Page 172 - Fundamentals of Geomorphology

WEATHERING AND RELATED LANDFORMS 155

saprock, which represents the first stages of weathering. silica. It occurs in humid and arid tropical environ-
Above the saprock lies saprolite, which is more weath- ments, and notably in central Australia and parts of
ered than saprock but still retains most of the structures northern and southern Africa and parts of Europe, some-
found in the parent bedrock. Saprolite lies where it was times in the same weathering profiles as ferricretes.
formed, undisturbed by mass movements or other erosive In more arid regions, it is sometimes associated with
agents. Deep weathering profiles, saprock, and sapro- calcrete. Calcrete is composed of around 80 per cent
lite are common in the tropics. No satisfactory name calcium carbonate. It is mostly confined to areas where
exists for the material lying above the saprolite, where the current mean annual rainfall lies in the range 200
weathering is advanced and the parent rock fabric is not to 600 mm and covers a large portion of the world’s
distinguishable, although the terms ‘mobile zone’, ‘zone semi-arid environments, perhaps underlying 13 per cent
of lost fabric’, ‘residuum’, and ‘pedolith’ are all used of the global land-surface area. Gypcrete is a crust of
(see Taylor and Eggleton 2001, 160). gypsum (hydrated calcium sulphate). It occurs largely
Weathering can produce distinct mantles. The intense in very arid regions with a mean annual precipitation
frost weathering of exposed bedrock, for instance, pro- below 250 mm. It forms by gypsum crystals growing
duces blockfields, which are also called felsenmeer, block in clastic sediments, either by enclosing or displacing
meer, and stone fields. Blockfields are large expanses of the clastic particles. Magnecrete is a rare duricrust made
coarse and angular rock rubble occurring within polar of magnesite (magnesium carbonate). Manganocrete
deserts and semi-deserts. Steeper fields, up to 35 ,are is a duricrust with a cement of manganese-oxide
◦
called blockstreams. An example is the ‘stone runs’ of the minerals.
Falkland Islands. Talus (scree) slopes and talus cones Hardpans and plinthite also occur. They are hard
are the result of weathering processes on steep rock faces layers but, unlike duricrusts, are not enriched in a specific
aided by some mass wasting. element.
Duricrusts are commonly harder than the materials
in which they occur and more resistant to erosion. In
Duricrusts and hardpans consequence, they act as a shell of armour, protecting
land surfaces from denudational agents. Duricrusts that
Under some circumstances, soluble materials precip- develop in low-lying areas where surface and subsurface
itate within or on the weathered mantle to form flows of water converge may retard valley down-cutting
duricrusts, hardpans, and plinthite. Duricrusts are to such an extent that the surrounding higher regions
important in landform development as they act like a wear down faster than the valley floor, eventually lead-
band of resistant rock and may cap hills. They occur ing to inverted relief (Box 6.1). Where duricrusts have
as hard nodules or crusts, or simply as hard layers. The been broken up by prolonged erosion, fragments may
chief types are ferricrete (rich in iron), calcrete (rich persist on the surface, carrying on their protective role.
in calcium carbonate), silcrete (rich in silica), alcrete The gibber plains of central Australia are an example of
(rich in aluminium), gypcrete (rich in gypsum), mag- such long-lasting remnants of duricrusts and consist of
necrete (rich in magnesite), and manganocrete (rich in silcrete boulders strewn about the land surface.
manganese).
Ferricrete and alcrete are associated with deep weath- Weathering landforms
ering profiles.They occur in humid to subhumid tropical
environments, with alcretes favouring drier parts of such Bare rock is exposed in many landscapes. It results
regions. Laterite is a term used to describe weathering from the differential weathering of bedrock and the
deposits rich in iron and aluminium. Bauxite refers to removal of weathered debris by slope processes. Two
weathering deposits rich enough in aluminium to make groups of weathering landforms are (1) large-scale cliffs
economic extraction worthwhile. Silcrete, or siliceous and pillars and (2) smaller-scale rock-basins, tafoni, and
duricrust, commonly consists of more than 95 per cent honeycombs.