Geology Reference
In-Depth Information
Figure 10.10.
Development of hollows on opposed upper and lower surfaces of a basal parting: Tcharkulda
Hill, near Minnipa, northwestern Eyre Peninsula, South Australia.
the fracture plane, and accumulates there in thin discontinuous covers of fragmented rock until
washed or blown away by wind or rain. For the case of surfaces of vertical or strongly inclined
starting surfaces, the development of both cavities is even. In the second or anisotropic stage (ii),
the active surface advances unequally, rapidly in alveoles and mamillated surfaces, less so in the
septa and ribs in between. In places the intervening rises form orthogonal patterns. Granular dis-
integration is still active, but thick topics of flakes are spectacularly developed at some sites. The
hollow is thus enlarged. The dryness of the interior walls is attested by the absence of lichens. Swifts
also build nests, and similarly, bats inhabit the hollows and their droppings, in places attain consid-
erable thickness, may contribute to the formation of speleothems. The third stage (iii) corresponds
to the moment in which overall, the rate of development of the tafoni decreases, though alveoles
continue to extend, causing the intervening septa to disappear.
Eventually, the extending hollow intersects the outer surface of the host block or boulder. The
apertures or windows thus formed differ in their morphology. In some, the outer and inner surfaces
of the confining shell are roughly parallel and the window takes the form of a bevelled opening.
Elsewhere the aperture in section looks like a truncated cone (
Fig. 10.2b),
narrowing toward the
outside; they have been referred to as ox-eyes.
The fourth stage (iv) is characterised by the breaching of the outer shell that causes marked
changes in the microclimate in the tafoni. At this stage the interior ceilings are scalloped.
Eventually in this stage, the water entrance from the exterior may take place, so that small
siliceous speleothems derived from the dissolution and reprecipitation of silica also appear (see
below). But because of increased air circulation and accession of moisture consequent on the
opening of the hollows, the walls are colonised by biota such as algae, lichens and mosses; and
these may accelerate extension due to the general breaching of the confining shell and the collapse
of the remains of the block or boulder. All that remains is a scatter of fragments and detritus.
10.4.4
Case-hardening and other veneers
Case-hardening is commonly invoked in explanation of the visors/outer shell enclosing tafoni. It
takes the form of a red-brown coating concentrated at the surface, but extending several crystals
deep beneath it that is commonly developed on exposed surfaces in arid and semi-arid lands. It is
