Page 108 - Sedimentology and Stratigraphy

Denudation and Landscape Evolution 95

move with ocean currents, heat absorbed by land obstacle abrasion occurs but on the down-flow side
masses and cold air over high glaciated mountain the ice dislodges blocks that range from centimetres to
regions. A complex and shifting pattern of regions of metres across. The blocks plucked by the ice and
high pressure (anticyclones) and low pressure subsequently incorporated into the glacier are often
(depressions) regions generates winds all over the sur- loosened by subglacial freeze–thaw action (6.4.1).
face of the Earth. Winds experienced at the present The landforms created by this combination of glacial
day range up to storm force winds of 100 km h 1 to abrasion and plucking are called roche moutone ´e,
hurricanes that are twice that velocity. apparently because they resemble sheep from a
Winds are capable of picking up loose clay, silt and (very) great distance.
sand-sized debris from the land surface. Wind erosion
is most effective where the land surface is not bound
by plants and hence it is prevalent where vegetation is 6.6 DENUDATION AND LANDSCAPE
sparse, in cold regions, such as near the poles and in EVOLUTION
high mountains, and dry deserts. Dry floodplains of
rivers, sandy beaches and exposed sand banks in The lowering of the land surface by the combination
rivers in any climate setting may also be susceptible of weathering and erosion is termed denudation.
to wind erosion. Eroded fine material (up to sand Weathering and erosion processes are to some extent
grade) can be carried over distances of hundreds or interdependent: it is the combination of these pro-
thousands of kilometres by the wind (Schutz 1980; cesses that are of most relevance to sedimentary geol-
Pye 1987). The size of material carried is related to ogy, namely the rates and magnitudes at which
the strength (velocity) of the air current. The pro- denudation occurs and the implications that this has
cesses of transport and deposition by aeolian processes on the supply of material to sedimentary environ-
are considered in Chapter 8. ments. Rates of denudation are determined by a com-
bination of topographic and climatic factors, which in
turn influence soil development and vegetation, both
6.5.4 Erosion and transport by ice of which also affect weathering and erosion. In addi-
tion, different bedrock lithologies respond in different
Glaciers in temperate mountain regions make a very ways to these combinations of physical, chemical and
significant contribution to the erosion and transport biological processes.
of bedrock and regolith. The rate of erosion is between
two and ten times greater in glaciated mountain areas
than in comparable unglaciated regions (Einsele 6.6.1 Topography and relief
2000). In contrast, glaciers and ice sheets in polar
regions tend to inhibit the erosion of material because A distinction needs to be made between the altitude
the ice is frozen to the bedrock: movement of the ice in of a terrain and its relief, which is the change in
these polar ‘cold-based’ glaciers is mainly by shearing the height of the ground over the area. A plateau
within the ice body (7.2.1). In temperate (warm- region may be thousands of metres above sea level
based) glaciers, erosion of the bedrock by ice occurs but if it is flat there may be little difference in the rates
by two processes, abrasion and plucking. of denudation across the plateau and a lowland
Glacial abrasion occurs by the frictional action of region with a comparable climate. With increasing
blocks of material embedded in the ice (‘tools’) on the relief the mechanical denudation rate increases as
bedrock. These tools cut grooves, glacial striae,in erosion processes are more efficient. Rock falls
the bedrock a few millimetres deep and elongate par- and landslides are clearly more frequent on steep
allel to the direction of ice movement: striae can slopes than in areas of subdued topography: stream
hence be used to determine the pathways of ice flow flow and overland water flow are faster across steeper
long after the ice has melted. The scouring process slopes and hence have more erosive power. A deeply
creates rock flour, clay and silt-sized debris that is incised topography consisting of steep sided valleys
incorporated into the ice. separated by narrow ridges provides the greatest
Glacial plucking is most common where a glacier area of steep slopes for bedrock and regolith to be
flows over an obstacle. On the up-flow side of the eroded.