The view from the ancient camp on the top of Ingleborough offers a striking example of the effect of rain-water in eroding the surface of the limestone. As you look down over the dark crags of millstone grit, great, grey, pavement-like masses of limestone strike the eye, standing above the heather, perfectly bare, and in the distance resembling clearings, and in rainy weather sheets of snow. On approaching them the surface of erosion becomes more and more apparent, and the shapes due to the mere accident of varying hardness in the rock, or the varying quantity of water passing over it, present a most astonishing variety. There are, however, general principles underlying the confusion. The lines of joints in the strata being lines of weakness, searched out by the acid-laden water, have been widened into chasms, sometimes of considerable depth; and as they cross at right angles, the whole surface is formed of rectangular masses, each insulated from its fellow, and some of them detached from the strata beneath so as to form rocking-stones. The mode in which the acid has attacked one of these joints in the limestone of Doveholes in Derbyshire is represented in Figure 7, the surface being honeycombed and worn into sharp points, solely by chemical action. The minute fossil-shells also, and fragments of crinoid standing out in bold relief, lead to the same conclusion—that the denuding agent is chemical and not mechanical. Each of the upper surfaces of the blocks is traversed by small depressions, which are valley systems in miniature, in which the tiny valleys converge into a main trunk leading into the nearest chasm. There are also tiny caves and hollows, that are sometimes mistaken for borings made by pholas. In the chasms the vegetation is most luxuriant, and the dark green fronds of harts-tongue, the delicate Lady-fern, and the graceful Asplenium nigrum, grow with a rare luxuriance.
In these pavements every feature of limestone scenery is represented on a minute scale. There are the valley systems on the surface, determined by the direction of the drainage; the long chasms represent the open valleys and ravines, and the caves and hollows, for the most part, run in the line of the joints.
The carbonic acid has left precisely the same kind of proof of its work within the caves as we find above-ground; and it would necessarily follow, that to it, as well as to the mechanical power of the waters flowing through them, their formation and enlargement must be due, as Professor Phillips has pointed out in his “Rivers, Mountains, and Sea Coast of Yorkshire,” pp. 30–1.
From the preceding pages it will be seen that caves in calcareous rocks are merely passages hollowed out by water, which has sought out the lines of weakness, or the joints formed by the shrinkage of the strata during their consolidation. The work of the carbonic acid is proved, not merely by the acid-worn surfaces of the interior of the caves, but also by the large quantity of carbonate of lime which is carried away by the water in solution. That, on the other hand, of the mechanical friction of the stones and sand against the sides and bottom of the water-courses, is sufficiently demonstrated by their grooved, scratched, and polished surfaces, and by the sand, silt, and gravel carried along by the currents. The generally received hypothesis, that they have been the result of a subterranean convulsion, is disproved by the floor and roof being formed, in very nearly every case, of solid rock; for it would be unreasonable to hold that any subterranean force could act from below, in such a manner as to hollow out the complicated and branching passages, at different levels, without affecting the whole mass of the rock. Nor is there cause for holding the view put forth by M. Desnoyers32 or M. Dupont,33 that they are the result of the passage of hydrothermal waters. The causes at present at work, operating through long periods of time, offer a reasonable explanation of their existence in every limestone district; and those which are no longer watercourses can generally be proved to have been formerly traversed by running water, by the silt, sand, and rounded pebbles which they contain. In their case, either the drainage of the district has been changed by the upheaval or depression of the rock, or the streams have searched out for themselves a passage at a lower level.
But if caves have been thus excavated, it is obvious that ravines and valleys in limestone districts are due to the operation of the same causes. If, for instance, we refer to Figures 1 and 6, we shall see that the open valley passes insensibly into a ravine, and that into a cave. The ravine is merely a cave which has lost its roof, and the valley is merely the result of the weathering of the sides of the ravine. There can be no manner of doubt but that, in both these cases, the ravine is gradually encroaching on the cave, and the valley on the ravine; and if the strata be exposed to atmospheric agencies long enough, the valley of the Axe will extend as far as Priddy (Fig. 1), and that of Dalebeck to the watershed above the Gatekirk cave (Fig. 6).
In the same manner the lofty precipice of Malham Cove, near Settle, in Yorkshire (Fig. 8), is slowly falling away and uncovering the subterranean course of the Aire. Eventually the ravine thus formed will extend as far as Malham Tarn, and the Aire flow exposed to the light of day from its source to the sea.34
This view is applicable to many if not to all ravines and valleys in calcareous rocks, such as the Pass at Cheddar, or the gorge of the Avon at Clifton, and those of Derbyshire, Yorkshire, and Wales. And since the agents by which the work is done are universal, and calcareous rock for the most part of the same chemical composition, the results are the same, and the calcareous scenery everywhere of the same type. In the lapse of past time, so enormous as to be incapable of being grasped by the human intellect, these agents are fully capable of producing the deepest ravines, the widest valleys, and the largest caves.
This view of the relation of caves to ravines was so strongly held by M. Desnoyers, that he terms the latter “cavernes à ciel ouvert.” I arrived independently at the same conclusion after the study of the scenery of limestone for many years.
In many cases, however, in northern latitudes and in high altitudes, the ravine or valley so formed has been subsequently widened and deepened by glacial action. That, for instance, of Chapel-en-le-Dale bears unmistakeable evidence of the former flow of a glacier, in the roches moutonnées and travelled blocks that it contains. To this is due the flowing contour and even slope of its lower portion.
The pot-holes and “cirques” in calcareous rocks with no outlet at the surface, may also be accounted for by the operation of the same causes as those which have produced caves. Each represents the weak point towards which the rainfall has converged, caused very generally by the intersection of the joints. This has gradually been widened out, because the upper portions of the rock would be the first to seize the atoms of carbonic acid, and thus be dissolved more quickly than the lower portions. Hence the funnel shape which they generally assume, and which can be studied equally in the compact limestone or in the soft upper chalk. They are to be seen on a small scale also in all limestone “pavements.” Sometimes, however, the first chance which the upper portions of the funnels have of being eroded by the acidulated water, is more than counter-balanced by the increased quantity converging at the bottom, and the funnel ends in a vertical shaft. If the area in the rock thus excavated be sufficiently large to allow of the development of a current of water, the mechanical action of the fragments swept along its course will have an important share in the work, as we have seen to be the case in Helln Pot.
In some few cases the lines of weakness which have been worn into caves, pot-holes, ravines, and valleys, may have been produced, as M. Desnoyers believes, by subterranean movements of elevation and depression; but in all those which I have investigated the faults do not determine the direction of the caverns. The mountain limestone of Castleton, in Derbyshire, offers an example of caves intersecting faults without any definite relation being traceable between them. The ramifications of the Peak cavern traverse the Speedwell Mine nearly at right angles, and the water flowing through it has been traced, Mr. Pennington informs me, to a swallow-hole near Chapel-en-le-Frith, running across two, if not three faults, which are laid down in the geological map. As a general rule caverns are as little affected by disturbance of the rock as ravines and valleys which have been formed in the main irrespective of the lines of fault.
M. Desnoyers points out the close analogy between caverns and mineral veins, and infers that both are due to the same causes. This, undoubtedly, exists in that class of veins which are known to miners as “pipe” and “flat veins;” and there is clear proof, in the majority of cases, that the cavities in which the minerals occur have been formed by the action of running water, and have subsequently been more or less filled with their mineral contents; and these have been deposited on the sides of the cavity by the same “incretionary35” action, as that by which dripstone is now being formed in the present caves from the solution of carbonate of lime. Such veins present every conceivable form of irregularity, and frequently contain silt, sand, and gravel, which have been left behind by their streams, and their history is identical with that of the caverns.
It is not so, however, with the second class of veins, the “rake,” “right running,” and “cross courses,” as the miners term them, or those which occupy lines of fault. The fissures which contain the ore are proved very frequently, by their scratched and grooved sides, and polished surfaces or slicken-sides, to have been the result of subterranean movements by which the rock has been broken by mechanical force. They have been subsequently modified, in various ways, by the passage of water, and filled with minerals, in the same manner as the preceding class. With this exception they present no analogy to the caverns, with which they contrast strongly in their rectilinear direction, as well as in their purely mechanical origin.
It is very probable that caves were formed in calcareous rocks from the time that they were raised to the level of the sea, since they abound in the Coral Islands. “Caverns,” writes Prof. Dana,36 “are still more remarkable on the Island of Atiu, on which the coral-reef stands at about the same height above the sea as on Oahu. The Rev. John Williams states—that there are seven or eight of large extent on the Island of Tuto; one he entered by a descent of twenty feet, and wandered a mile in one only of its branches, without finding an end to ‘its interminable windings.’ He says—‘Innumerable openings presented themselves on all sides as we passed along, many of which appeared to be equal in height, beauty, and extent to the one we were following. The roof, a stratum of coral-rock fifteen feet thick, was supported by massy and superb stalactitic columns, besides being thickly hung with stalactites from an inch to many feet in length. Some of these pendants were just ready to unite themselves to the floor, or to a stalagmitic column rising from it. Many chambers were passed through whose fret-work ceilings and columns of stalactites sparkled brilliantly, amid the darkness, with the reflected light of our torches. The effect was produced not so much by single objects, or groups of them, as by the amplitude, the depth, and the complications of this subterranean world.’”
Calcareous rocks might, therefore, be expected to contain fissures and caves of various ages. In the Mendip Hills they have been proved by Mr. Charles Moore to contain fossils of Rhætic age, the characteristic dog-fishes, Acrodus minimus, and Hybodus reticulatus, the elegant sculptured Ganoid fish, Gryrolepis tenuistriatus, and the tiny marsupials, Microlestes and its allies. This singular association of terrestrial with marine creatures is due to the fact, that while that area was being slowly depressed beneath the Rhætic and Liassic seas, the remains were mingled together on the coast-line, and washed into the crevices and holes in the rock.
The older caves and fissures have very generally been blocked up by accumulations of calc-spar or other minerals, and they are arranged on a plan altogether independent of the existing systems of drainage.
It is a singular fact that no fissures or caves should, with the above exception, contain the remains of animals of a date before the Pleistocene age. There can be but little doubt that they were used as places of shelter in all ages, and they must have entombed the remains of the animals that fell into them, or were swept into them by the streams. Caves there must have been long before, and the Eocene Palæotheres, and Anoplotheres met their death in the open pit-falls, just as the sheep and cattle do at the present time. The Hyænodon of the Meiocene had, probably, the same cave-haunting tastes as his descendant, the living Hyæna, and the marsupials of the Mesozoic age might be expected to be preserved in caves, like the fossil marsupials of Australia. The chances of preservation of the remains when once cemented into a fine breccia, or sealed down with a crystalline covering of stalagmite, are very nearly the same as those under which the Pleistocene animals have been handed down to us. The only reasonable explanation of the non-discovery of such remains seems to be, that the ancient suites of caves and fissures containing them, and for the most part near the then surface of the rock, have been completely swept away by denudation, while the present caverns were either then not excavated or inaccessible.
Such an hypothesis will explain the fact that the no ossiferous caverns are older than the Pleistocene age, not merely in Europe, but in North and South America, Australia, and New Zealand. The effect of denudation in rendering the geological record imperfect, may be gathered from the estimate, which Mr. Prestwich has formed, of the amount of rock removed from the crests of the Mendips and the Ardennes, which is in the one case a thickness “of two miles and more,” and in the other as much as “three or four miles.”37 Under these conditions we could not expect to find a series of bone caves reaching far back into the remote geological past, since the caves and their contents would inevitably be destroyed.
We must now consider the condition under which caves become filled up with various deposits. If the velocity of the stream in a water-cave be lessened, the silt, sand, or pebbles it was hurrying along will be dropped, and may ultimately block up the entire watercourse. In bringing this to pass, however, the carbonate of lime in the water plays a most important part. If the excess of carbonic acid by which it is held in solution be lost by evaporation, it immediately reassumes its crystalline form, and shoots over the surface of the pool like plates of ice, or is deposited in loose botryoidal masses at their sides and on their bottoms; and, since the atmospheric water very generally percolates through the crannies in the rock, the sides and roof of the channel, above the level of the water, are adorned with a stony drapery of every conceivable shape. The rate at which this accumulation takes place depends upon the free access of air necessary for evaporation, and is therefore variable,—as in the case of the Ingleborough cave. In all the caves which I have examined there is a free current of air. If a water-channel becomes blocked up by either or both these causes, the joints and fissures in the rock offer an outlet to the drainage, more or less free, at a lower level, as in the Ingleborough cave, Poole’s cave, near Buxton, and many others. Sometimes, however, owing to the increased rain-fall, or to the obstruction of the lower channels, the water re-excavates the old passages, as we shall see to have been the case with the famous caverns of Kent’s Hole and Brixham. In the summer of 1872, a sudden rain-fall not merely opened out for itself a new passage into a swallow-hole close to Gaping Gill, on the flanks of Ingleborough, but forced its way out through the old entrance of the Ingleborough cave, breaking up the calcareous breccia, and removing the large stones in its course. A cave obviously may become dry, either by the drainage passing along a lower level, or by the elevation of the district by subterranean energy. After it has been forsaken by the stream, the particles brought down by the atmospheric water percolating through the joints, tend to fill it up on the surface, and these may be either of clay, loam, or sand.
These actions may be studied in this country in the well-known caves of Ingleborough, Buxton, Cheddar, Wookey Hole, and a great many others in Derbyshire, Yorkshire, Staffordshire, Durham, Cumberland, and Wales.
Among the most beautiful stalactite caverns in this country is that on the island of Caldy, immediately opposite to Tenby in Pembrokeshire, discovered some years ago in the limestone cliff, and explored by Mr. Ayshford Sanford and the Rev. H. H. Winwood, in 1866, and subsequently by the writer in 1871 and 1872. On creeping through a narrow entrance with an outlook to the sea on a precipitous side of a quarry, a passage leads to a chamber of considerable horizontal extent, the bottom being covered with silt, on which stand pedestals of dripstone from an inch to two feet in length, each rising from a thin calcareous crust which does not altogether conceal the silt below. From it a low entrance leads into a fairy-like chamber, the floor consisting of a rich red, crystalline pavement, perfectly horizontal, and studded here and there with round bosses (Figs. 9, 10, 11), either red or snow-white. From the roof hang stalactites offering the same beautiful contrast of colours, forming a delicate canopy of tassels, or passing downwards to the floor and constituting slender shafts about three feet long, and about the diameter of straws. Each of these is hollow, translucent, and more or less traversed by water, and in some places each stood next its fellow, almost as close as the straws in a cornfield. Sometimes the shaft stands on a cone (Fig. 11) of dripstone, more or less raised above the floor. Small pools of water occupy hollows in the pavement, each lined with glittering crystals of calcite (Fig. 12), which are slowly shooting over the surface, and converting some of the open hollows into bottle-shaped cavities (Fig. 13). Their sides and bottoms are covered with a crystalline growth of singular beauty, of which an idea may be formed by woodcut 14, which represents the edge. Where the drip happened to fall into a shallow pool, it gradually built up for itself a cone, on the lower portion of which the varying water-level is marked by horizontal rings of crystals (Fig. 15), and the normal waterline by the upper horizontal plate. Sometimes these were united to the roof by a slender straw-shaft. In Figure 11 the original shaft has been broken away, and as the direction of the drip has slightly shifted, a new one gradually descended, until finally it became cemented to the side of the cone.
The history of these structures is very evident. The straw-like stalactites were formed by the evaporation of the carbonic acid from the surface of each drop of water, as it accumulated in one spot, and the consequent deposit of carbonate of lime around its circumference. It could not be formed in the centre, because of the continual movement of the successive drops in falling. By a circumferential growth of this kind a small crystal tube, of the diameter of a drop, is slowly developed, which continues to lengthen until the result is one of the straw-columns, with a hole in the centre for the passage of the water, which cannot readily part with its carbonic acid till it arrives at the end of the tube. Sometimes the hole has been subsequently blocked up by calc-spar, or the general surface been covered over with successive layers, until it becomes a mass of considerable diameter. If the drop fell into a deep pool, the straw-column was continued down to the water-line; if in shallow water, or on the floor, a pedestal was built up, as is represented in the preceding figures. The crystallization going on in the pools is greater at the surface than below, because of the greater evaporation, and consequently the stalagmitic film is gradually extending over it on every side from the edges (Figs. 12, 13).
As I broke my way into some of the unexplored recesses, through the thickly planted straw-shafts, and scene after scene of fairy beauty, unsullied by man, opened upon my eyes, the ringing of the fragments on the crystalline floor that accompanied almost every movement made me feel an intruder, and sorry for the destruction.
In some places, where the drip was continuous, and the calcareous basin which it had built up for itself shallow, small spherical bodies of calcite were so beautifully polished by friction in the agitated water, that they deserve the name of cave-pearls from their lustre. In Fig. 16 I have represented a tiny basin with its pearly contents. Where the drip had ceased to be continuous each of these formed a nucleus for the deposit of calcite crystals, by which they were united to the bottom of the basin.
In the principal chamber in the cave, which is very nearly free from drip, the upper surfaces of the stones and stalagmites on the floor are covered with a peculiar fungoid-like deposit of calcite, consisting of rounded bosses, attached to the general surface by a pedicle (see Figs. 17, 18) sometimes not much thicker than a hair. They stood close together at various levels, following the inequalities of the surface of attachment, and being on an average about 0·2 inch long. Several microscopical sections (Fig. 17) showed that each was formed originally on a slight elevation of the general surface, which would cause a greater evaporation of water than the surrounding portions, and therefore be covered with a greater deposit of calcite. This process would go on until the height was reached to which the water slowly passing over the general surface would no longer rise. Hence the remarkable uniformity of the height of the bosses. The evaporation is greater at the point furthest removed from the general surface, and therefore the apex is larger than the base (see Fig. 17). In Figure 18 they stand as thickly together as trees in a virgin forest, and are developed in greatest vigour where the small eminences cause a greater evaporation than the small depressions, and are stoutest and strongest at the free edges. Some of the pedicles, as in the figure, present traces of erosion, the outer layers having been eaten away by acid-laden water.
Some of these singular little bosses may have been moulded on minute fungi, such as those in the cave of Ingleborough, but their presence is not revealed by the microscope.
I met with this remarkable kind of calcareous deposition in a second cave in the neighbourhood of Tenby. When examining the Black-rock quarries in 1871, the workmen pointed out a small opening which they believed to be the entrance of a cave, but which was too small for them to enter. By knocking off, however, a few sharp angles, I got into a small chamber about five feet high, with sides, roof, and bottom covered with massive dripstone. A few loose stones rested on the bottom. The whole surface, even including the stones upon the floor, one of which is figured (Fig. 18), was so completely covered with these peculiar fungoid bodies, that it was impossible to move without destroying hundreds of them. All were about the same height, 0·2 inches, snow-white, or of a rich reddish brown, and conformed to the unequal surface on which they stood. It is quite impossible to describe the effect of a whole chamber bristling with these peculiar structures. The only author by whom they are mentioned, Mr. John Beaumont—who described the caves of Mendip in 1680, considered them to be veritable plants of stone.38 The beautiful forms assumed by the dripstone in the caves of Caldy and Black-rock are by no means uncommon, but I have never met with them anywhere else in such perfection. They may be studied in all stalactitic caverns.
A small portion only of the carbonate of lime is deposited as tufa or dripstone in the neighbourhood of the rock from which it has been derived, as compared with that carried by the streams into the rivers, and the rivers into the sea. An idea of this quantity may be formed from the calculation of the solid matter conveyed down by the Thames, given by Mr. Prestwich in his Presidential Address to the Geological Society in 1871, p. lxvii.
“Taking the mean daily discharge of the Thames at Kingston at 1,250,000,000 gallons, and the salts in solution at nineteen grains per gallon, the mean quantity of dissolved mineral matter there carried down by the Thames every twenty-four hours is equal to 3,364,286 lbs., or 150 tons, which is equal to 548,230 tons in the year. Of this daily quantity about two-thirds, or say 1,000 tons, consist of carbonate of lime and 238 tons of sulphate of lime, while limited proportions of carbonate of magnesia, chlorides of sodium and potassium, sulphates of soda and potash, silica and traces of iron, alumina, and phosphates, constitute the rest. If we refer a small portion of the carbonates and the sulphates and chlorides chiefly to the impermeable argillaceous formations washed by the rain-water, we shall still have at least ten grains per gallon of carbonate of lime, due to the chalk, upper greensand, oolitic strata, and marlstone, the superficial area of which, in the Thames basin above Kingston, is estimated by Mr. Harrison at 2,072 square miles. Therefore the quantity of carbonate of lime carried away from this area by the Thames is equal to 797 tons daily, or 290,905 tons annually, which gives 140 tons removed yearly from each square mile; or, extending the calculation to a century, we have a total removal of 29,090,500 tons, or of 14,000 tons from each square mile of surface. Taking a ton of chalk, as a mean, as equal to fifteen cubic feet, this is equal to the removal of 210,000 cubic feet per century for each square mile, or of 9/100 of an inch from the whole surface in the course of a century, so that in the course of 13,200 years a quantity equal to a thickness of about one foot would be removed from our chalk and oolitic districts.”
This destructive action, operating through long periods of time, destroys not merely the general surface of the limestone, but, where it is localized by the convergence of water, is capable of excavating the deepest gorges and the longest caves. The quantity of material carried away in solution is a measure of the power of carbonic acid in the general work of denudation.
The circulation of carbonate of lime in nature presents us with a never-ending cycle of change. It is conveyed into the sea to be built up into the tissues of the animal and vegetable inhabitants. It appears in the gorgeous corallines, nullipores, calcareous sea-weeds, sea-shells, and in the armour of crustaceans. In the tissues of the coral-zoophytes it assumes the form of stony groves, of which each tree is a colony of animals, and in the wave-defying reef it reverts to its original state of limestone. Or, again, it is seized upon by tiny masses of structureless protoplasm, and fashioned into chambers of endless variety and of infinite beauty, and accumulated at the bottom of the deeper seas, forming a deposit analogous to our chalk. In the revolution of ages the bottom of the sea becomes dry land, the calcareous débris of animal and vegetable life is more or less compacted together by pressure and by the infiltration of acid-laden rain-water, and appears as limestone of various hardness and constitution. Then the destruction begins again, and caves, pot-holes, and ravines are again carved out of the solid rock.
The air in caves is generally of the same temperature as the mean annual temperature of the district in which they occur, and therefore cold in summer and warm in winter. This would be a sufficient reason why they should be chosen by uncivilized peoples as habitations.
The very remarkable glacières, or caves containing ice instead of water, in the Jura, Pyrenees, in Teneriffe, Iceland, and other districts of high altitude and low temperature, in which the temperature even in summer does not rise much above freezing-point, may be explained by the theory advanced independently by De Luc and the Rev. G. F. Browne. “The heavy cold air of winter,” writes the latter, “sinks down into the glacières, and the lighter, warm air of summer cannot on ordinary principles dislodge it, so that heat is very slowly spread in the caves; and even when some amount of heat does reach the ice, the latter melts but slowly, since a kilogramme of ice absorbs 79° C. of heat in melting; and thus when ice is once formed, it becomes a material guarantee for the permanence of cold in the cave. For this explanation to hold good it is necessary that the level at which the ice is found should be below the level of the entrance to the cave; otherwise the mere weight of the cold air would cause it to leave its prison as soon as the spring warmth arrived.” It is also necessary that the cave should be protected from direct radiation and from the action of wind. These conditions are satisfied by all the glacières explored by Mr. Browne.39 The apparent anomaly that one only out of a group of caves exposed to the same temperatures should be a glacière, may be explained by the fact that these conditions are found in combination but rarely, and if one were absent there would be no accumulation of perpetual ice. It is very probable that the store of cold laid up in these caves, as in an ice-house, has been ultimately derived from the great refrigeration of climate in Europe in the Glacial Period.
In this chapter we have examined the physical history of caves, their formation, and their relation to pot-holes, cirques, and ravines; and we have seen that they are not the result of subterranean disturbance, but of the mechanical action of rain-water and the chemical action of carbonic acid, both operating from above. We have seen that cave-hunting is not merely an adventurous amusement, but also a quest that brings us into a great laboratory, so to speak, in which we can see the natural agents at work that have carved out the valleys and gorges, and shaped the hills wherever the calcareous rocks are to be found.
The rest of this treatise will be devoted to the evidence which they offer as to the former inhabitants, both men and animals, of Europe.