Page 228 - Fundamentals of Geomorphology

KARST LANDSCAPES 211

which were formed by tectonic, diagenetic (miner- 2 Joints, fractures, and faults. Joint networks greatly
alization), or artesian processes, being flooded and facilitate the circulation of water in karst. Large
enlarged by karst groundwater, so forming a cave joints begin as angular, irregular cavities that become
conduit. rounded by solution. Cave formation is promoted
3 The hypergene view imagines hydrothermal waters when the joint spacing is 100–300 m, which allows
charged with carbon dioxide, hydrogen sulphide, or flowing water to become concentrated. Some pas-
other acids producing heavily mineralized cavities, sages in most caves follow the joint network, and
which are then overrun by cool karst waters to create in extreme cases the passages follow the joint net-
larger and more integrated cavities or networks. work fairly rigidly to produce a maze cave, such
as Wind Cave, South Dakota, USA. Larger geo-
All or any of these three processes may have operated in logical structures, and specifically faults, affect the
anycaveduringitshistory.Inallcases,itisusuallythecase complex pattern of caves in length and depth.
that, once an initial cave conduit is formed, it dominates Many of the world’s deepest known shafts, such as
thenetworkofpassagesandenlarges,becomingaprimary 451-m-deep Epos in Greece, are located in fault
tube that may adopt a variety of shapes (from a simple zones. Individual cave chambers may be directed by
meandering tube to a highly angular or linear conduit) faults, an example being Gaping Ghyll in Yorkshire,
depending on rock structure. England. Lubang Nasib Bagsu (Good Luck Cave),
Mulu, Sarawak is at 12 million cubic metres the
Cave form world’s largest known underground chamber and
Cavern systems can be very extensive. Mammoth Cave, owes its existence to a combination of folding and
Kentucky, USA, comprises over 800 km of subterranean faulting.
hollows and passages arranged on several levels, repre- 3 Cave breakdown and evaporite weathering. Lime-
senting major limestone units with a vertical depth of stone is a strong rock but brittle and fractures
110 m. At 563,270 m, the cave system is the longest in easily. Cave wall and ceiling collapse are impor-
the world. The form of caverns – their plan and cross- tant in shaping passages and chambers. Collapse
section – depends upon the purity of the limestone in is common near the cave entrance, where stress
which they are formed and the nature of the network of caused by unloading (p. 50) produces a denser
fissures dissecting the rock, as well as their hydrological joint network. Rock weathering by gypsum and
setting. halite crystallization (exsudation) may alter pas-
The shape of caves is directed by lithology, by the sage form. Water rich in soluble material seeping
pattern of joints, fractures, and faults, and by cave through the rocks evaporates upon reaching the
breakdown and evaporite weathering: cave wall. The expansion of crystals in the bed-
ding planes or small fissures instigates sensational
1 Lithology. Caves often sit at changes of lithology, spalling.
with passages forming along or close to litholog-
ical junctions, for example the junctions between Caves may also be classified in relation to the water table.
pure and impure limestones, between limestones The three main types are phreatic, vadose, and water table
and underlying shales, and between limestones and caves (Figure 8.18a, b, c). Vadose caves lie above the
igneous rocks. Passages may have a propensity to water table, in the unsaturated vadose zone, water table
form in a particular bed, which is then known as the or epiphreatic or shallow phreatic caves lie at the water
inception horizon (Lowe 1992). For instance, in table, and phreatic caves below the water table, where
the Forest of Dean, England, caves start to form the cavities and caverns are permanently filled with water.
in interbedded sandstones and unconformities in the Subtypes are recognized according to the presence of cave
Carboniferous limestone. loops (Figure 8.18d, e, f).