Geology Reference
In-Depth Information
Figure 10.7.
(c) In Corsica with scallops.
locally as dew holes, are reported from the Bohemian Massif. But they do not appear to be
developed on the Dartmoor granite, southwestern England, for example.
At a local scale, the distribution of tafoni is puzzling with some blocks hollow and others,
immediately adjacent and apparently identical, intact. Such contrasted developments are found, for
example, in exposures of the Begur granite, in the Costero-Catalana Range of northeastern Spain.
In the Pindo granitic massif of Galicia, tafoni are scarce, though those that are present are very
well-developed. Again, although tafoni are widespread in the Upper South East and Eyre Peninsula
regions of South Australia, not all boulders exhibit tafoni.
10.4.2 Process
The process responsible for tafoni can be considered under three headings: initiation, growth or
development, and the nature of the visor/outer shell. Initiation
There is no doubt that some tafoni are initiated by soil moisture attack beneath the land surface.
Boyé and Fritsch (1973) reported hollows in the fresh rock surface occupied by grus and already
present on the underside of sheet structure in a newly exposed quarry face at Ebaka, in southern
Cameroon. Similarly, the outlines of tafoni were noted already developed during the excavation of
the foundations of a dam at Xallas River, at Ézaro, near A Coruña, in Galicia NW Spain. By its
very nature, such direct evidence of subsurface initiation is rare.
In addition, tafoni and flared slopes commonly merge. Examples have been noted in the side-
walls of a joint cleft on Scholz Rock and on boulders at Murphys Haystacks, both on northwest-
ern Eyre Peninsula, South Australia, at Kokerbin Hill, in the southwest of Western Australia (Figs
10.3c), and on Bloedkoppie Dome in central Namibia. Pronounced hollows are associated with
basally fretted slopes at several sites on Eyre Peninsula (Figs 8.11a and d), and in the Pietersburg
area of the Northern Transvaal ( Fig. 10.5b).
Alternatively, many tafoni appear to originate at the land surface. Some workers have attributed
tafoni to preferential weathering under epigene, or subaerial, conditions. They argue that the
hollows were originally occupied by material that was different from, and presumably weaker
than, that which remains. At most sites it is difficult to test this hypothesis because the alleged
Search WWH ::

Custom Search