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and spread evenly to give the smooth surface characteristic of pediments. Third, because of the low
permeability of fresh granite, the weathering front is usually sharp, so that distinctive platforms or
rock pediments are readily initiated and exposed. Finally, because of the marked contrast between
massive and well-fractured compartments and because the latter are defined by fractures, the pied-
mont angle is better developed in granitic than in many other lithological environments.
4.2.3
Relationship between pediment and peneplain
The fringing mantled pediments that are developed around such inselbergs as Ucontitchie Hill,
Eyre Peninsula, South Australia, merge without topographic break at their lower extremities with the
rolling surface of the peneplain
(
Fig. 4.2b).
Both forms are developing concurrently. Moreover, there
are enough boulder clusters and other low residuals to suggest that when the latter are eventually
eliminated the pediments that now surround them will extend and coalesce in convex crests that
are integral parts of the peneplain.
Thus, rock and mantled pediments are not genetically or temporally distinct from peneplains.
They commonly coexist. They are particular types of plains developed on fresh rock and in the
piedmont of inselbergs. Pediments are not fundamentally distinct from peneplains: they are parts
of a larger whole, namely planation surfaces. Moreover, though pediments are well and widely
developed and preserved in arid and semi-arid lands, they occur also in other climatic contexts.
Pediments are typical of the arid and semi-arid tropics but they are not, as Blackwelder (1931,
p. 138) suggested, merely “the desert-inhabiting species of the genus peneplain” for both pediments
and peneplains are developed on granitic rocks in various climatic contexts. Peneplains are dis-
sected pediments and reflect the concomitant downstream increase in volume of run-off and evo-
lution of the drainage network, resulting in the concentration of flow in fewer larger channels.
These cause dissection and the conversion of the surface from smooth to undulating or rolling,
depending on stream spacing.
4.3
ETCH PLAINS IN GRANITE
Many deep weathering profiles developed on granitic rocks are reported from the various shield
lands which have been relatively stable for long periods. Thus it is not surprising that plains are
well and widely developed on older granite masses. High plains are also characteristic of several
Palaeozoic uplands, such as Dartmoor, southwestern England, the Meseta of the Iberian Peninsula
and the Central Massif of France, which are, however, not of simple epigene origin but are due to
a more complex sequence of events.
Where the weathered mantle developed on a surface of low relief has been stripped away usu-
ally by rivers but also by other epigene processes, the weathering front has been exposed as an etch
plain or platform. That exposed near Platja d'Aro, Girona, in northeastern Spain, provides a good
example (
Fig. 4.9)
.
The platform is cut in granite and its level is coincident with that of the weath-
ering front preserved beneath the soil-regolith cover. The platform has been exposed as a result of
the stripping of the regolith by waves during the last eustatic rise of sea level (Twidale, Bourne and
Twidale, 1977). But etch planation is not necessarily an indication of such baselevel movement for
rivers may develop lower gradients in debris reduced in size by weathering and may thus erode
into the regolith. Also, lowering could result from the subsurface flushing and evacuation in solu-
tion of the products of weathering, as suggested by Ruxton (1958) and Trendall (1962).
Weathering and stripping are in some regions incomplete, as for example in the Ávila-Villacastín
region of central Spain. There, the landscape is characterised by plains carrying a discontinuous
recently developed regolith, with numerous boulders, clusters of boulders and even small nubbins
and koppies. Remnants of an older regolith are preserved in low mesas and plateaux which stand
a few metres above the general plain level (
Fig. 4.10)
.
A similar partly-stripped etch surface with
many boulders (corestones) has been exposed just east of O Cadramón, in northeastern Galicia,
