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So what is it which makes limestones so spectacularly cavernous? To understand the process in which caves are formed, we must begin by examining the origins, nature and structure of the limestone rock itself. Limestone is a sedimentary rock, that is, it began life as suspended particles in an ancient sea, gradually settling to the ocean floor millimetre by millimetre over millions of years. During this inconceivably long period, there were intervals when conditions changed enough to interrupt the steady downward rain of lime, allowing some other type of deposit to intervene briefly in the sequence of otherwise pure calcium carbonate. Aeons later, and now hardened to rock, these geological glitches have become ‘fossilized’ as bedding planes – horizons of weakness between the solid layers of limestone.

The Carboniferous Limestones of the British Isles were laid down somewhere in the tropical seas of the southern hemisphere. Pushed along on a northward-drifting chunk of continental crust, they have had a bumpy ride. Some, like those of the Yorksire Dales, have survived their 340 million year journey the right way up, though somewhat fragmented by massive vertical faults. Others have fared less well. The Mendip limestones lie like a wrecked car, buckled and perched at a steep angle, so that the bedding planes dip downhill at an average gradient of 50° or so. In all cases, the rough ride has produced vertical stress cracks, called joints, which link with faults and with the original bedding weaknesses of the limestone to form a boxwork of crevices reaching from the highest hilltop to beneath the deepest valley.

Limestone is a strong rock and so frequently forms upland regions. Solid limestone is impervious to water, but water is able to flow through the cracks within it. It is these cracks which are the key to understanding the origins of caves. Limestone caves form principally by means of a simple chemical reaction in which hydrogen ions from groundwaters, acidified with dissolved carbon dioxide, act on the relatively insoluble carbonate ions in the limestone to produce soluble bicarbonate 10ns which are then flushed away. The reaction renders the limestone 25 times more soluble than it would be in pure water and the result is holes.

Some of the carbon dioxide in groundwaters is collected by raindrops falling through the atmosphere, and some from the breakdown of organic material picked up as the rain then trickles down through the soil. Immediately beneath the soil, the weathered surface layers of rock are more fractured than those at a greater depth and the acidified, aggressive soil waters have their maximum impact here – so that at any one time, up to 15% of the volume in the top three metres of limestone may be occupied by air-, or water-filled spaces (Stearns, 1977 – figures for Central Tennessee, USA). These mesocaverns act like the guttering beneath the roof of a house – collecting soil water and quickly conveying it to natural drainpipes, often developed on the intersections of major joint fractures, or steeply inclined bedding planes.

The flow of water downwards into the limestone carries with it sediments which contain particulate organic material, dissolved organic acids and microorganisms (bacteria and fungi). Decomposition continues way below the soil in the cracks and crevices of the limestone. There, to paraphrase Hoover’s famous advertising slogan, groundwater micro-organisms ‘eat-as-they-seep-as-they-clean’, mopping up the organic impurities and excreting CO

– in effect arming their liquid medium with chemical teeth. In the larger conduits it may take up to 50 days and several kilometres of flow before the bacteria finish mopping up the water-borne food, and even longer and further before the chemical aggression of the cave water is finally spent. All this time, limestone is being steadily corroded, and the cracks along which the water travels widened, resulting in the slow opening of a complex drainage network reaching deep below the water table.

The initial pattern of flow within the flooded cracks is dictated by the structural geology of the limestone and the shape of the land surface. Between them, these two factors determine where water will escape from the rocks as a spring, and lay down the blueprint for the caves to come. If the rock strata lie horizontally, as they do in the Yorkshire Dales, water is forced to follow a rectangular course down vertical joints and along short sections of horizontal bedding, producing the characteristic stepped profile of a ‘pothole’ system. If the strata are inclined, as in the Mendip Hills, drainage will alternate between short joint-controlled shafts and longer bedding-plane slopes, producing caves with a steep profile, but few vertical sections.

At or below the water table, the course taken by percolation water is determined by the direction of the hydrological gradient between where the water goes into the permanently-flooded system of cracks known as the ‘phreas’ and where it comes out again as a spring. Within the phreas, water is free to follow along, down or up the 3-D maze of cracks to produce the smoothest possible overall flow. Where the rock beds lie horizontally the smoothest profile may be along just one bed, producing a horizontal cave which may run for several kilometers (Yorkshire has many such systems). Where the rocks dip steeply in a direction which does not coincide with the hydrologically-determined direction of flow, the groundwater may be forced into a series of vertical z-bends, down joints and up the bedding, to tack its way to the spring (a classic example is Wookey Hole in the Mendip Hills). Where the limestone beds are trapped in a syncline or U-bend beneath impermeable rocks, water may be forced to travel down to great depths following the configuration of the rock strata. Chinese geomorphologist Yuan Daoxian has recently reported the discovery of a substantial cave at a depth of 2900 m below the water table in the Sichuan Basin in China. When perforated by drilling, the hole gushed water. Caves at this depth are, however, extremely rare and probably have little or no biological significance.

As the profile of the cave takes shape, under the twin controls of hydrology and geology, the ‘best route’ is inevitably favoured with a greater rate of flow, which promotes more rapid corrosion of its boundary walls – which in turn results in a still greater flow capacity. In this way, the initially diffuse drainage within the limestone is gradually simplified with increasing time (and depth) into a pattern of coalescing collectors of increasing diameter. Over the aeons, the vagaries of geology and hydrology may conspire to favour one particular route in preference over all others, opening it out to form a major trunk conduit which drains the entire subterranean aquifer to its spring, or ‘resurgence’.

I have suggested so far that cracks in the limestone can be enlarged to form mesocavernous conduits by corrosive water trickling gently down towards the water table under the influence of gravity, or by slow creeping but aggressive groundwaters draining towards a resurgence. However, chemical solution is not the full story – once the cave becomes large enough to be an efficient drain, fast-moving waters can carry sharp-edged grit, adding physical claws to chemical teeth. Grains of quartz (a mineral much harder than calcite) provide a particularly effective scouring agent, and are easily collected by streams running over the Millstone Grit which butts onto much of our cavernous limestone. Where such a stream is captured by a well-developed Yorkshire joint, the outcome is a vertical pothole; on Mendip, an inclined swallet cave results.

Erosion of the river valleys which drain a limestone block may lower the water table within it, so that a conduit formed within the flooded phreas eventually becomes emptied of water and filled with air. One such system, GB Cave in the Mendip Hills, has received intensive study in recent years – in turn by Drs Tim Atkinson, Pete Smart and Hans Friederich of Bristol University. Their work has demonstrated the extent and importance of the system of mesocavernous conduits which overlies and feeds water and sediment into GB Cave. The conduits can be divided into three types. First there is the dense network of ‘subcutaneous’ cracks in the top three metres of rock beneath the soil. These have a large total volume and drain rapidly into the underlying cave, via mesocavernous collectors. They operate intermittently for up to two days following rain, but do not store a great deal of water. Two other types of inlets feed into the cave. They are small-diameter seeps, which stay full of water all the time and show little variation in flow discharge (flow being constrained by their small diameter); and vadose flows, which have higher and more variable discharges than do the seeps. Smart and Friedrich report that inlets are more frequent into shallower cave passages, and less frequent at depth, but do not give estimates of the porosity represented by these inlets, for which we have to consult Stearns on his Tennessee karst. Thus, at 10 m depth, the initially high porosity within the zone of subcutaneous flow (15%) may have fallen to about 1.5%, with a mainly downward flow via mesocavernous shafts and vadose cracks. At 30 m depth, porosity is down to 0.002%, and flow (at least in massively-bedded limestones) is increasingly confined within macrocavernous collector passages, fed by a converging network of smaller collectors of mesocavernous size. Thus, the main concentration of cave habitats (of mesocavernous dimensions) lies within the top three metres of bedrock.

Fig. 2.5 The main streamway in Poulnagollum Pot, Co. Clare – a classic vadose passage cut down through the limestone beds by an allogenic stream flowing off the eastern flank of Slieve Elva (compare with Fig. 2.1). (Chris Howes)

Phreatically-formed passages can be recognized by their characteristic circular or at least symmetrical cross-section, produced by equal corrosion of the walls, ceiling and floor (a classic example is Peak Cavern in Derbyshire). As soon as they develop an air surface, corrosion and erosion become channelled downwards and the cave stream begins to cut a trench into the floor, as is seen in the Ogof Ffynnon Ddu streamway in South Wales. This may eventually result in a canyon-shaped passage many metres deep.

As the cave matures, the enlargement of joints and bedding planes may weaken whole blocks of the ceiling and floor, resulting in their collapse. Where the cave contains an aggressive stream, such material may be removed as it falls, creating a chamber whose size is limited only by the structural strength of the overlying rock. Given enough time, the roof will eventually fail, opening the cave to the sky; if the collapsed blocks are not removed by water, the cave fills up with rubble.

The same chemical process which resulted in the removal by solution of limestone in cracks close to the surface, can go into reverse when acidified, lime-laden waters drip through cracks into an underground passage full of fresh air. As CO

in the hanging water droplet diffuses out into the air of the cave, the resulting transformation of bicarbonate to carbonate ions forces lime to precipitate from solution. The drip falls, a rim of lime remains on the cave ceiling, and hey presto, a few thousand years later the cave is festooned with sparkling stalactites. Given an aeon more, these may completely fill up and block the passage.

Caves may also become filled with insoluble sediments from outside. This is frequently the case in Britain, where sediments enter the cave by one of three main mechanisms: large masses of unsorted clay-and-rubble have slumped into caves as a half-frozen mush during the glacial advances of the Pleistocene; sands and gravels are washed in by cave streams and dumped as the waters slow down in more gently graded stretches of cave; and fine mud is often trickled downwards by percolation water until it completely fills passages right up to the roof.

The worst ravages of collapse and sedimentation may be reversed by a little-known bio-chemical phenomenon known as ‘digging’. First noted in Mendip caves, it appears to be caused by large sweaty men in overalls, armed with plastic buckets, and generally takes place at weekends in the pursuit of new explorations and discoveries underground. The chemical component of the process consists in a sparing application of ‘Dr Nobel’s Linctus’ to otherwise immovable blockages, with sometimes impressive results. We shall consider the conservation implications of digging and blasting in caves in a later chapter.

Types of cave habitat (#ulink_abf86299-82dc-55ec-8454-fb94917111a1)

In the previous section, we traced the process of cave formation in limestones by the gradual chemical enlargement of bedding cracks and joints, their subsequent drainage, further enlargement, collapse and infilling by sediment, breakdown and speleothems. In the course of this ‘life cycle’, limestone caves present a series of distinct habitats which are each exploited by a characteristic biota.

In recent years, similar biotas have been found in equivalent habitats in a wide range of other rocks (lava, gypsum, mudstone, shale, chert, breccia, tuff, rhyolite, diorite, quartzitic sandstone, etc) and sediments. This has led to a plethora of exotic technical terms in the literature – it seems that any biologist who finds a new habitat wants to give it an impressive, polysllabic label, an urge doubtless born of years of grappling with latin species names. My favourite examples include ‘parafluvial nappes’ (as worn by water-babies?), ‘hypotelminorheic biotope’ and ‘the petrimadicolous biocoenose’ (etymologically-inclined readers may enjoy trying to sort out these little beauties). In the section which follows, I have attempted to keep things simple by stressing the the similarities between habitats, rather than the differences between them.

Fig. 2.6 Cave habitats and structural features in a limestone upland.

1. Dolines or shakeholes 2. SUC or subcutaneous zone 3. Mesocavernous seeps and conduits 4. Air-filled macrocavern or dry cave passage 5. Vadose macrocavern or wet cave passage 6. Rimstone gour pools 7. Cave sediment 8. Breakdown blocks 9,10. Riffle and pool in cave system 11. Perched siphon or sump 12. Water table 13. Water-filled macrocavern or sump in the phreas below water table 14. Water-filled mesocaverns in the phreas 15. Talus, below a scar or limestone cliff.

Air-filled macrocaverns (dry caves)

The longest, biggest and, by any criteria, the best macrocaverns are those developed in limestone. The world’s longest cave is the Mammoth Cave system in Kentucky at over 540 kilometres, while the deepest is the Réseau Jean Bernard in the French Pyrenees at over 1600 m deep. Our longest cave, the Ease Gill system in Yorkshire, 70 km long, can manage only 15th place in the world ratings. The ages of caves vary considerably. Recent estimates suggest that certain limestone cave passages in North America could have formed 10 million years ago, while some tropical limestone caves are estimated to be only around 100,000 years old.

Substantial caves are not by any means confined to limestone. Other notable caves are formed by solution of marble, dolomite and non-carbonate rocks such as gypsum (e.g. Optimisticheskaja in the Ukraine, 183 km long); rock salt (in arid regions); and even quartzitic sandstone (e.g. Sima Aonda in Venezuela, 362 m deep) – while lava tubes are formed by the crusting-over and subsequent draining of molten lava flows (e.g. Kazumura Cave in Hawaii, over 20 km long). The latter include the youngest of all caves, formed on the slopes of Kilauea Volcano in Hawaii within the last five years. Other significant caves may be formed in a range of different rocks and within accumulations of large talus boulders (e.g. Lost Creek in Colorado, USA), by gravity sliding (gull caves), tectonic movements, wind-, or wave-blasting and other mechanisms. Most bizarre of all is Kitum Cave on Kenya’s Mount Elgon, which has been literally eaten for 200 m horizontally into a bed of volcanic ash by generations of salt-hungry elephants.

In caving parlance, a ‘dry cave’ is one which a human visitor can explore without getting wet. While such caves may provide an easy route into an underground world of great beauty, budding cave biologists may be greatly disappointed not to find every surface festooned with strange, eyeless arthropods, armed with sweeping antennae, stalking around on matchstick legs. As stated earlier in this chapter, dry passages in tropical caves may contain huge populations of bats or birds and a wealth of guano-associated cavernicoles, but in Britain and Ireland, cave-roosting bats and the species which depend on their presence have become increasingly scarce in recent years. Nevertheless a surprisingly large number of our macrocaverns do still harbour small populations of bats and a few associated ‘battelite’ species, which will be considered in Chapters 4 (#litres_trial_promo) and 5 (#litres_trial_promo).

Fig. 2.7 Mexican Free-tailed Bats, Tadarida brasiliensis, stream out of the entrance of Carlsbad Caverns at dusk. (Chris Howes)

The main biological interest of our dry caves is that they may contain the only accessible populations of cavernicoles primarily adapted to other, relatively inaccessible habitats. Thus, drip- or seepage-fed pools, gour pools and wet flowstone may harbour aquatic mesocavernicoles; and rock piles, flowstone pockets and ‘deep cave’ or ‘stagnant air’ environments may give us a rare glimpse of the terrestrial mesocavernicolous fauna.

Vadose macrocaverns (wet caves)

Cave streams may contain water derived from permanently-flowing surface streams, swallowed through an open sinkhole, or percolation water which has collected gradually from a large number of mesocavernous inputs. Those fed entirely by percolation water or seasonal run-off tend to hold more biological interest, as the populations they contain (being genetically isolated from surface habitats) may have evolved specialized features.

Surface streams arrive in the cave complete with a biota of their own, most of which will survive quite well in darkness for some considerable time, though few species may succeed in reproducing. Apart from the lack of growing green plants, and a tendency to experience less seasonal temperature change, such streams do not differ significantly as a habitat from streams above ground. The terrestrial habitat in stream passages is usually draughty and moist. Its fauna may include the adult stages of aquatic insect larvae swept in with the sinking stream, and various predators, such as spiders, which feed on them, plus a range of detritivorous cavernicoles.

Where stream passages enter the phreas (cavers’ ‘sumps’, marking the end of exploration to all but divers) it is not unusual to find accumulations of organic detritus deposited by floods. Given a suitable microclimate, such places may be well populated by specialized cavernicoles, or ‘troglobites’. Indeed, in many food-poor cave systems (such as those in mountainous areas of the Pyrenees and Cantabrians) this may be the only habitat in which cavernicoles occur in any numbers.

Water-filled macrocaverns (phreatic caves)

The chemistry, food supply and fauna of phreatic waters is determined to a large extent by their mode of entry into the cave. Underground waters are classed as autogenic (originating as through-soil percolation via diffuse seeps and subcutaneous flow) or allogenic (sinking streams). The former tend to be lower in organic content, less chemically aggressive by the time it reaches the phreas and may carry a lower sediment load. Phreatic waters may have a very different chemistry to the vadose waters which feed them, because they are unable to de-gas the carbon dioxide produced by microbial oxidation of organic materials, or to replenish oxygen used up in such decomposition.

The rate of flow in large-diameter phreatic tubes is generally greater than in phreatic mesocaverns, but is still often sluggish enough to accommodate small, slow-swimming cavernicoles (such as Niphargus fontanus), which avoid fast-moving vadose streamways.

In recent years cave divers have penetrated great distances into freshwater phreatic macrocaverns, and to considerable depths, but to date no detailed studies have been made of the biota of this remote and fascinating environment.

Water-filled mesocaverns

Most of our knowledge about the structure of mesocaverns comes from looking at exposed joints or bedding planes in limestone quarries, or occasionally caves, and from the work of karst hydrologists. We know that the very earliest stages of cave development occur under phreatic conditions, eventually creating inter-connected systems of conduits which may be in any plane, from horizontal to vertical. Phreatic mesocaverns may take the form of wiggly networks of small-diameter tubes (anastomoses), or thin, but laterally-extensive cracks, or narrow shafts – and are probably as common in other cavernous rocks, such as chalk or gypsum, as in limestone. Evidence that such spaces are inhabited comes from the animals found in well-water over the centuries. It is ironic that the earliest records of our cave fauna should be from a habitat about which little more is known today than a century-and-a-half ago, when Philip Henry Gosse wrote:

“recently, investigations in various parts of the world have revealed the curious circumstance of somewhat extensive series of animals inhabiting gloomy caves and deep wells, and perfectly deprived even of the vestiges of eyes … even in this country we possess at least four species of minute shrimps [all of which] have been obtained from pumps and wells in the southern counties of England, at a depth of thirty or forty feet from the surface of the earth.”

There is no way at present of collecting information directly about how aquatic cavernicoles use mesocavernous bedrock cracks, but it seems likely that great local variation exists within this habitat in terms of oxygen concentrations, pH and food supply. Such factors are likely to influence the distributions of the fauna, and will be discussed in Chapter 4 (#litres_trial_promo). Fortunately there are other, more accessible types of water-filled mesocavernous habitat which are easier to study. They include the deeper interstices of stream-bed cobbles (phreatic nappes), and a peculiar sub-soil phreas of mesocavernous dimensions which occurs on the surface of impervious silt or clay deposits in mountainous areas of Europe (the hypotelminorheic medium). In both these habitats, the food supply comprises dissolved or finely particulate organic material, and the waters tend to be rather low in oxygen; and both contain faunas very similar to those of limestone mesocaverns.

Amphibious mesocaverns

As no detailed investigation of the biology of air-filled mesocavernous habitats has yet been attempted, we are forced to infer what we can about the conditions within them from studies of limestone caves and other similar habitats.

As soon as mesocaverns develop an airspace, they become available for colonization by terrestrial cavernicoles. However, cracks and anastomoses are extremely flood-prone, often filling up with water each time there is heavy rainfall at the surface. Vertical cracks probably flush more violently, but remain water-filled for shorter periods than horizontal cracks, and this may result in some differences in their faunas. Less immersion-tolerant organisms may tend to inhabit the wider, better-drained vertical cracks and humid terrestrial cave habitats, while the more aquatic organisms may prefer horizontal cracks or cave pools. It is likely that particulate organic material accumulates at specific points within the cracks (perhaps at the upstream ends of permanently flooded sections), so that some patches of habitat will be better supplied with food than others. Some areas may be too anoxic to support any life other than anaerobic micro-organisms, while some patches may harbour relatively large concentrations of detritivorous invertebrates. As previously discussed, there may be an almost complete overlap in distribution between the ‘terrestrial’ and ‘aquatic’ components of the fauna of such habitats.

The French biospeleologists Juberthie, Delay and Bouillon consider that mesocavernous spaces in fractured rock immediately below the soil constitute a habitat which is separate from caves, which they have termed the ‘Superficial Underground Compartment’ (SUC). Their claim rests on differences between the fauna found here and that found in deep caves. They consider the primary cause of such differences to be the greater temperature variation experienced in the ‘SUC’ compared with the ‘Deep Underground Compartment’ (DUC) represented by deep fissures and caves. While this may be so in SUC habitats beneath shallow soils of regions which experience a strongly seasonal temperate climate, I would doubt that the microclimate in deeply-buried SUC habitats or those of tropical karst differs a great deal from that of the ‘DUC’ – and there is evidence that cave faunas migrate up into the SUC periodically in order to exploit the resources they contain. For the purposes of this classification, I propose therefore to treat the ‘SUC’ as part and parcel of other intermittently-flooding mesocavernous spaces, whether they be immediately below the soil, within cave passages, or connecting one with the other.

There would seem to be little doubt that the SUC within calcareous rocks is by far the most extensive and important of all cave habitats in terms of the numbers and diversity of its biota. Since their ‘discovery’ of the ‘SUC’ (an environment previously well-known to karst hydrologists as the ‘subcutaneous zone’), Juberthie and Delay have gone on to show that this habitat and its biota not only occurs in limestone and other cavernous rocks, but also in ‘non-cavernous’ shales, granites, schist, gneiss, sandstones, etc. My first reaction on reading the paper announcing this discovery was to attack the bottom end of my garden with a pick and shovel. There, to my delight and amazement, I found tiny-eyed cave spiders (Porrhomma egeria) frolicking among the fractured chunks of Pennant Sandstone just one metre beneath the wreckage of the flower bed. As far as I know there has been no systematic investigation to date of the fauna of ‘SUC’ habitats in Britain and Ireland – an extraordinary gap in our knowledge which surely must be remedied before long.

A better-known mesocavernous habitat is contained in talus, or scree, whose surface can frequently become covered with vegetation and soil, turning it into a fair imitation of Juberthie and Delay’s SUC. When not sealed by soil, the upper levels of talus are unsuitable as a habitat for cavernicoles, being too cold in winter, too hot in summer and too dry for much of the time. However, if the scree is deep enough, the lower levels must surely provide exactly the conditions favoured by cavernicoles, though I know of no work on this deep-talus habitat in Britain.

I know of only two accessible ‘DUC’ mesocavernous habitats within caves. One is in the spaces within rock piles (underground talus), the other is in speleothem pockets. Rock piles may, or may not provide a suitable habitat for mesocavernicoles. If the pile is in an old, dry ‘fossil’ passage, as most rock piles tend to be, it is unlikely to contain enough food to support life (unless the cave contains bats, or other vertebrates, in which case the rocks may be over-run by guano-beasts). On the other hand, if the pile is sufficiently extensive, and is traversed by percolation water carrying organic material, it is likely to harbour a rich fauna of mesocavernicoles – although the depth within it at which a searching biologist can expect to ‘strike bugs’ will increase with the increasing dryness or breeziness of the surrounding cave atmosphere, precisely as would be the case with above-ground talus. Juberthie (1983) gives an interesting example of a schist-boulder pile in the great Salle de la Verna chamber in France’s Pierre Saint Martin cave. It is inhabited by a typical SUC fauna of Aphaenops beetles which appear to be quite oblivious of the fact that their schist scree habitat lies the best part of 1000 m underground.

Speleothem pockets are essentially just spaces of mesocavernous dimensions like all the others described in this section, but where they occur in the ‘deep cave’ or ‘stagnant air’ zone of caves (see the microclimate (#ulink_2fd43709-4d59-5025-a277-de571652d52b) section, later in this chapter), they will often prove to be the very best places to look for mesocavernicoles. Speleothems occur where percolation water, rich in dissolved lime, intersects a cave passage. Where such deposits are laid down over mud, pockets often form between the two; and if deposition is still in progress, trickling water maintains just the microclimate conditions favoured by cavernicoles, while also supplying a source of food. In short, they perfectly reflect conditions in the mesocaverns. The late A. Vandel, in his famous book Biospeologie la biologie des animaux cavernicoles (published in 1964), described such a habitat in the Grotte de Sainte-Catherine, at Balaguères, in the Ariège region of France:

“One side of the chamber is formed by a stalagmitic wall which is covered by a thin layer of water which flows from an opening in the roof … The constant flow brings in organic material from the exterior which nourishes the Collembola and nematoceran Diptera on which Aphaenops (a blind cave beetle) feeds … The stalagmitic covering is separated from the wall by a space of a few millimetres which contains the products of dissolution of the rock: clotted red and black clay, and black magnesium deposits.