is again exposed capping the hill near Stoodley Knowle, and forming a scarp above Bishop's Walk where the path crosses the seaward slope south of Anstey's Cove. There is however a slight complication, for the whole of the limestone on the Stoodley Knowle side of the valley is either inverted, in which case it is lying on its back with the purple, grey and green Gurrington Slates (Upper Devonian), which should be on the top of it, beneath it, or it has been thrust laterally over these younger Gurrington Slates.
It was largely in an attempt to explain this phenomenon that the detailed mapping of the area was carried out. This mapping indicated, in my opinion, that the Stoodley Knowle exposure forms part of the recumbent middle-limb of a large, local overfold and that this overfold is cut off from the normally disposed limestone of Asheldon Copse and Kents Cavern by an important fault which runs along the contiguous portion of the Ilsham Valley. This is in fact the line of weakness along which this portion of the valley has been eroded.
The nature of this fault is of great importance in connection with the caves, because if the Asheldon Copse exposures should be thrust over the original roots of the overfold along a fault of low hade (the angle which the fault makes with the vertical plane), then the limestone of the cavern might overlie the limestone in the lower limb of the overfold, and the resultant repetition of the beds might allow considerable vertical extension of the ramifications of the caves and there might be undiscovered chambers at lower depths.
The fault can however be traced northwards behind Devil's Point and along the face of the cliff at the back of Redgate Beach. Here the fault can be seen in places, sometimes with patches of purplish Upper Devonian marly clay still adhering to the limestone wall. Further north it runs into the cleft in the limestone through which the footpath from the beach climbs up to Walls Hill. Here it can be seen to be a nearly vertical fault giving little or no horizontal displacement.
In the lower part of Ilsham Road, south-east of the caves, there are exposures of limestone which must underlie the purplish Upper Devonian red clays near Ilsham manor. This limestone therefore forms part of the lower limb of the overfold, and occurs further east than would be expected from the dips in the recumbent limb. Thus it appears that the inverted Stoodley Knowle beds have been slightly displaced westwards along an almost horizontal thrust of only minor structural significance, which locally replaces the axis of the overfold.
Thus, as the limestone exposures of Stoodley Knowle form part of an entirely different structural unit to those near Kents Cavern, and as the two units are separated by a fault of high hade, the limestone of Kents Cavern cannot overlie any portion of the overfold. It follows that there is no possibility of repetition of the limestone below the caves, and therefore no likelihood of any considerable downward extension of them.
South of the Caves the ground rises to the eastern end of Lincombe Hill, which is formed by very distinctive grit bands and shales of Lower Devonian age. Topographically these lie at a much higher level than the limestone, though stratigraphically they lie below it. It is therefore clear that the limestone outcrop is truncated southwards by a considerable fault. Exposures are bad in this area, but it has been possible to locate this fault about 350 feet southward from the entrance to the caves and to follow it eastward to Smugglers Cove, where it brings the Dolerite of Black Head, which is here intruded into the axis of the Stoodley Knowle overfold, against the Lower Devonian of Kilmorie Hill. This fault appears to merge with the Walls Hill-Ilsham Valley fault, and to form the southerly termination of the Stoodley Knowle overfold.
As can be seen on the map the ramifications of the caves extend in a south-westward direction nearly up to the face of this fault, but nowhere do they cross it. In fact they nowhere actually reach it, for careful search in the most southerly passages failed to reveal any sign of the presence of Lower Devonian grits."
Based on Vachell, E.T. 1953. 'Kents Cavern: its origin and history.' Torquay Natural History Society Transactions and Proceedings, XI (2), 51 - 73.