Page 202 - Fundamentals of Geomorphology

KARST LANDSCAPES 185

stalactites, straws, flowstones, columns, and curtains, all coloured in white, orange (from iron oxide), and blue-grey
(from manganese oxide). These formations are created from lime waste from an old quarry tip above the cave. The
iron has coated the tips of stalagmites to give them the appearance of poached eggs. At the far end of the Poached
Egg Chamber are thousands of straws and stalactites, with a cascade of new flowstone on top of an old one known as
the ‘Frozen Waterfall’. Above this formation is the ‘Big Drip’, a 0.45-m-high stalagmite that is very active, splashing
drips around its sides, so making itself thicker. At this point, bedding planes in the limestone show signs of cavern
collapse.Turning to the left, the ‘Mary Queen of Scots Pillar’, a 2-m-high stalactite boss, presents itself.This feature is
said to have been named by Mary Queen of Scots when she visited the cavern in 1582. In the last chamber, the River
Wye can be seen emerging from the 15-m-high boulder choke that blocks the rest of the cavern system. A beautiful
flowstone structure in this chamber was named the ‘Sculpture’ by a party of local schoolchildren in 1977, and above
it is the ‘Grand Cascade’, another impressive flowstone formation stained with oxides of iron and manganese.

KARST ENVIRONMENTS and diverse group of rocks (Figure 8.3). Limestone
is a rock containing at least 50 per cent calcium
What is karst? carbonate (CaCO 3 ), which occurs largely as the mineral
calcite and rarely as aragonite. Pure limestones con-
Karst is the German form of the Indo-European word tain at least 90 per cent calcite. Dolomite is a rock
kar, which means rock. The Italian term is carso, and containing at least 50 per cent calcium–magnesium
the Slovenian kras. In Slovenia, kras or krš means ‘bare carbonate (CaMg(CO 3 ) 2 ), a mineral called dolomite.
stony ground’ and is also a rugged region in the west Pure dolomites (also called dolostones) contain at least
of the country. In geomorphology, karst is terrain in 90 per cent dolomite. Carbonate rocks of interme-
which soluble rocks are altered above and below ground diate composition between pure limestones and pure
by the dissolving action of water and that bears distinc- dolomites are given various names, including magnesian
tive characteristics of relief and drainage (Jennings 1971, limestone,dolomiticlimestone,andcalcareousdolomite.
1). It usually refers to limestone terrain characteristically Karst features achieve their fullest evolution in beds of
lacking surface drainage, possessing a patchy and thin fairly pure limestone, with more than 80 per cent calcium
soil cover, containing many enclosed depressions, and carbonate, that are very thick, mechanically strong, and
supporting a network of subterranean features, includ- contain massive joints. These conditions are fulfilled in
ing caves and grottoes. However, all rocks are soluble to the classic karst area of countries bordering the eastern
some extent in water, and karst is not confined to the sideoftheAdriaticSea.Chalk,althoughbeingaverypure
most soluble rock types. Karst may form in evaporites limestone, is mechanically weak and does not favour the
such as gypsum and halite, in silicates such as sand- formation of underground drainage, which is a precon-
stone and quartzite, and in some basalts and granites dition for the evolution of medium-scale and large-scale
under favourable conditions (Table 8.1). Karst features surface-karst landforms.
may also form by other means – weathering, hydraulic
action, tectonic movements, meltwater, and the evac-
uation of molten rock (lava). These features are called
pseudokarst as solution is not the dominant process in KARST AND PSEUDOKARST
their development (Table 8.1). PROCESSES
Extensive areas of karst evolve in carbonate rocks
(limestones and dolomites), and sometimes in evaporites, Few geomorphic processes are confined to karst land-
which include halite (rock salt), anhydrite, and gypsum. scapes, but in areas underlain by soluble rocks some
Figure 8.2 shows the global distribution of exposed car- processes operate in unique ways and produce charac-
bonate rocks. Limestones and dolomites are a complex teristic features. Solution is often the dominant process