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Surface Processes
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Youthful rivers cut gorges in hard, jointed rocks and V-shaped valleys in softer rocks, Fig. 3.11a, and are characteristic of many upland areas. A youthful valley frequently follows a zig-zag course, leaving overlapping spurs which project from either side of the valley. Debris, loosened by frost, rain or insolation, falls from the valley sides, to be carried away by the stream and assist in its work of abrasion. Rain-wash and soil creep (p. 59) also contribute material from the slopes, especially in more mature stages of valley development, when a mantle of soil has been formed on a land surface. Large movements in the form of landslides also contribute to the erosion of a valley by transporting material to the river (Fig. 3.11b). Gradually as the headwaters of a river cut back, increasing the length of the river’s course, its valley is deepened and widened into a broader V; slope movement causes the valley sides to recede and deposits debris on the valley floor (Fig. 3.11c). Small scree slopes form at the base of the valley sides. The deepening and widening, if uninterrupted, continue as shown in the figure until the stage of maturity is reached, when there is maximum topographical relief (i.e. the difference in height between valley floor and adjacent ridge tops). Eventually the tributaries of one catchment will be separated by only a thin ridge from those of its neighbour, and when denudation reduces the height of the ridges the river can be considered to have entered its old age: the valley comes to have a wide, flat floor over which the river follows a winding course, the upper slopes may be convex and the depth of relief is less. Such a sequence would be followed by a river in a temperate climate, if uninterrupted. Under other conditions, as in a drier climate, the valley slopes stand at a steeper angle because they are less affected by atmospheric weathering and slope instability, particularly landslides: the ridges between valleys are then sharper. Flat-topped hills may be left in partly denuded horizontal strata where a layer of hard rock such as sandstone, or a dolerite sill, forms the capping of the hill; the term mesa (= table) is used for this topographical form. Continued weathering and erosion reduce the mesa to isolated, steep-sided, pillar-like hills (Fig. 3.11e,f).
Geomorphic provinces and regolith-landform evolution of the Capricorn Orogen, Western Australia
Published in Australian Journal of Earth Sciences, 2021
R. L. Thorne, S. C. Spinks, R. R. Anand
The landscape process that have formed the regolith in Transect 3 are modelled in Figure 7. A coherent lateritic horizon would have developed initially. The thicknesses of the lateritic residuum and underlying saprolite vary in accordance with the underlying lithology. Higher abundances of more stable minerals such as quartz within the primary sediment will result in shallower weathering profile development. As weathering continues iron segregations develop in a clay zone and characteristic mottled zones develop. Ferricrete develops in the upper portions of weathering profiles as Fe2+ is mobilised. As the climate becomes more arid or uplift occurs, regolith is stripped from the landscape and in situ weathering profiles remain where they have been protected by formation in topographic lows and by ferricrete caps. Colluvium and alluvium are deposited in broad drainages, and the concentration of Si-bearing fluids in these lows associated with drainages results in the formation of silcrete (Taylor & Eggleton, 2017). Continued weathering and erosion lead to topographic inversion and the formation of silcrete mesas.