Geology Reference
In-Depth Information
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
rock-basins expand, they may coalesce to form com-
pound forms.
Solution pools
(pans, solution basins,
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
(Plate 6.2), the term being originally used to describe hol-
lows excavated in granites on the island of Corsica. They
tend to form in vertical or near-vertical faces of rock.
They can be as little as 0.1 m to several metres in height,
width, and depth, with arched-shaped entrances, concave
walls, sometimes with overhanging hoods or visors, espe-
cially in case-hardened rocks (rocks with a surface made
harder by the local mobilization and reprecipitation of
minerals on its surface), and smooth and gently slop-
ing, debris-strewn floors. Some tafoni cut right through
boulders or slabs of rock to form rounded shafts or win-
dows. The origins of tafoni are complex. Salt action
is the process commonly invoked in tafoni formation,
but researchers cannot agree whether the salts promote
selective chemical attack or whether they promote phys-
ical weathering, the growing crystals prising apart grains
Plate 6.1
Weathering pit on Clach Bhàn, Ben Avon, in the
eastern Cairngorms, Scotland.
(
Photograph by Adrian M. Hall
)
of rock. Both processes may operate, but not all tafoni
contain a significant quantity of salts. Once formed,
tafoni are protected from rainwash and may become the
foci for salt accumulations and further salt weathering.
Parts of the rock that are less effectively case-hardened
are more vulnerable to such chemical attack. Evidence
also suggests that the core of boulders sometimes more
readily weathers than the surface, which could aid the
selective development of weathering cavities. Tafoni are
common in coastal environments but are also found in
arid environments. Some appear to be relict forms.
Honeycomb weathering
is a term used to describe
numerous small pits or
alveoli
, no more than a few
centimetres wide and deep, separated by an intricate
network of narrow walls and resembling a honeycomb
(Plate 6.3). They are often thought of as a small-
scale version of multiple tafoni. The terms
alveolar
