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