Geology Reference
In-Depth Information
10
Caves and tafoni
10.1
GENERAL STATEMENT
Caves are underground openings connected with the atmosphere, in places by wide openings,
elsewhere by narrow apertures or vertical shafts, but at some sites only by narrow gaps in the sur-
rounding rock. Granite masses are not noted for cave developments, but several have been
reported from various parts of the world. On the other hand, tafoni, or relatively shallow inverse or
laterally disposed hollows, are widely developed on granitic rocks.
The term pseudokarst has been applied to forms which mimic features developed by solution in
carbonate or gypsiferous strata but which are developed in rocks not as obviously or rapidly soluble.
Thus, various so-called pseudokarstic forms, including caves, are developed in acid plutonic rocks
in several parts of the world. Used in this context, the prefix pseudo is disputed by those who point
out that solution contributes significantly to the formation of flutings, gutters, basins and caves in
granitic rocks (see also Chapters 8 and 9). Others, while not denying the efficacy of solution as a
major weathering process acting on all minerals, including all the essential minerals of granite,
nevertheless argue that other mechanisms such as abrasion are more important here than in car-
bonate rocks. In any event, an alternative term such as silicate karst (hence Silikatrillen,
Granitrillen) might be more appropriate (Kastning, 1976).
10.2
CAVES ASSOCIATED WITH CORESTONES AND GRUS
Occasionally, the preferential subsurface flushing or evacuation of friable, weathered rock has left
irregular, tubular voids, largely defined by corestones set in grus and known as boulder caves.
Such subsurface erosion is achieved by streams that are diverted underground and re-emerge a
short distance downstream.
Thus, Labertouche Cave, near Neerim South, in Victoria, is about 200 m long and essentially
straight, though irregular in detail. It begins in a sinkhole in a blind valley and its point of emer-
gence is marked by a pronounced re-entrant. Others of a similar kind have been reported from the
uplands of eastern Victoria, in the Girraween National Park, New South Wales, and from central
and southeastern Queensland. They are also developed in Spanish granite massifs in the Pyrenees,
and in Galicia (in the Pindo, Louro-Os Profundos and Barbanza uplands) where the well-fractured
rock has been, subject to Holocene periglacial activity. Cave systems in granite are also reported
from various parts of the Western Cordillera of the USA.
Granite caves present the same hazards as karst caves. In 1980 a caver died from injuries
received when he fell and suffered immersion in a boulder cave, one of the Lost Creek Granite
Caves, in Colorado, which, though up to 15 m high, are also characterised by boulder falls and by
streams that from time to time run at high velocity.
A cave system is developed on the granitic Makatau inselberg in the Rupununi savannas of
Guyana, where the form of the openings in plan is clearly influenced by the orthogonal joint system.
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