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304 DOLINES DOLINES A doline is a natural enclosed depression found in karst landscapes. Dolines are also sometimes known as sinkholes, particularly by engineers and especially in North America. They are usually subcircular in plan and tens to hundreds of metres in diameter, though their width can range from a few metres to about a kilometre. From the lowest point on their rim, their depths are typically in the range of a few metres to tens of metres, although some can be more than a hundred metres deep and occasionally even 500 m. Their sides range from gently sloping to vertical, and their overall form can range from saucer-shaped to conical or even cylindrical. Their lowest point is often near their centre, but can be close to their rim. Dolines are especially common in terrains underlain by carbonate rocks, and are widespread on evaporite rocks. Some are also found in siliceous rocks such as quartzite. Dolines have long been considered a diagnostic landform of karst, but this is only partly true. Where there are dolines there is certainly karst, but karst can also be developed subsurface in the hydrogeological network even when no dolines are found on the surface. Doline derives from “dolina”, a word of Slav origin meaning valley, possibly because these were the most common hollows in the landscape of the Dinaric karst, where there are few fluvial valleys. “Slepe dolina” means blind valley, where a stream disappears underground and so its valley ends in a steep face. The word doline entered international scientific literature largely through the writings of Cviji[c3] (1893), despite the more accurate local term vrta[c2]a being current in the “classical” Karst of Slovenia. However, the usage of the word doline is so embedded in karst literature now that it would be fruitless to try to change it. The term sinkhole is sometimes used to refer both to dolines (especially in North America and in the engineering literature) and to depressions where streams sink underground, which in Europe are described by separate terms (including ponor, swallow hole, and stream-sink). Thus the terms doline and sinkhole are not strictly synonymous. Hence, to avoid the ambiguity that sometimes arises in general usage, further qualification is required, such as solution sinkhole or collapse sinkhole. Indeed, the international terminology that is used to refer to dolines that are formed in different ways can also be very confusing. Table 1 lists the terms employed by different authors, the range of terms partly reflecting the extent to which genetic types are subdivided, and Figure 1 illustrates six main doline types. It is widely recognized that enclosed depressions in karst can be formed by four main mechanisms: dissolution, collapse, suffosion, and regional subsidence (Table 2). However, in practice the complexity of natural processes often results in more than one mechanism being involved, in which case the doline is polygenetic in origin. A typical case is a depression formed initially by dissolution that later in its development is subject to collapse of its floor into an underlying cave, following a combination of downwards dissolution and upwards stoping of the cave roof. In such a case, the gentler upper slopes of the doline were formed by dissolution and the steeper lower slopes by collapse. Table 2 describes the different processes responsible for doline formation, the various styles of doline landforms produced, and the names used in English to refer to them. Beck (1984), Waltham (1989), White (1988), and Ford and Williams (1989) provide numerous examples of doline form and evolution. Solution Dolines The bowl-shaped form of a typical doline (Figure 2) indicates that more material has been removed from its centre than from around its margins. Where the principal process responsible for this is dissolution of the bedrock, it follows that there is a mechanism that focuses chemical attack. The amount of limestone that can be removed in solution depends upon two variables: first, the concentration of the solute and, second, the volume of the solvent (in this case the amount of water draining through the doline). Variations in either or both of these quantities could be responsible for the focusing of dissolution near the centre of the depression, but if local variation in solute concentration alone were sufficient to explain the occurrence of solution dolines, then they would be found on every type of limestone in a given climatic zone. This is not the case, as illustrated by comparison of landscapes formed on Devonian, Carboniferous, Jurassic, and Cretaceous limestones in England, where dolines are most frequently found on Carboniferous limestones and practically absent on Cretaceous and Jurassic limestones. It follows, therefore, that local spatial variations in water flow must be responsible for focusing corrosional attack. The development of dolines of all kinds depends on the ability of water to sink into and flow through karst rocks to outlet Dolines: Table 1. Doline / sinkhole English language nomenclature as used by various authors (modified from Waltham & Fookes, 2002). Dolineforming Processes Ford & Williams (1989) Dissolution Collapse solution collapse Caprock collapse Dropout Suffosion Burial subsidence White (1988) solution collapse solution collapse – subjacent collapse subsidence cover collapse suffosion cover subsidence – Jennings (1985) – – Bögli (1980) solution collapse (fast) or subsidence (slow) alluvial – Sweeting (1972) Culshaw & Waltham (1987) solution collapse solution collapse solution subsidence alluvial – – 5603$$000D 04-01-03 06:57:33 subsidence – Beck & Sinclair (1986) Other Terms in Use solution collapse interstratal collapse cover collapse cover subsidence – ravelled, shakehole filled, paleo- DOLINES 305 Dolines: Figure 1. Six main types of dolines (after Waltham & Fookes, 2003). Note that dropout dolines and suffosion dolines are two types of subsidence doline. springs. The initiation of karst drainage is explained in Ford and Williams (1989) but, essentially, the exposure of limestones by erosion provides an input boundary for infiltration of water and a valley incised into the limestone provides an output boundary. The hydraulic gradient between the two sets up a groundwater circulation and dissolved limestone is discharged at springs. Streams and seepages flowing from non-limestone cover rocks (allogenic runoff) into exposed inliers of underlying limestone sink into the limestone. The recharge is centripetal, converging on the inlier, and so focuses solutional attack with the result that closed depressions are formed at such sites. Consequently, such depressions can be considered to be point recharge depressions. The floors of these dolines are usually in limestone, but their sides may be developed in clastic (non-carbonate rocks such as shales or sandstones) cover beds. These landforms are genetically transitional to stream-sinks. When all cover beds have been removed, the exposed limestone is subject to diffuse recharge and solutional attack from direct precipitation (autogenic recharge). Rainwater is acidified in the atmosphere and infiltrating water is further acidified in the soil. On percolating downwards this water accomplishes most of its dissolutional work within 10 m of the surface. The highly corroded zone near the surface is termed the epikarst or subcutaneous zone. Within this zone fissures in the limestone are found to be especially enlarged by corrosion near the surface but taper with depth. Consequently, infiltration into the karst is rapid, but vertical water flow encounters increasing resistance with depth as fissures become narrower and less frequent. This produces a bottleneck effect after particularly heavy rain, resulting in temporary storage of percolation water in a perched epikarstic aquifer. Joints, faults, and bedding planes vary spatially within the rock because of tectonic history and variations in lithology, consequently the frequency and interconnectedness of fissures available to transmit flow also varies. Some fissures are more favourable for percolation than others, for example where several joints intersect, and as a result these develop as principal drainage paths. Water in the epikarstic aquifer flows towards them and as a result they are subjected to still more dissolution by a positive feedback mechanism and so vertical permeability is enhanced. The piezometric surface (water table) of the epikarstic aquifer draws down over the preferred leakage path similar to the cone of depression in the water table over a pumped well; streamlines adjust and resulting flow is centripetal and convergent on the drainage zone. By this means solvent flow is focused and, as the surface lowers, the more intensely corroded zones begin to obtain topographic expression as solution dolines. The diameters of neighbouring solution dolines are determined by the radii of intersecting draw-down cones (Figure 2). Small dolines develop in areas with high fissure frequency, typically in thinly bedded and closely jointed limestones, whereas particularly large dolines develop in rocks that are massive and have widely spaced joints. Dolines formed by the focusing of dissolution either by recharge or by draw-down can occur in any climatic zone where water exists in a liquid state, provided there is unimpeded underground flow from recharge to discharge zones. Particularly large solution depressions often occur in the humid tropics where corrosion processes were uninterrupted by Pleistocene glaciations. In these places the term cockpit is sometimes applied to them after a particular style of landscape in Jamaica, where depressions are incised between intervening conical hills (see Cockpit Country Cone Karst, Jamaica). Although small solution dolines have formed in 15 000 years or so in some mid to high latitude areas that were glaciated in the late Pleistocene, several tens to hundreds of thousands of years are required to develop large solution dolines in limestone. Once formed they may persist in the landscape for several million years provided there is sufficient thickness of limestone for their continued incision. Individual dolines may merge to form compound closed depressions (known as uvalas) and large dolines may subdivide internally into smaller second generation basins. Where all the available space is occupied by depressions, rather like an egg box, the landscape is termed polygonal karst, because the topographic divides of the adjoining solution depressions have a polygonal pattern when viewed in plan (Williams, 1971). However, not all dolines within polygonal karst are necessarily of dissolutional origin, because a small proportion of collapse 5603$$000D 04-01-03 06:57:33 306 DOLINES Dolines: Table 2. Types of doline, the processes that form them, and terms used in English to describe them. Process 1. Dissolution: Chemical dissolution (corrosion) of carbonate bedrock or physical solution of evaporite rocks. 2. Collapse: This can occur in unconsolidated coverbeds or in compact caprock, and may progress rapidly or slowly. (i) Sudden or progressive collapse of the roof of a cave into the underlying cavern. Collapse is entirely within karst rocks and may be propagated upwards from tens to hundreds of metres beneath the surface. (ii) Sudden or progressive collapse of non-karst caprock into an underlying cave. A ceiling collapse within a cave located in interstratified karst rocks stopes upwards into, and through, overlying consolidated caprock causing the surface to collapse. The upwards stoping may occur over tens to hundreds of metres. (iii) Sudden failure in mechanically weak unconsolidated sediments overlying a subsurface cavity, giving rise to a depression at the surface. Sediments are evacuated downwards through solution pipes in the bedrock creating a void at the bedrock-sediment contact that enlarges by upwards propagation through the clastic coverbeds. The ceiling dome above the void eventually fails. 3. Suffosion and subsidence: Gradual subsidence of superficial coverbeds (as opposed to sudden collapse) giving rise to a depression at the surface). Formative processes involve the evacuation of superficial unconsolidated sediments through underlying solution pipes. Because the nature of cover deposits can vary considerably from uniform clays, silts, and sands through mixed alluvial facies to heterogeneous glacial deposits, the mechanisms involved in their evacuation also vary. Suffosion is one of the processes and involves the gradual physical and chemical downwashing of fines through underlying solution pipes in the karstified bedrock. Plastic flow of clays and silts can also occur with cover sediments sometimes being extruded as a wet slurry into underlying caves. Overlying sediments slowly settle and deform in response to gradual undermining. Coarse materials such as boulders and scree move downwards by gravity as underlying sediments are removed. 4. Regional subsidence: This process involves gentle, progressive settling of the ground surface over large areas as a consequence of gradual dissolution of deep, underlying interstratal evaporite beds by groundwaters. Karst Landforms Produced Terms Used to Describe Them Bowl-, saucer-, or funnel-shaped enclosed depressions in bedrock, usually with a soil cover. Depressions usually less than 1 km in diameter and often much smaller. Solution doline / sinkhole Cockpit (in a humid tropical context) Cylindrical to steep-sided enclosed depressions in karst rocks with debris-filled floor, sometimes with sheer and even overhanging rock walls tens to hundreds of metres high. Some open near their base into the underlying cave; some contain lakes. Up to a few hundred metres in diameter, but often much less. Cylindrical to steep-sided enclosed depressions in caprock with debris-filled floor, sometimes but not always revealing underlying karst rock. Up to a few hundred metres in diameter, but often much less. Collapse doline Cave-collapse sinkhole Cenote (only when they contain a lake) Steep-sided enclosed depressions (sometimes cylindrical when freshly formed) in unconsolidated cover sediments with debrisfilled floor, usually a few metres to tens of metres in diameter. Such depressions are often found in sediment fill near the bottom of a solution doline. Dropout doline Collapse doline Cover-collapse sinkhole Dimpled surface of enclosed depressions in coverbeds, sometimes exposing windows of underlying bedrock beneath. Such depressions are often found in superficial sediments such as glacial drift, alluvium, loess, and sand. They are usually only a few metres in diameter, but larger forms can be produced if the removal of coverbeds exhumes buried dolines in bedrock. Suffosion doline, especially in finergrained coverbeds. Cover-subsidence sinkhole is used in the United States to describe depressions formed by the gradual settling of unconsolidated clastic coverbeds. Sometimes the sediments have completely buried a pre-existing doline, the form of which is being exhumed as the subsidence occurs. Shakehole is a term used in England to describe depressions in bouldery deposits such as glacial drift overlying limestone. Strike-aligned depressions are formed when beds are dipping, but otherwise gently sloping depressions are distributed across an undulating surface. Depressions are often of several square kilometres in area and so are very much larger than is generally understood by the term doline. Nevertheless, some smaller depressions are formed in this way. Subsidence depression Solution trough 5603$$$CHD 04-01-03 06:57:33 Caprock doline Subjacent karst collapse doline Interstratal collapse doline / sinkhole DOLINES 307 when the importance of dissolution compared to other geomorphic processes has waxed and waned (see also buried dolines below). Collapse Dolines Dolines: Figure 2. Solution dolines develop in the uppermost weathered zone of the karst rock, termed the epikarst or subcutaneous zone. Drainage within that zone is achieved by leakage of water down fissures such as joints and faults. Major fissures capture most of the flow and therefore are the foci of solvent attack on the bedrock. This results in more rock being removed in solution from these locations than elsewhere, and this gradually gains topographic expression as enclosed depressions as the overall surface is lowered by chemical denudation (from Williams, 1983). depressions is also likely to be included. The mesh sizes of different polygonal karsts can vary widely and is a direct reflection of the characteristic radii of individual solution dolines, in turn determined by the epikarstic processes described above. Dense fields of dolines constituting polygonal karsts are known from the mid latitudes (e.g. New Zealand and Tasmania) to the tropics, but none has been reported polewards of 50⬚ latitude, probably because of the destructive influence of periglacial and glacial processes on surface landforms during the Pleistocene. There are few examples of dolines that have survived severe glaciation, although large solution dolines at High Mark near Malham, England, clearly formed before the last glaciation and contain drift deposits in which many small suffosion dolines have developed. In contrast, there are numerous cases of dolines formed prior to the last glaciation and located beyond the ice limits that have been strongly subjected to processes other than dissolution. In many cases they have been partly filled by scree and others have been covered, or even completely buried, by loess. Post-glacial conditions have resulted in some of these materials being removed and in the re-expression and continued development of the underlying doline form. This is the case in the Sinkhole Plain of Kentucky, where many dolines were buried by fine clastic sediments. It follows, therefore, that solution dolines, like valleys in a fluvial landscape, can have long and complex histories over time periods when climates have changed and Whereas most authors agree with the designation solution doline, there is considerable variation in nomenclature concerning depressions formed mainly by mechanical processes (Table 1). This is largely because of the variety of materials and processes involved, and the tendency of some authors to group types and of others to subdivide types. Collapse refers to rapid downward movement of the ground, whereas subsidence refers to gradual movement sometimes without even ripping the surface. These processes can occur in karst bedrock (collapse dolines sensu stricto), in caprock that may stratigraphically overly it (caprock collapse dolines), and in veneers of unconsolidated sediments (dropout dolines). In all cases the collapse has to be preceded by dissolution of the karst rock to form a void into which material can fall. The movement may involve little water or may be promoted by lubrication by water and the kind of landforms produced depends upon which of the various materials and processes were involved. Where collapse dolines form in karst bedrock the void is commonly part of a cave system. Collapse may occur following undermining from below as the roof of a cavity stopes upwards, ultimately causing the surface above to collapse or following dissolution from above that weakens the span of a cave roof, causing it to collapse. For example, solutional attack by drainage water near the bottom of a solution doline may combine with upwards stoping of an underlying cave roof to weaken a span from above and below, thereby causing the doline floor to collapse into a cave. The natural process of ground surface lowering inevitably means that caves will eventually be unroofed and, as the final stage involves failure of their thinning roof, the depressions produced must be regarded as a sub-type of collapse doline. When limestones or evaporites are overlain by a caprock, a void may form by dissolution at the lithological junction between caprock and karst rock. Stoping may then work upwards through the cover rocks and lead to the formation of a caprock collapse doline, which may be entirely within rocks such as shales, sandstones, or even basalt (Figure 3). Such features propa- Dolines: Figure 3. Caprock collapse doline in basalt overlying limestone near Timahdite, south of Fes, Morocco. (Photo’ by John Gunn) 5603$$000D 04-01-03 06:57:33 308 DOLINES gated from interstratal dissolution were termed subjacent collapse dolines by Jennings (1985), and good examples are found in Namurian sandstones around the North Crop of the South Wales coalfield (Thomas, 1974). While many of the South Wales dolines have formed following dissolution at the limestone–sandstone unconformity, some result from collapse propagated upwards from deeper caves that developed entirely within the limestone (Bull, 1980). Stoping has propagated through more than 1000 m of cover rocks at sites in Canada and Russia. When recently developed, collapse and caprock collapse dolines are steep sided, even cylindrical in form, but over time their sides degrade and bottoms infill with debris, so that superficially they assume the bowl-shaped morphology of solution dolines for which they can be easily mistaken. Only excavation, drilling, or geophysical survey will reveal their true origin. Collapse dolines are on average smaller in diameter than solution dolines, although particularly large examples 700 m along their largest axis and up to 400 m deep are known in the Nakanai Mountains of New Britain (see Nakanai entry, with photo). In China, the name tiankeng (roughly meaning skyhole) is applied to some of these very large dolines, including Xiaozhai over Di Feng Dong (see Di Feng Dong, with photo) and those at Xingwen (see China entry). Progressive collapse has clearly been important in their genesis, but not all tiankengs are associated with appropriately massive river cave development, and there is debate over whether they are merely very large ordinary dolines or if they represent a different mechanism of doline formation. Sometimes a collapse extends from a cave below the modern water-table level, in which case the collapse doline will contain a lake. Spectacular cylindrical collapses that descend below the water table are known as cenotes after the type-site in the Yucatán Peninsula of Mexico, although similar features are found elsewhere, such as in southeast Australia. The deepest known case of a collapse doline containing a lake is the Crveno Jezero (Red Lake) in Croatia, which is 528 m deep from its lowest rim, the bottom of the collapse extending 281 m below the modern level of the nearby Adriatic Sea. The collapse diameter at the surface is about 350 m and at lake level is about 200 m. Recent diving has found an active subterranean river that crosses the doline near its floor. The whole feature is thought to have formed by progressive upwards collapse of the cave roof, much of the collapse debris having been transported away by the underground river. The capacity of a cave roof to resist collapse depends on the width of the roof span and on factors determining rock mass strength such as the thickness of beds, the composition, texture, and compressive strength of the rock, and the frequency and continuity of fissures. If stress exceeds the rock’s strength then it will fail. In the simplest case where bedding is horizontal, each bed can be considered to act as a beam, but if it is fractured then it has less strength and acts as a cantilever. Collapse occurs where a critical span is exceeded for a given thickness and strength. White and White (1969) illustrate this with examples from West Virginia, where 0.5 m beds of carbonate rock can support roofs up to 25 m wide where they are unbroken beams, but only about 10 m where they act as cantilevers. Beams of more thickly bedded carbonates can support spans of 35 m or more, though cantilevered beds as massive as 5 m thick will fail with a span of 30 m or less. Massive reef limestones support the largest spans, which exceed 50 m in the Big Room of Carlsbad Cavern, New Mexico. In the Mulu karst of Malaysia, Sarawak Chamber is more than 250 m wide and about 500 m long. Its ceiling is actively collapsing and working upwards, but with few bedding weaknesses it is developing an arched profile that is stable in compression. Its ultimate evolution into an enormous collapse doline will be due more to surface lowering than to roof stoping. Another process that increases the effective stress on rock arches and domes is removal of buoyant support by water-table lowering, which increases the effective weight on the span of the roof, resulting in its strength being exceeded and so in its failure and collapse. This occurs because in a fully saturated medium the buoyant force of water is 1 t mⳮ3, and if the water table is lowered by 30 m, the increase in the effective stress on the rocks is 30 t mⳮ3 (Hunt, 1984). A gradual lowering of the water table occurs naturally as karstification proceeds, because of the increase in subterranean void space within the rock, but it occurs more rapidly when valley incision occurs, because springs are lowered too, and with them the level of the saturated zone that feeds them. More rapid still is the lowering of the level of saturation caused by sea-level lowering, a process that occurred frequently in the Pleistocene because of repeated glacio-eustatic (vertical movement of sea level caused by glaciation and deglaciation) fluctuations, although this only affected karsts well connected to the coast such as in Florida, southeastern Australia, and Yucatán, where it probably was a significant influence in the development of cenotes. If unconsolidated coverbeds are drained by water-table lowering, then consolidation and compression occurs, leading to subsiding of the surface and collapse where clastic sediments span de-watered unsupported arches. This has been a common process in Florida where porous sandy formations overlie karstified limestones, but has been much exacerbated by groundwater pumping for water supplies, which has still further reduced buoyant support (Beck, 1984). This process and the resulting incidence of collapse attains dangerous hazardous proportions in karstified areas extensively de-watered by mining activities. These dolines in unconsolidated coverbeds have been referred to as cover collapse sinkholes (Table 1) by Beck and Sinclair (1986) and White (1988). They are genetically transitional to subsidence dolines. Subsidence (Suffosion/Dropout) Dolines When unconsolidated deposits such as alluvium, glacial moraine, loess, or sand mantle karstified rock, the sediments are sometimes evacuated downwards through corrosionally enlarged pipes in the underlying karst resulting in gradual or rapid subsidence of the surface. Hence, the term subsidence doline is sometimes used for any doline in unconsolidated deposits although the term is also used for depressions formed by regional subsidence (see below). The main process by which the sediment moves is known as suffosion and involves the gradual winnowing and downwashing of fines by a combination of physical and chemical processes. The topographic consequence of this activity depends on whether the material is cohesive or non-cohesive. In cohesive sediments evacuation of material may proceed for some time without any surface expression. However, a void is formed that enlarges and stopes upwards resulting in a sudden, and sometimes catastrophic, failure of the ground surface. The depression thus formed is called a dropout doline or cover collapse doline 5603$$000D 04-01-03 06:57:33 DOLINES 309 tion the evaporite beds are dissolved away and gradual subsidence of the surface results. Waltham (1989) refers to this process as salt subsidence and provides a discussion of the mechanisms involved and the effects produced. When the beds are dipping, large depressions are produced along the strike. These can be many square kilometres in area and so are too large to be designated dolines and are best considered solution troughs. However, there is a continuum of forms and more localized subsidence can produce doline-sized features that could be termed subsidence dolines, although collapse is also sometimes implicated in their formation. Many have been mapped in the vicinity of Ripon in north Yorkshire, England. Ford and Williams (1989) provide further details and examples. Mapping of Dolines Dolines: Figure 4. Dropout doline, Santang, Guizhov, China. (Photo by John Gum) (Figure 4). As the final process is one of collapse, some authors group these dolines with those formed by collapse in bedrock (Tables 1 and 2). Where the sediment is non-cohesive, the clayey fraction will tend to move as a slurry into underlying cavities, producing dejection fans and mudslides in cave passages, whereas the coarser fraction will tend to remain closer to the surface and line small bouldery depressions known as suffosion dolines that are usually only a few metres in diameter and depth. In Britain suffosion dolines formed in glacial boulder clay overlying limestone are widely referred to as shakeholes, although the term is sometimes applied to any doline. Similar features but in more uniform finer grained materials are referred to as cover subsidence sinkholes in the United States. Often a combination of processes is involved in the development of subsidence dolines including corrosion and collapse of the underlying bedrock, as well as suffosion, mudflow, and void collapse in the mantling materials. Soil piping and surface collapse can also occur in unconsolidated non-karstic sediments in badland country, leading to a style of topography termed suffosional pseudokarst by White (1988). Buried Dolines Changes in the environment may result in bedrock dolines, however formed, becoming filled with sediment. There may be no evidence of such dolines at the ground surface, their presence only being revealed by geophysical prospecting or geotechnical survey. Good examples have been found in the central lowlands of Ireland where the lowest fills are of Tertiary age. Some buried dolines contain sediment of economic importance such as karst bauxites. Where they are no longer hydrologically active they may be regarded as paleokarst but if karstification continues, or is reactivated, in the underlying bedrock then there is likely to be gradual removal of material leading to surface subsidence. Depressions Formed by Regional Subsidence Particularly soluble rocks such as evaporites are often interbedded with clastic rocks. During the course of groundwater circula- For various reasons we may wish to map dolines. This is particularly the case when karst collapses may be hazardous to human activity (Beck & Pearson, 1995). Hence, it is necessary to know how to recognize different genetic types of dolines and their boundaries. In the case of solution dolines, which depend for their development on hydrological and chemical processes in the epikarst, their perimeters are the topographic divides (watersheds), within which water drains centripetally towards the doline bottoms. In the case of collapse dolines, where the main forcing process is undermining of the surface by cavern collapse, the surface boundary is the outermost surface inflection or break of slope that marks the limit of zone of influence of the failure. This may not be static even in the short term, but can move outwards until stability is attained. The boundaries of suffosion dolines and subsidence depressions are also defined by topographic inflections and breaks of slope at the outer limits of influence of the subsurface processes that formed them. Bearing in mind the polygenetic nature of some dolines, it is possible to map nested features such as collapse depressions within the boundary of a larger solution depression. Doline Hydrology Solution dolines have a similar function in karst landscapes to the drainage basin in non-karstic lithologies, they channel water to an outlet at the lowest point in the doline. Lateral movement is by overland flow and throughflow if there is cover material over the bedrock and by subcutaneous flow. Vertical movement is by shaft flow, vadose flows, and vadose seeps, three points on a continuum of flow routes (Gunn, 1981). Suffosion dolines also form small drainage basins, but the hydrological function of collapse and caprock collapse dolines depends on their form. Where they are narrow and steep sided they may be virtually inert, particularly when they are formed in essentially flat landscapes. However, as they widen the sides usually become less steep and an internal drainage system may develop. PAUL WILLIAMS Works Cited Beck, B.F. (editor) 1984. Sinkholes: Their Geology, Engineering and Environmental Impact, Rotterdam: Balkema Beck, B.F. & Pearson, F.M. (editors) 1995. Karst Geohazards: Engineering and Environmental Problems in Karst Terrane, Rotterdam: Balkema Beck, B.F. & Sinclair, W.C. 1986. Sinkholes in Florida: An Introduction, Florida Sinkhole Research Institute Report 85–86–4 5603$$000D 04-01-03 06:57:33 310 DOLINES Bull, P. 1980. The antiquity of caves and dolines in the British Isles. Zeitschrift für Geomorphologie, Supplementband 36: 217–32 Cviji[c3], J. 1893. Das Karstphänomen [The karst phenomenon]. Geographische Abhandlung, 5(3): 218–329 (The section on dolines, pp.225–76, is translated into English in Karst Geomorphology, edited by M.M. Sweeting, Stroudsburg, Pennsylvania: Hutchinson-Ross, 1981) Ford, D.C. & Williams, P.W. 1989. Karst Geomorphology and Hydrology, London and Boston: Unwin Hyman Gunn, J. 1981. Hydrological processes in karst depressions. Zeitschrift für Geomorphologie, 25(3): 313–31 Hunt, R.E. 1984. Geotechnical Investigation Engineering Manual, New York: McGraw-Hill Jennings, J.N. 1985. Karst Geomorphology, Oxford: Blackwell Thomas, T.M. 1974. The South Wales interstratal karst. Transactions of the British Cave Research Association, 1: 131–52 Waltham, A.C. 1989. Ground Subsidence, Glasgow: Blackie and New York: Chapman and Hall Waltham, A.C. & Fookes, P.G. 2003. Engineering classification of karst ground conditions. Geotechnical Engineering White, W.B. 1988. Geomorphology and Hydrology of Karst Terrains, Oxford and New York: Oxford University Press White, E.L. & White, W.B. 1969. Processes of cavern breakdown. Bulletin of the National Speleological Society of America, 31(4): 83– 96 Williams, P.W. 1971. Illustrating morphometric analysis of karst with examples from New Guinea. Zeitschrift für Geomorphologie, 15: 40–61 Williams, P.W. 1983. The role of the subcutaneous zone in karst hydrology. Journal of Hydrology (Netherlands), 61: 45–67 See also Cone Karst; Karst Evolution DRAENEN, OGOF DRAENEN, WALES Located in 1994, Ogof Draenen is Britain’s most significant caving discovery in recent years. It is already one of the longest caves in Britain (68 km) and contains the longest continuous streamway (⬎ 2.5 km) and some of the country’s largest passages. It also has abundant gypsum flowers, rare gypsum needles and anthodites, plus significant bat-guano deposits. But it is its geomorphic interest that sets it apart and makes it worthy of recognition. Above and up dip from the streamway lie several tiers of abandoned cave passage, most of which are unrelated to modern hydrology. These preserve evidence that the cave system has behaved like a hydrological see-saw, with the flow switching from south to north and then south again in response to landscape evolution and valley incision. Ogof Draenen lies under the interfluve between the Usk and Afon Lwyd valleys, on the northeastern margin of the South Wales coalfield (Figure 1). The entrance lies close to the hamlet of Pwll Ddu, six kilometres southwest of Abergavenny. First investigated by the Cwmbran Caving Club, the entrance to Ogof Draenen was a small, choked but draughting phreatic tube, almost buried under an old coal tip. The main digging breakthrough occurred on 6 October 1994; by 28 November, over 15 km of passage had been explored and surveyed (Kendall, 1994). Over the next few years, the rate of exploration averaged a remarkable 2 km per month, with many clubs and individuals contributing to numerous major discoveries. A comprehensive detailed survey, initiated by the Chelsea Spelaeological Society, had mapped over 68 km of passage by April 2002. The first geomorphological study of the cave was published by Simms et al. (1996) and expanded on in Waltham et al. (1997). Ogof Draenen is developed entirely within the Carboniferous Dinantian Limestone, although some of the larger boulder chokes stope up into the overlying Namurian Millstone Grit. The limestone succession was deposited on a carbonate ramp close to the contemporary shoreline. This, coupled with preNamurian erosion and overstep by the overlying Millstone Grit, has created a wedge of limestone less than 25 m thick along the eastern margin of the escarpment, thickening westwards to c.80 m in the Pwll Ddu area. The limestone crops out in a narrow band around the escarpment overlooking the Usk Valley and as a re-entrant up the Afon Lwyd Valley. Much of the cave is overlain by less-permeable Millstone Grit, and, in some places, by the Coal Measures, which protect the system from erosion. The geological structure of the area is relatively simple. The rocks dip to the southwest at between 10⬚ and 28⬚, and are cut by several minor faults, which can be identified underground by lines of boulder chokes. The dominant joint set which strongly influences cave development runs at 150–330⬚, slightly oblique to the strike and mirroring the main fault orientations. Some joints have been mineralized with barite and copper minerals. Ogof Draenen has had a complicated multiphase origin. Evidence of at least four phases of cave development, associated with major reversals in flow direction and resurgence location, can be identified (Figure 2). Essentially, the system can be thought of as four separate vertically stacked cave systems, linked by more recent vadose inlets and passage intersections. Some passages are multiphase and are thus usually much larger than single-phase passages. Major sediment influxes have occurred throughout much of the cave’s development, modifying and controlling passage genesis and evolution and creating some fine paragenetic passages. The major trunk conduits trend northwest–southeast along the dominant joint set. Because this coincides with the maximum hydraulic gradient, long unidirectional quasi-horizontal passage segments develop (Figure 3). This permits passage segments to rise or fall stratigraphically while remaining at a constant elevation. Draining into these are downdip vadose tributaries. In places maze networks occur, usually in response to a change in passage orientation. The earliest phase of cave development is represented by a major conduit system which extends southeastwards from a major northern sink at an elevation of at least 390 m, to a resurgence in the Usk Valley at c.370 m. Once this system was established, a series of down-dip captures began in response to incision in the Afon Lwyd Valley down dip to the west. This shift increased the down-dip component of underground drainage, causing a gradual migration in the conduit orientation to the southsoutheast. These can be identified as series of maze networks which diverge off south from the initial conduit. The second phase of development occurred in response to rapid incision of the Clydach Gorge, effectively reversing the hydraulic gradient and causing a fundamental 180⬚ re-orientation of the 5603$$000D 04-01-03 06:57:33