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scouring and erosion. The granite and gneiss outcrops studied are, however, and to a greater or
lesser degree, covered by foliose lichen and moss. Though lichens can cause rock disintegration the
lichen present on the rims could also conceivably protect the surface, as required by Blank's
(1951) working hypothesis.
If the areas marginal to rock basins were favourable to lichen growth because of their dampness,
it could be argued that this ring of vegetation could divert flow and wash around the basin, induce
increased turbulence and scour, and thus produce the annular rim that is essential to the formation
of the doughnuts under investigation. Unfortunately for this suggestion, there is no evidence of
pronounced lichen growth on the Texas doughnuts.
Blank (1951) also considered whether the doughnuts could have originated as small, low topo-
graphic domes left behind in relief as remnants of circumdenudation by the development of gut-
ters on the sloping granite surface. He visualised basins developing in the crests of these, leaving
the resultant rims as rock doughnuts. The initiation of basins in such situations, and the develop-
ment of doughnuts as a consequence of such local relief inversion, is highly unlikely because
water runs off upstanding areas. In special circumstances, such as the fortuitous exposure of a con-
centration of weak materials on the crest of an upstanding rock mass, such development can be
envisaged, but this must surely be a rare occurrence; and doughnuts, covered by lichen, have now
been located on several granite residuals on northwestern Eyre Peninsula, South Australia, includ-
ing some on recent, artificially exposed, platforms.
9.6.3
Evidence and argument
Several general arguments can be marshalled against such localised relief inversion giving rise to,
and developing rock doughnuts. Yet forms and processes which substantially corroborate this last
of Blank's (1951) suggestions have been noted in the field. First, as noted earlier, in several places
isolated blocks and boulders rest on plinths, which, though in physical continuity with the under-
lying granite, nevertheless stand higher than the adjacent slopes. They can evidently form in the
subsurface or under the influence of epigene processes, for at both Tcharkuldu Hill and Mt Hall, on
western Eyre Peninsula, South Australia, angular blocks protrude above the level of a regolith-covered
platform.
Second, boulders standing on plinths commonly develop basal tafoni as a result of moisture
attack. Basins are also formed in the plinths (see Chapter 10, and particularly Fig. 10.11). Allowing
that the boulder must eventually disintegrate, so exposing the basins set into the plinths, doughnuts
could be formed in this way. Thus, the deducible consequences of the working hypothesis involv-
ing relief inversion are found in the field. The explanation offered is consistent with the evidence
that rock doughnuts are residual forms due to the weathering and erosion of a roughly circular
zone inside them, and of an annular ring outside the rim.
Another possible explanation is suggested by the earlier interpretation of rock levees (above). It
was suggested that the forms could reflect the contrasted susceptibility to weathering of moist,
regolith-covered surface on the one hand and dry, bare surfaces on the other. The same mechanism
could account for rock doughnuts
(
Fig. 9.19)
.
The bedrock areas exposed around basins are rela-
tively dry and stable, whereas the bedrock beneath adjacent bedrock-covered platforms and slopes
is weathered more rapidly, leaving the circular area around the basin in relief. This hypothesis
finds support in the occurrence of two small doughnuts on a recently cleared rock platform at
Kwaterski Rocks, north of Minnipa, Eyre Peninsula, South Australia.
9.7
FONTS
In some granitic areas in Portugal, Galicia and Catalonia, Spain (Vidal Romaní and Twidale, 1998;
Roqué and Pallí, 1991), rock doughnuts and basins occur on the crests of pedestals or small
towers, together making fonts or benitiers which are commonly up to one metre, but in the Sierra
Guadarrama of central Spain (Centeno, 1988), up to 4 metres high (
Fig. 9.21)
.
The forms are due
