Page 224 - Fundamentals of Geomorphology

KARST LANDSCAPES 207

b
a
() N ()
Poague Poague
Run
Run
Cedar
Creek
L I M E ST ON E B E LT Natural
Bridge
Active
tributary

James James
River
River
0 3 km

Figure 8.15 Natural Bridge, Cedar Creek, Virginia, USA. (a) Landscape before river piracy. (b) Landscape after river
piracy.
Source: Adapted from Woodward (1936)

Natural bridges lake waters in association with plants are called tufa
(Colour Plate 5, inserted between pages 208 and 209).
Natural bridges are formed of rock and span ravines Compact, crystalline, often banded calcium carbon-
or valleys. They are productions of erosion and are ate deposits precipitated from spring, river, or lake
commoner in karst terrain than elsewhere. Three mech- water are called travertine, or sometimes calc-sinter
anisms seem able to build natural bridges in karst areas. (Plate 8.13). However, some geomorphologists use the
First, a river may cut through a very narrow band of terms tufa and travertine interchangeably. Tufa and
limestone that crosses its path. Second, cave roofs may
collapse leaving sections still standing. Third, rivers may travertine deposition is favoured in well-aerated places,
which promote plant growth, evaporation, and car-
capture each other by piracy (Figure 8.15). This hap- bon dioxide diffusion from the air. Any irregularity
pens where meander caves on one or both sides of a
meander spur breach the wall of limestone between them
(Figure 8.15).

Tufa and travertine deposits

Karst rivers may carry supersaturated concentrations of
carbonates. When deposited, the carbonates may build
landforms. Carbonate deposition occurs when (1) water
is exposed to the atmosphere, and so to carbon diox-
ide, on emerging from underground; (2) when evap-
oration supersaturates the water; and (3) when plants
secrete calcareous skeletons or carbonate is deposited Plate 8.13 Hot spring travertine terraces at Pummukale,
around their external tissues. Porous accumulations Turkey.
of calcium carbonate deposited from spring, river, or (Photograph by Derek C. Ford )