How long ago this state of things existed is a matter for geological calculation. Suffice it that the earliest historical records show us no wild beasts existing in the land except Bears and Wolves, along with the Red Deer which is with us to this day. Now there is no sign at Wookey Hole of the time when the Bear and Wolf alone remained and all else had become extinct from the land. There is no trace whatever in the Hyæna Den of the pottery which we find in the entrance of the great cave. Without a doubt, the latest deposits here are vastly older than the most ancient deposits there. The commingling of northern, temperate, and southern forms gives evidence of oscillations in temperature such as demand a vast time to have taken place. Yet the whole of these remains accumulated between the time when the entrance to the Den was left exposed by the gradual destruction and retreat of the cliff face up the valley, and the infilling and choking of the entrance by the accumulating gravel which eventually blocked it. It is only within the last few years that the gravel arch which was first formed, and then undermined in the search after bones, has collapsed, revealing the true configuration of the cavern. Here we must again postulate a great antiquity for our caverns, since these deposits exist in what is really an insignificant fragment of the great cavern, and are only an incidental part of the material which an exposed cavity is sure to receive. But when purely geological evidences are taken into account, the demand for time becomes still more imperative. The subterranean Axe occupies, as its present channel, vast chambers formed by the excavation of thousands of tons of the hard Conglomerate, great halls over 70 feet in height and of fine proportions. The process which formed these is still at work enlarging them, till in the course of time they must collapse; yet no change is ever visible, no signs of recent action can at any point be seen. The rarely occurring great flood serves but to remove one film of sand from the floor and to leave another in its place as the waters subside. So slow is the undermining action that no eye can ever detect a change though the waters rise ever so high. Yet this channel is but one of five distinct levels which the river has occupied from time to time, until it has found in turn a lower course, leaving its sands as a record upon each, here and there sealed down beneath a mass of stalagmite. What untold ages have elapsed since first the river flowed through these upper channels!
PLAN AND SECTION OF WOOKEY HOLE CAVERN.
(Click on map to see a larger version. Not available on all devices.)
But an examination of the top of the Mendips points to a vaster antiquity still. The published horizontal section No. 17 of the Geological Survey gives an excellent idea of the plateau of Mendip, which stretches from immediately north of Wells to the neighbourhood of Compton Martin. This plane of denudation would never have been reached save by the long-continued action of subterraneous streams, an assumption supported by the existence of the great depression crossed by the road from Wells to Priddy. That depression of nearly 100 feet in depth and several miles in length, hollowed in the hard Carboniferous Limestone, here dotted with every known type of swallet or swallow hole, has been obviously formed by the slow action of swallet streams prolonged through vast periods of time. Every atom of the millions of tons of solid rock represented by this depression has been borne down the course of the subterranean Axe. Tributary to this depression a little valley has been eroded across the Old Red Sandstone anticline immediately to the north, and in it are deposited masses of Dolomitic Conglomerate, the component pebbles of which were derived from the surrounding rocks. The same valley existed, therefore, in pre-Triassic time, and as there was obviously no other outlet for its water, the cavities into which it flowed—that is to say, the swallets and subterranean channels—must have existed also, and are therefore pre-Triassic in date. Though at first sight this appears impossible, inasmuch as the known course of the resulting Axe River is through Triassic Conglomerate, I propose to show that such a conclusion is necessary and inevitable. Long ago I was struck with the fact that at Wookey Hole the Triassic Conglomerate attains an abnormal thickness, and measurements have shown that at the far end of the cavern there is certainly a thickness of over 350 feet of this rock. As there is no sign of any approach to the Limestone against which it must abut, nor any change in the character of the Conglomerate itself at this point, I think that we may fairly conclude that the total thickness of it must be at least 500 feet. Now this is a vast deposit, far exceeding any known to exist elsewhere, and it requires a special explanation to account for it. Only one explanation is possible. The Conglomerate is here filling in some great pre-existing valley in the Mountain Limestone. That is just what I should expect.
The great Limestone cavern formed by the action of the swallet streams in early Triassic times collapsed, and formed a Limestone ravine, into which was rolled a great accumulation of fragments of the Limestone derived from the slopes and crags above. With the whole of this part of England these beds were subsequently submerged, remaining so during the deposit of the whole of the Secondary beds; and on their emerging once more from beneath the sea the lines of drainage were re-established along the old courses, where these had not been choked with sedimentary material. Forcing a way through the Conglomerate which then impeded its flow, the river formed those cavities which we see. Indeed, it may well be that the successive levels cut by the Axe through the Conglomerate may represent stages in the uplifting of the land, the lowest channel being the last and largest, as it has been formed during an extended period of stability. But we are not without evidences of another sort as to the existence of some of our swallet ways at that remote period. The cavities found in the Holwell quarries, near Frome, filled in with Rhaetic material containing bones and teeth of fishes; those of Gurney-Slade, near Radstock; and numbers which from time to time are laid bare in the Limestone quarries, all filled in with Triassic sediment, show that penetrating waterways of considerable size then existed. There was, too, at Charterhouse-on-Mendip, north of Cheddar, a fissure, possibly a swallet, which, being open, received an infilling of Liassic material that is known to extend to a depth of 300 feet. Had these channels been closed by a narrow aperture temporarily blocked, no infilling but by water would have taken place when the land sank beneath the waters of the Triassic and Liassic seas.
Furthermore, in the position of the entrances of many of our swallets there is corroborative evidence to the same effect. The great circular swallet on Rookham, near Wells, situated far from any existing line of drainage, yet withal one of the largest cavities on Mendip, shows that great changes have taken place since it was an active waterway. The position of the caverns of Compton Bishop and of Banwell, far removed from any stream or any line of drainage possible with the present contours, proves that the configuration of the country has utterly changed since they formed the points of engulfment of any streams. The Coral Cave (as we have called it) at Compton Bishop descends abruptly into the earth, and its outlet must have been far below the level where now the Triassic Marl forms an impervious barrier. The waters of Banwell Pond rise through the Marl, forced upwards through beds which do not yield water and ordinarily retard its passage. Doubtless the Marl when it was deposited covered some earlier outlet from the Limestone. The waters of St. Andrew's Well, at Wells, are forced upwards through Dolomitic Conglomerate and overlying Pleistocene gravel, the former of which was doubtless deposited upon what was once a free and unimpeded outlet from the Mountain Limestone, similar to that of Cheddar. The water of Rickford, near Burrington, resulting from the streams engulfed at and around Burrington, is forced up through the Secondary beds, which have been similarly deposited upon the pre-existing outlet. All these things help to demonstrate that what I contend is true, viz. that our caverns as a whole are pre-Triassic in age. The Long Hole at Cheddar, high in the cliffs above Gough's Cave, lends its evidence too. Contrary to all the other caves at Cheddar, it was a channel of intake for the water which formed it. Doubtless it is a fragment of a larger cavern, which, before the gorge of Cheddar itself was formed, existed in the mass of rock occupying the whole area. At the northern end of the Limestone defile of Ebbor, near Wells, the ravine is carved through Dolomitic Conglomerate, which has been much worked for iron ore. The fact that this Conglomerate was deposited in a depression in the land, at the head of the present ravine, yet without entering it, suggests that here was an entrance to a series of caverns, the collapse of which produced the gorge.