Page 110 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 93

and chelation. The chemical and mechanical action of Eroded materials eventually come to rest. Deposition
animals and plants bring about biological weathering. occurs in several ways to produce different classes
Rock weathering manufactures debris that ranges in size of sediment: clastic (solid fragments), chemical (pre-
from coarse boulders, through sands and silt, to col- cipitated materials), or biogenic (produced by living
loidal clays and then solutes. Weathered debris may things). Sediments accumulate in three main environ-
move downslope under its own weight, a process called ments: the land surface (terrestrial sediments); around
mass wasting. Gravity-driven mass wasting is determined continental edges (shallow marine sediments); and
largely by the relationships between stress and strain in on the open ocean floor (deep marine sediments).
Earth materials, and by the rheological behaviour of brit- Humans are potent geomorphic agents and currently
tle solids, elastic solids, plastic solids, and liquids. Mass move more material than natural processes. Mining
movements occur in six ways: creep, flow, slide, heave, and construction, agricultural practices and land-
fall, and subsidence. Half-mountain-sized mass move- use, and dam building have significant impacts upon
ments are the subject of gravity tectonics.Water runs over sediment fluxes.
the land surface, through the soil and rock (sometimes
emerging as springs), and along rills and rivers. Streams ESSAY QUESTIONS
are particularly effective landform-makers.They conduct
material along their beds, keep finer particles in suspen- 1 Evaluate the relative importance of
sion, and carry a burden of dissolved substances. They factors that affect weathering.
wear away their channels and beds by corrosion, cor-
rasion, and cavitation, and they erode downwards and 2 How does ice flow?
sideways. They lay down sediments as channel deposits, 3 Compare and contrast sediment
channel margin deposits, overbank floodplain deposits, transport by wind and by water.
and valley margin deposits. Episodes of continued depo-
sition and valley filling (alluviation) often alternate with
periods of erosion and valley cutting. Ice abrades and
fractures rock, picks up and carries large and small
rock fragments, and deposits entrained material. Glaciers FURTHER READING
carry rock debris at the glacier base (subglacial debris),
in the ice (englacial debris), and on the glacier surface Allen, P. A. (1997) Earth Surface Processes. Oxford:
(supraglacial debris). They also deposit sediment under, Blackwell Science.
on, and by the side of the moving ice. Meltwater issu- An outstanding account of geomorphic processes.
ing from glacier snouts lays down proglacial sediments. Butler, D. R. (1995) Zoogeomorphology: Animals as
Wind erodes dry, bare, fine-grained soils and sediments. Geomorphic Agents, Cambridge: Cambridge University
It is most effective in deserts, sandy coasts, and alluvial Press.
plains next to glaciers. Wind erodes by deflating sedi- An engaging account of the role of animals in land-
ments and sandblasting rocks. Particles caught by the scape development.
wind bounce (saltation), hop (reptation), ‘float’ (suspen-
sion), or roll and slide (creep). Wind deposits particles Goudie, A. (1995) The Changing Earth: Rates of
by dropping them or ceasing to propel them along the Geomorphological Process. Oxford and Cambridge,
ground. Weathering and wave erosion destroy coast- Mass.: Blackwell.
lines, while sediment deposition, reef-building corals, A good survey of spatial and temporal variations in the
and the mangal and marsh builders create them. rates at which geomorphic processes operate.