Page 173 - Fundamentals of Geomorphology

156 PROCESS AND FORM

Box 6.1

INVERTED RELIEF

Geomorphic processes that create resistant material Stage 1: Duricrust forms in valley
in the regolith may promote relief inversion. Duri-
crusts are commonly responsible for inverting relief. Duricrust
Oldvalleybottomswithferricreteinthemresisterosion
and eventually come to occupy hilltops (Figure 6.2).
Even humble alluvium may suffice to cause relief inver-
sion (Mills 1990). Floors of valleys in the Appalachian Stage 2: Erosion of unmantled saprock leading
Mountains, eastern USA, become filled with large to scarp retreat and relief inversion
quartzite boulders, more than1min diameter. These
boulders protect the valley floors from further erosion
by running water. Erosion then switches to sideslopes
of the depressions and, eventually, ridges capped with
bouldery colluvium on deep saprolite form. Indeed,
the saprolite is deeper than that under many uncapped Figure 6.2 Development of inverted relief associated
with duricrust formation.
ridges.

pillars, the undercutting produces mushroom, perched
Cliffs and pillars
or pedestal rocks. Processes invoked to account for the
Cliffs and crags are associated with several rock types, undercutting include (1) the presence of softer and more
including limestones, sandstones, and gritstones. Take effortlessly weathered bands of rock; (2) abrasion by
the case of sandstone cliffs (Robinson and Williams wind-blown sand (cf. p. 301); (3) salt weathering brought
1994). These form in strongly cemented sandstones, about by salts raised by capillary action from soil-covered
especially on the sides of deeply incised valleys and talus at the cliff base; (4) the intensified rotting of the
around the edges of plateaux. Isolated pillars of rock are sandstone by moisture rising from the soil or talus; and
also common at such sites. Throughout the world, sand- (5) subsurface weathering that occurs prior to footslope
stone cliffs and pillars are distinctive features of sandstone lowering.
terrain. They are eye-catching in arid areas, but tend to
be concealed by vegetation in more humid regions, such
as England. The cliffs formed in the Ardingly Sandstone, Rock-basins, tafoni, and honeycombs
south-east England, are hidden by dense woodland.
Many cliffs are dissected by widened vertical joints Virtually all exposed rock outcrops bear irregular surfaces
that form open clefts or passageways. In Britain, such that seem to result from weathering. Flutes and runnels,
widened joints are called gulls or wents, which are terms pits and cavernous forms are common on all rock types
used by quarrymen. On some outcrops, the passageways in all climates. They are most apparent in arid and semi-
develop into a labyrinth through which it is possible arid environments, mainly because these environments
to walk. have a greater area of bare rock surfaces. They usually
Many sandstone cliffs, pillars, and boulders are under- find their fullest development on limestone (Chapter 8)
cut towards their bases. In the case of boulders and but occur on, for example, granite.