Geology Reference
In-Depth Information
Box 6.1
INVERTED RELIEF
Geomorphic processes that create resistant material
in the regolith may promote
relief inversion
. Duri-
crusts are commonly responsible for inverting relief.
Old valley bottoms with ferricrete in them resist erosion
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
Mountains, eastern USA, become filled with large
quartzite boulders, more than1mindiameter. 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
ridges.
Stage 1:
Duricrust forms in valley
Duricrust
Stage 2:
Erosion of unmantled saprock leading
to scarp retreat and relief inversion
Figure 6.2
Development of inverted relief associated
with duricrust formation.
pillars, the undercutting produces
mushroom, perched
or
pedestal rocks
. Processes invoked to account for the
undercutting include (1) the presence of softer and more
effortlessly weathered bands of rock; (2) abrasion by
wind-blown sand (cf. p. 301); (3) salt weathering brought
about by salts raised by capillary action from soil-covered
talus at the cliff base; (4) the intensified rotting of the
sandstone by moisture rising from the soil or talus; and
(5) subsurface weathering that occurs prior to footslope
lowering.
Cliffs and pillars
Cliffs and crags are associated with several rock types,
including limestones, sandstones, and gritstones. Take
the case of sandstone cliffs (Robinson and Williams
1994). These form in strongly cemented sandstones,
especially on the sides of deeply incised valleys and
around the edges of plateaux. Isolated pillars of rock are
also common at such sites. Throughout the world, sand-
stone cliffs and pillars are distinctive features of sandstone
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,
south-east England, are hidden by dense woodland.
Many cliffs are dissected by widened vertical joints
that form open clefts or passageways. In Britain, such
widened joints are called
gulls
or
wents
, which are terms
used by quarrymen. On some outcrops, the passageways
develop into a labyrinth through which it is possible
to walk.
Many sandstone cliffs, pillars, and boulders are under-
cut towards their bases. In the case of boulders and
Rock-basins, tafoni, and honeycombs
Virtually all exposed rock outcrops bear irregular surfaces
that seem to result from weathering. Flutes and runnels,
pits and cavernous forms are common on all rock types
in all climates. They are most apparent in arid and semi-
arid environments, mainly because these environments
have a greater area of bare rock surfaces. They usually
find their fullest development on limestone (Chapter 8)
but occur on, for example, granite.
