KARST LANDSCAPES - Fundamentals of Geomorphology - page 211

which were formed by tectonic, diagenetic (miner-
alization), or artesian processes, being flooded and
enlarged by karst groundwater, so forming a cave
conduit.

2 Joints , fractures , and faults . Joint networks greatly
facilitate the circulation of water in karst. Large
joints begin as angular, irregular cavities that become
rounded by solution. Cave formation is promoted
when the joint spacing is 100-300 m, which allows
flowing water to become concentrated. Some pas-
sages in most caves follow the joint network, and
in extreme cases the passages follow the joint net-
work fairly rigidly to produce a maze cave , such
as Wind Cave, South Dakota, USA. Larger geo-
logical structures, and specifically faults, affect the
complex pattern of caves in length and depth.
Many of the world's deepest known shafts, such as
451-m-deep Epos in Greece, are located in fault
zones. Individual cave chambers may be directed by
faults, an example being Gaping Ghyll in Yorkshire,
England. Lubang Nasib Bagsu (Good Luck Cave),
Mulu, Sarawak is at 12 million cubic metres the
world's largest known underground chamber and
owes its existence to a combination of folding and
faulting.
3 Cave breakdown and evaporite weathering . Lime-
stone is a strong rock but brittle and fractures
easily. Cave wall and ceiling collapse are impor-
tant in shaping passages and chambers. Collapse
is common near the cave entrance, where stress
caused by unloading (p. 50) produces a denser
joint network. Rock weathering by gypsum and
halite crystallization ( exsudation ) may alter pas-
sage form. Water rich in soluble material seeping
through the rocks evaporates upon reaching the
cave wall. The expansion of crystals in the bed-
ding planes or small fissures instigates sensational
spalling.

3 The hypergene view imagines hydrothermal waters

charged with carbon dioxide, hydrogen sulphide, or
other acids producing heavily mineralized cavities,
which are then overrun by cool karst waters to create
larger and more integrated cavities or networks.

All or any of these three processes may have operated in
any cave during its history. In all cases, it is usually the case
that, once an initial cave conduit is formed, it dominates
the network of passages and enlarges, becoming a primary
tube that may adopt a variety of shapes (from a simple
meandering tube to a highly angular or linear conduit)
depending on rock structure.

Cave form

Cavern systems can be very extensive. Mammoth Cave,
Kentucky, USA, comprises over 800 km of subterranean
hollows and passages arranged on several levels, repre-
senting major limestone units with a vertical depth of
110 m. At 563,270 m, the cave system is the longest in
the world. The form of caverns - their plan and cross-
section - depends upon the purity of the limestone in
which they are formed and the nature of the network of
fissures dissecting the rock, as well as their hydrological
setting.
The shape of caves is directed by lithology, by the
pattern of joints, fractures, and faults, and by cave
breakdown and evaporite weathering:

1 Lithology . Caves often sit at changes of lithology,
with passages forming along or close to litholog-
ical junctions, for example the junctions between
pure and impure limestones, between limestones
and underlying shales, and between limestones and
igneous rocks. Passages may have a propensity to
form in a particular bed, which is then known as the
inception horizon (Lowe 1992). For instance, in
the Forest of Dean, England, caves start to form
in interbedded sandstones and unconformities in the
Carboniferous limestone.

Caves may also be classified in relation to the water table.
The three main types are phreatic, vadose, and water table
caves (Figure 8.18a, b, c). Vadose caves lie above the
water table, in the unsaturated vadose zone, water table
or epiphreatic or shallow phreatic caves lie at the water
table, and phreatic caves below the water table, where
the cavities and caverns are permanently filled with water.
Subtypes are recognized according to the presence of cave
loops (Figure 8.18d, e, f ).