Geology Reference
In-Depth Information
Figure 9.4.
(i) Schematic evolution of tafoni and basin in two blocks in contact along a fracture, (ii) load
concentrations, and (iii) the resultant forms. The figures indicate stability coefficient (see Vidal
Romaní, 1989).
(a)
Figure 9.5.
(a) Elongate pit formed along fracture, Ebaka Dome, French Cameroon (M. Boyé).
Several workers have argued that water collecting in chance crevices and depressions in
exposed rock surfaces could enlarge those initial depressions to form rock basins. Hedges (1969)
went so far as to state that “They are never found to be developed beneath a soil cover in road cuts
and other excavations”. Watson and Pye (1985) make similar assertions regarding the basins
developed on the Mdzimba Hills in Swaziland, though, as with flutings or grooves (see Chapter
8.6), it is relevant to consider the origin of the host landform.
Some rock basins, like that developed on the upper surface of the artificially-erected menhir at
St Uzek in Brittany (Lageat, Sellier and Twidale, 1993 and see Fig. 8.23), are undoubtedly of epi-
gene origin. But many rock basins (like other granite forms, major and minor) are demonstrably
initiated in the subsurface. Some basins have been revealed by excavations of road cuts and of dam
foundations in northern Spain. Similar evidence has been revealed at sites at Midrand, between
