Geoscience Reference
In-Depth Information
mean maximum flood level and up to 10 km from the river. Each of these puzzles has
now been explained (Williams et al.,
2000
; Williams et al.,
2006b
; Williams et al.,
2010b
;and
Chapter 10
).
It is important to realise that non-climatic factors are often involved in soil formation
in semi-arid and seasonally wet tropical areas on the margins of deserts. For example,
in the seasonally wet tropics of Africa, Australia, South America and Asia, three-
layered soils are widespread but defy classification into the conventional A-, B- and
C-horizons. They consist of a sandy top-soil (M-horizon), an underlying stone layer
(S-horizon) and a highly weathered clay-rich subsoil (W-horizon). Buried stone layers
are characteristic of strongly weathered granites containing resistant quartz veins, and
are widespread in seasonally wet tropical Asia, Africa, South America and in the
Piedmont region of the United States (Parizek and Woodruff,
1956
; Vogt and Vincent,
1966
; Troitsky et al.,
1968
;Segalen,
1969
). They have been attributed to soil creep
(Ireland et al.,
1939
; Eargle,
1940
), burial of stony colluvium, alluvium or erosional
lag gravels (Ruhe,
1956
; Parizek and Woodruff,
1957
; Ruhe,
1959
; Marchesseau,
1967
; Fairbridge and Finkl,
1984
), slope retreat (Segalen,
1969
), the swelling of clay
soils (Jessup,
1960a
; Mabbutt,
1965a
) and termite activity (Charter,
1950
;Nye,
1955
;
Watson,
1962
; Williams,
1968c
; Lee and Wood,
1971
; Williams,
1978
).
Although a variety of mechanisms are capable of producing buried stone layers,
one of the most effective is through the activity of mound-building termites (
Figure
15.2
). Termites (of which there are more than 2,600 species) are related to cockroaches
and, thanks to special microorganisms in their gut, can feed almost exclusively on
cellulose. In northern Australia, where termites play a major role in savanna ecosystem
dynamics, there are more than 150 species of termite (Andersen et al.,
2005
). Of the
four broad groups of termites (wood-feeders, soil-feeders, debris-feeders and grass-
harvesters), only the latter need concern us here. The grass-harvesting species collect
dry grass, cut it into small pieces and store the pieces in the galleries inside their
mounds. Two of the more common mound-building species are
Tumulitermes hastilis
and
Nasutitermes triodiae
(
Figure 15.3
).
T. hastilis
builds quite small mounds up to
75 cm high that are usually abandoned after about three years, but the mounds of
N.
triodiae
can exceed 6 m in height and attain ages of up to 100 years, although less
than half that age is more common. Williams (
1968c
;
1978
) monitored rates of soil
erosion by slopewash and soil creep on granite hill slopes in seasonally wet tropical
northern Australia and found that current rates of surface lowering are balanced by
top-soil replenishment from termite mound breakdown and redistribution across the
slope. At existing termite mound densities and current rates of slope erosion, a surface
coarse sand horizon 30-50 cm thick and a buried stone layer 30-50 cm thick would
only need about 10,000-15,000 years to form.
The distinction is important because some workers have interpreted the buried
stone layers that are widespread in parts of semi-arid South America as being caused
by an abrupt change of climate, in which a surface lag gravel formed under arid
