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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
–
–
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subsidence
–
Beck &
Sinclair
(1986)
Other
Terms in
Use
solution
collapse
interstratal
collapse
cover collapse
cover subsidence
–
ravelled,
shakehole
filled, paleo-
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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
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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
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Caprock doline
Subjacent karst collapse doline
Interstratal collapse doline / sinkhole
DOLINES
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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)
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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
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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
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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
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