Page 165 - Fundamentals of Geomorphology

148 STRUCTURE

no signs of deep weathering, and stripping and scarp fade with time. Flat sedimentary beds and folded sed-
retreat seem unlikely as formative mechanisms. A third imentary rocks produce distinctive suites of structural
possibility is mantle planation (Ollier 1978). In this landforms. Flat beds tend to form plateaux, mesas, and
environment, weathering attacks any rock protruding buttes. Folded beds produce a range of landforms includ-
above the ground surface, levelling them off to create ing anticlinal hills, cuestas, and hogbacks.They also have
a plane surface littered with a mantle of debris. Succes- a strong influence on the courses of some rivers and pat-
sive bevelling episodes of mantle planation would reduce terns of drainage. Faults and joints are foci for weathering
the level of the plains, leaving pockets of more durable and produce large-scale landforms. Dip-slip faults may
rock as high-standing residuals with their boundaries produce fault scarps, grabens, horsts, and tilted blocks.
corresponding with geological boundaries. Interestingly, Strike-slip faults are sometimes connected with shutter
therefore, three different suites of processes may pro- ridges, sag ponds, and offset drainage. Joints have a strong
duce the same suite of landforms, a case of convergent influence on many landforms, including those formed
landform evolution. on granite. Characteristic forms include bornhardts
Tors, which are outcrops of rock that stand out on all and tors.
sides from the surrounding slopes, are probably formed
in a similar way to bornhardts.They are common on crys-
talline rocks, but are known to occur on other resistant
rock types, including quartzites and some sandstones. ESSAY QUESTIONS
Some geomorphologists claim that deep weathering is a
prerequisite for tor formation. They envisage a period 1 Explain the landforms associated
of intense chemical weathering acting along joints and with folding.
followed by a period when environmental conditions 2 Explain the structural landforms
are conducive to the stripping of the weathered mate- associated with rifting.
rial by erosion. Other geomorphologists believe that tors
can develop without deep weathering under conditions 3 To what extent do landforms result
from ‘tectonic predesign’?
where weathering and stripping operate at the same time
on rocks of differing resistance.

SUMMARY
FURTHER READING
Plutonic and hypabyssal forces intrude molten rock
(magma) into the deep and near-surface layers of the Ollier, C. D. (1981) Tectonics and Landforms. Harlow,
Earth respectively, while volcanic forces extrude it on Essex: Longman.
to the Earth’s surface. Volcanic and plutonic landforms Old, given the pace of developments in the subject, but
arise from the injection of magma into rocks and the still a good read for the novice geomorphologist.
effusion and ejection of magma above the ground.
Intrusions include batholiths and lopoliths, dykes and Scheidegger, A. E. (2004) Morphotectonics. Berlin:
Springer.
sills, laccoliths and phacoliths, all of which may express Looks at the relationships between landform and tecton-
themselves in topographic features (hills, basins, domes, ics. Unusual and interesting.
and so on). Extrusions and ejections produce volca-
noes of various types, which are tectonic landforms. Sparks, B. W. (1971) Rocks and Relief. London:
Impacts by asteroids, meteoroids, and comets pock the Longman.
Earth’s surface with craters and impact structures that Very old by almost any criterion, but worth a look.