Page 174 - Fundamentals of Geomorphology

WEATHERING AND RELATED LANDFORMS 157

Flutes, rills, runnels, grooves, and gutters, as they
are variously styled, form on many rock types in many
environments. They may develop a regularly spaced
pattern. Individual rills can be 5–30 cm deep and
22–100 cm wide. Their development on limestone is
striking (pp. 191–4).
Rock-basins, also called weathering pits, weather-
pits or gnammas, are closed, circular, or oval depressions,
a few centimetres to several metres wide, formed on flat
or gently sloping surfaces of limestones, granites, basalts,
gneisses, and other rock types (Plate 6.1). They are com-
monly flat-floored and steep-sided, and no more than a
metre or so deep, though some are more saucer-shaped.
The steep-sided varieties may bear overhanging rims and
undercut sides. Rainwater collecting in the basins may
overflow to produce spillways, and some basins may
contain incised spillways that lead to their being perma-
nently drained. Rock-basins start from small depressions
in which water collects after rainfall or snowmelt. The
surrounding surfaces dry out, but the depression stays
moist or even holds a small pool for long periods,
so providing a focus for more rapid weathering. In
consequence, the rock-basin expands and deepens. As Plate 6.1 Weathering pit on Clach Bhàn, Ben Avon, in the
rock-basins expand, they may coalesce to form com- eastern Cairngorms, Scotland.
pound forms. Solution pools (pans, solution basins, (Photograph by Adrian M. Hall )
flat-bottomed pools) occur on shore platforms cut in
calcareous rocks.
Tafoni (singular tafone) are large weathering features
that take the form of hollows or cavities on a rock surface of rock. Both processes may operate, but not all tafoni
(Plate 6.2), the term being originally used to describe hol- contain a significant quantity of salts. Once formed,
lows excavated in granites on the island of Corsica. They tafoni are protected from rainwash and may become the
tend to form in vertical or near-vertical faces of rock. foci for salt accumulations and further salt weathering.
They can be as little as 0.1 m to several metres in height, Parts of the rock that are less effectively case-hardened
width,anddepth,witharched-shapedentrances,concave are more vulnerable to such chemical attack. Evidence
walls, sometimes with overhanging hoods or visors, espe- also suggests that the core of boulders sometimes more
cially in case-hardened rocks (rocks with a surface made readily weathers than the surface, which could aid the
harder by the local mobilization and reprecipitation of selective development of weathering cavities. Tafoni are
minerals on its surface), and smooth and gently slop- common in coastal environments but are also found in
ing, debris-strewn floors. Some tafoni cut right through arid environments. Some appear to be relict forms.
boulders or slabs of rock to form rounded shafts or win- Honeycomb weathering is a term used to describe
dows. The origins of tafoni are complex. Salt action numerous small pits or alveoli, no more than a few
is the process commonly invoked in tafoni formation, centimetres wide and deep, separated by an intricate
but researchers cannot agree whether the salts promote network of narrow walls and resembling a honeycomb
selective chemical attack or whether they promote phys- (Plate 6.3). They are often thought of as a small-
ical weathering, the growing crystals prising apart grains scale version of multiple tafoni. The terms alveolar