262
Climatic Geomorphology
phenomenon of formation can be produced by freeze pushing in cold deserts. In hot deserts
the presence of expansive clays is necessary. These swell with an increase in humidity and
lead to a rise in ground level. During dry stages the soils contract and crack. Therefore,
pavement generation under these circumstances is favoured by enhanced humid/dry
cycles.
In general, mechanical surface weathering processes have been proposed for the
development of boulder hamadas, which break up the rocks and produce a residual
pavement of angular blocks. It is also believed that sub-surface weathering can play a part
in pavement development (Mabbutt, 1977). This author indicates that the higher subsurface humidity content, especially in the presence of chlorides and sulphates, leads to a
more efficient weathering and, therefore, coarse grains disintegrate gradually. On the
contrary, at the surface the water availability is much lower and the pavement stones
hardly weather. The end result is that the pavement persists.
2.3. Development
From the analysis of pavement-generating processes it can be deduced that genesis via
water or aeolian erosion is rapid, unlike pavements produced by movements in the soil
interior. Sub-aerial erosion produces a descent and levelling of the surface after long and
continual activity (Mabbutt, 1977). Moreover, as the pavement develops the availability of
fine material diminishes. The pavement is subjected to a slow evolution, over thousands of
years, during which time the stones weather and the material generated is partially
exported (Cooke, 1970a). This slowness in their development was shown in the study of a
group of terraces in the Dead Sea region (Hunt and Mabey, 1966), in which the stonecover percentage increases with terrace age. On the contrary, the oldest pavements in
Death Valley, California (Hunt and Mabey, 1966) are characterised by having a higher
percentage of fine particles and being more compact than the more recent pavements. In
this case the evolution is the inverse of the previous example, perhaps due to a more
intense surface weathering.
In addition, it has to remarked that as pavements evolve, their salt content can increase,
as the salts get moved by capillary action and stay trapped between the stones. Equally,
pavements can experience a continuous contribution of aeolian material. In any case, once
formed the pavement is relatively stable, as deflation decreases with an increase in stone
coverage, which tends to increase with water erosion. These circumstances mean that the
pavement is a fairly stable morphology in which both erosional and depositional processes
act, although with low intensities that can also counterbalance each other. This indicates
that pavements are resistant to erosion, because they protect the underlying material that is
prone to water and aeolian erosion.
3. Patterned ground
We have already indicated in the section related to periglacial morphoclimatic zones that
patterned ground (circles, polygons, nets, steps, stripes) reach a great development in both
Climatic Geomorphology
phenomenon of formation can be produced by freeze pushing in cold deserts. In hot deserts
the presence of expansive clays is necessary. These swell with an increase in humidity and
lead to a rise in ground level. During dry stages the soils contract and crack. Therefore,
pavement generation under these circumstances is favoured by enhanced humid/dry
cycles.
In general, mechanical surface weathering processes have been proposed for the
development of boulder hamadas, which break up the rocks and produce a residual
pavement of angular blocks. It is also believed that sub-surface weathering can play a part
in pavement development (Mabbutt, 1977). This author indicates that the higher subsurface humidity content, especially in the presence of chlorides and sulphates, leads to a
more efficient weathering and, therefore, coarse grains disintegrate gradually. On the
contrary, at the surface the water availability is much lower and the pavement stones
hardly weather. The end result is that the pavement persists.
2.3. Development
From the analysis of pavement-generating processes it can be deduced that genesis via
water or aeolian erosion is rapid, unlike pavements produced by movements in the soil
interior. Sub-aerial erosion produces a descent and levelling of the surface after long and
continual activity (Mabbutt, 1977). Moreover, as the pavement develops the availability of
fine material diminishes. The pavement is subjected to a slow evolution, over thousands of
years, during which time the stones weather and the material generated is partially
exported (Cooke, 1970a). This slowness in their development was shown in the study of a
group of terraces in the Dead Sea region (Hunt and Mabey, 1966), in which the stonecover percentage increases with terrace age. On the contrary, the oldest pavements in
Death Valley, California (Hunt and Mabey, 1966) are characterised by having a higher
percentage of fine particles and being more compact than the more recent pavements. In
this case the evolution is the inverse of the previous example, perhaps due to a more
intense surface weathering.
In addition, it has to remarked that as pavements evolve, their salt content can increase,
as the salts get moved by capillary action and stay trapped between the stones. Equally,
pavements can experience a continuous contribution of aeolian material. In any case, once
formed the pavement is relatively stable, as deflation decreases with an increase in stone
coverage, which tends to increase with water erosion. These circumstances mean that the
pavement is a fairly stable morphology in which both erosional and depositional processes
act, although with low intensities that can also counterbalance each other. This indicates
that pavements are resistant to erosion, because they protect the underlying material that is
prone to water and aeolian erosion.
3. Patterned ground
We have already indicated in the section related to periglacial morphoclimatic zones that
patterned ground (circles, polygons, nets, steps, stripes) reach a great development in both