Page 104 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 87

which may be thousands of kilometres from the point the surface and come to rest. Coarse grains are often
of entrainment. Indeed, dust particles can be carried associated with erosional surfaces, as the fine grains are
around the world (in less than 80 days!) (p. 37). Dust is winnowed by the wind. Fine grains tend to occur on
a somewhat loose term but can be taken as a suspension depositional surfaces. Coarse particles may also move to
of solid particles in the air (or a deposit of such parti- the ground surface from below.
cles, familiar to anyone who has done housework). Most
atmospheric dust is smaller than 100 micrometres and
a large portion is smaller than 20 micrometres. COASTAL PROCESSES

Wind deposition Coastal landforms are fashioned by weathering, by sedi-
ment erosion and transport associated with wave action
Wind moves much sediment around the globe, although and tides, and by sediment deposition. For expediency,
by no means so much as the sediment moved by rivers. it is helpful to distinguish degradational processes from
Some of this sediment, representing 10 per cent of that aggradational processes.
carried by rivers, is delivered to the oceans. The rest
2
falls on land. In Israel, the average fall is 0.25 kg/m /yr
2
but falls of as much as 8.3 kg/m /yr are recorded after Degradational processes
storms. Shoreline weathering
Wind deposition may take place in three ways
(Bagnold 1941): (1) sedimentation, (2) accretion, and The same weathering processes act upon shore environ-
(3) encroachment. Sedimentation occurs when grains ments as upon land environments, but the action of
fall out of the air or stop creeping forward. For sand seawater and the repeated wetting and drying of rocks
grains, this happens if the air is moving with insufficient and sediments resulting from tides are extra factors with
force to carry the grains forward by saltation or to move big effects. Direct chemical attack by seawater takes place
other grains by creep. For silt and clay, this happens if par- on limestone coasts: solution of carbonate rocks occurs,
ticles are brought to the ground by air currents or if the but as seawater is normally supersaturated with respect
air is still enough for them to settle out (dry deposition), to calcium carbonate it presumably takes place in rock
or if they are brought down by rain (wet deposition). pools, where the acidity of the seawater may change. Salt
Wet deposition appears to be significant where dust weathering is an important process in shoreline weath-
plumes pass over humid regions and out over the oceans. ering, being most effective where the coastal rocks are
It is the main processes bringing down Saharan dust in able to absorb seawater and spray. As tides rise and fall,
the Mediterranean region (Löye-Pilot and Martin 1996). so the zone between the low-water mark and the highest
Wet deposition may give rise to blood rains and red limit reached by waves and spray at high tide is wet-
rains. Measured deposition rates on land range from ted and dried. Water-layer weathering is associated with
2
2
3.5 t/km /yr to 200 t/km /yr (Goudie 1995; Middleton these wetting and drying cycles. Biological erosion, or
1997). Accretion occurs when grains being moved by bioerosion, is the direct action of organisms on rock.
saltation hit the surface with such force that some grains It is probably more important in tropical regions, where
carry on moving forward as surface creep, but the major- wave energy is weak and coastal substrates are home to
ity come to rest where they strike. Accretion deposits are a multitude of marine organisms. Tactics employed by
thus moulded by the combined action of saltation and organisms in the erosive process are chemical, mechan-
surface creep. Encroachment takes place when deposi- ical, or a mixture of the two. Algae, fungi, lichens, and
tion occurs on a rough surface. Under these conditions, animals without hard parts are limited to chemical attack
grains moving as surface creep are held up, while saltat- through secretions. Algae, and especially cyanobacteria,
ing grains may move on. Deposition by encroachment are probably the most important bioerosional instru-
occurs on the front of a dune when grains roll down ments on rock coasts. Many other animals secrete fluids