Page 176 - Fundamentals of Geomorphology

WEATHERING AND RELATED LANDFORMS 159

Table 6.1 Honeycomb weathering grades on sea walls at rise in temperature speeds chemical reactions, especially
Weston-super-Mare, Avon, UK sluggish ones, and some biological reactions by a factor
of two to three, a fact discovered by Jacobus Hendricus
Grade Description
van’t Hoff in 1884. The storage and movement of water
0 No visible weathering forms in the regolith is a highly influential factor in deter-
1 Isolated circular pits mining weathering rates, partly integrating the influence
2 Pitting covers more than 50 per cent of the area of all other factors. Louis Peltier (1950) argued that
3 Honeycomb present rates of chemical and mechanical weathering are guided
4 Honeycomb covers more than 50 per cent of the by temperature and rainfall conditions (Figure 6.3).
area
5 Honeycomb shows some wall breakdown The intensity of chemical weathering depends on the
6 Honeycomb partially stripped availability of moisture and high air temperatures. It is
7 Honeycomb stripping covers more than 50 per minimal in dry regions, because water is scarce, and in
cent of the area cold regions, where temperatures are low and water is
8 Only reduced walls remain scarce (because it is frozen for much or all of the year).
9 Surface completely stripped
Mechanical weathering depends upon the presence of
Source: Adapted from Mottershead (1994) water but is very effective where repeated freezing and
thawing occurs. It is therefore minimal where tempera-
tures are high enough to rule out freezing and where it is
so cold that water seldom thaws.
weathering, stone lattice, and stone lace are synonyms.
Honeycomb weathering is particularly evident in semi-
arid and coastal environments where salts are in ready Leaching regimes
supply and wetting and drying cycles are common. Climate and the other factors determining the water bud-
A study of honeycomb weathering on the coping stones get of the regolith (and so the internal microclimate of a
of the sea walls at Weston-super-Mare, Avon, England, weathered profile) are crucial to the formation of clays by
suggests stages of development (Mottershead 1994).The weathering and by neoformation.The kind of secondary
walls were finished in 1888. The main body of the walls clay mineral formed in the regolith depends chiefly on
is made of Carboniferous limestone, which is capped twothings:(1)thebalancebetweentherateofdissolution
by Forest of Dean stone (Lower Carboniferous Pennant of primary minerals from rocks and the rate of flush-
sandstone). Nine weathering grades can be recognized on ing of solutes by water; and (2) the balance between the
the coping stones (Table 6.1). The maximum reduction rate of flushing of silica, which tends to build up tetra-
of the original surface is at least 110 mm, suggesting a hedral layers, and the rate of flushing of cations, which
minimum weathering rate of 1 mm/yr. fit into the voids between the crystalline layers formed
from silica. Manifestly, the leaching regime of the regolith
is crucial to these balances since it determines, in large
WEATHERING AND CLIMATE measure, the opportunity that the weathering products
have to interact. Three degrees of leaching are associ-
Weathering processes and weathering crusts differ from ated with the formation of different types of secondary
place to place. These spatial differences are deter- clay minerals – weak, moderate, and intense (e.g. Pedro
mined by a set of interacting factors, chiefly rock type, 1979):
climate, topography, organisms, and the age of the
weathered surface. Climate is a leading factor in deter- 1 Weak leaching favours an approximate balance
mining chemical, mechanical, and biological weathering between silica and cations. Under these conditions
rates. Temperature influences the rate of weathering, but the process of bisiallitization or smectization creates
seldom the type of weathering. As a rough guide, a 10 C 2 : 2 clays, such as smectite, and 2 : 1 clays.
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