Geology Reference
In-Depth Information
in a stream profile is a prime site. A barrier
slowly builds up, on the front side of which frothing and
bubbling encourage further deposition. The end result
is that a dam and waterfall form across a karst river.
The waterfall may move down the valley leaving a fill of
travertine in its wake. Travertine may cover large areas. In
Antalya, south-west Turkey, a travertine complex, con-
structed by the supersaturated calcareous waters of the
Kirkgöz spring group, occupies 600 km
2
and has a max-
imum thickness of 270 m (Burger 1990). A sequence of
tufa dams in the Korana Valley, Croatia, impounds the
impressive Plitvice Lakes.
Karst forms on quartzite
It was once thought that quartzites were far too insolu-
ble to be susceptible to chemical weathering. Starting in
the mid-1960s with the discovery of quartzite karst
in Venezuela (White
et al
. 1966; see also Wirthmann
2000, 104-9), karst-like landforms have been found
on quartzose rock in several parts of the tropics. The
quartzitic sandstone plateau of the Phu Hin Rong
Kla National Park, north-central Thailand, bears fea-
tures found in limestone terrain - rock pavements,
karren fields, crevasses, and caves - as well as weath-
ered polygonal crack patterns on exposed rock surfaces
and
bollard-shaped rocks
(Doerr 2000). The crevasses,
which resemble grikes, occur near the edge of the plateau
and are 0.5-2 m wide, up to 30 m deep, and between
1 and 10 m apart. Smaller features are reminiscent of
solution runnels and solution flutes. Caves up to 30 m
long have been found in the National Park and were
used for shelter during air raids while the area was
a stronghold for the communists during the 1970s.
Some of the caves are really crevasses that have been
widened some metres below the surface, but others
are underground passages that are not associated with
enlarged vertical joints. In one of them, the passage is
0.5-1 m high and 16 m long. The bollard-shaped rock
features are found near the plateau edge (Plate 8.14).
They are 30-50 cm high with diameters of 20-100 cm.
Their formation appears to start with the development
of a
case-hardened surface
and its sudden cracking
under tensile stresses to form a polygonal cracking pat-
tern (cf. Williams and Robinson 1989; Robinson and
Plate 8.14
Bollard rocks formed in quartzitic sandstone,
north-central Thailand.
(
Photograph by Stefan Doerr
)
Williams 1992). The cracks are then exploited by weath-
ering. Further weathering deepens the cracks, rounding
off the tops of the polygonal blocks, and eventually
eradicates the polygonal blocks' edges and deepens
and widens the cracks to form bollard-shaped rocks
(Figure 8.16).
Karst-like landforms also exist on the surfaces of
quartzite table mountains (Tepuis) in south-eastern
Venezuela (Doerr 1999). At 2,700 m, the Kukenan Tepui
is one of the highest table mountains in South America
(Plate 8.15). The topography includes caves, crevasse-
like fissures, sinkholes, isolated towers 3-10 m high, and
shallow karren-like features. Evidence points to corro-
sion, rather than to erosive processes, as the formative
agent of these landforms (see pp. 189).
