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 CO2 – 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 CO2 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
