36 / Basics of Environmental Science
(CLARK, 1977, p. 6). In sheltered areas, plants rooted in the mud trap further sediment. In this way
mangroves extend some tropical coastlines seaward and, in temperate regions, sediment trapped by
saltmarsh vegetation raises the surface until it is beyond the reach of the tides and becomes dry land.
Beach material is transported by currents produced mainly by waves in directions determined by the
angle between the shoreline and prevailing movements of the water. Tides have much less effect. If
the angle between the approaching waves and the shore is less than 90°, water will flow parallel to
the shore, carrying loose beach material with it. This is longshore drift. Its mechanics are complex,
but its effect is to shift material to one end of the beach, where it may be trapped against resistant
material, such as a cliff or groynes built to prevent beach erosion, swept away entirely (and commonly
deposited elsewhere), or deposited where the shoreline turns into the mouth of a bay or estuary and
the current becomes turbulent. Sand or shingle accumulating in this way may eventually rise above
the water as a spit (HOLMES, 1965, pp. 820–822). Longshore drift moves material in such a way as
to rearrange the shoreline so it is at right angles to prevailing waves. This is why waves often appear
to meet the shore at right angles.
Waves are produced mainly by wind. This is obvious enough in the case of the huge waves thrown
against the coast during storms, but less so in the case of the gentle swell that moves against ocean
shores on even the calmest days and in fact is caused by strong winds 1500 km or more from land.
The size of sea waves depends on wind force and the length of time during which it blows, the
distance over which the water is affected (called the ‘fetch’), and the influence of waves that were
generated elsewhere. Some waves are also driven gravitationally. Low atmospheric pressure allows
the sea level to rise beneath it and water flows downhill towards areas where higher pressure produces
a lower sea level.
Beaches are built by waves, and especially by ‘spilling breakers’ which move a long way up the
beach before breaking, spilling a little water from their crests. ‘Plunging breakers’, which collapse
earlier, produce a strong backwash that tends to erode the beach. Where the usual wave pattern is of
spilling breakers arriving at about six to eight a minute, the uppermost part of the beach is likely to
end in a raised mound of coarse sand or pebbles (a ‘berm’).
Where waves crash against a vertical rock face, their force is considerable. It has been measured at
up to 25 t m-2 (SMALL, 1970, p. 438). If the rock is soft, or has many joints or fissures, this is
enough to erode it. In harder rock, it is enough to violently compress air held in crevices and allow
the air to expand again as the water recedes. This weakens the rock and may produce further cracks
into which air enters, until eventually sections are detached as boulders.
Sea cliffs are the result of the wave erosion of hills and as what used to be a hill is cut back, the base
becomes a gently sloping wave-cut platform and the eroded material accumulates just below the
low-tide limit as a wave-built terrace. Figure 2.9 illustrates the process and shows that it is the
ultimate fate of sea cliffs to be eroded completely, until the land slopes gently from the upper limit of
wave action to the low-tide line.
How long this takes depends on the resistance offered by the rock, the degree to which it is exposed
to the full force of the waves, and the topography of the original high ground. In north Cornwall,
Britain, the very impressive sea cliffs have taken around 10000 years to reach their present condition.
The process may never be completed, because the sea level may reverse its present trend and fall
again. This could happen were the ice sheets to advance in a new glaciation. Alternatively, cliff
erosion may accelerate as the sea level rises. In some places the present sea-level rise is due to
erosion, so it is really a sinking of the land rather than a rising of the sea. It is also believed to be
due, in some places exclusively, to the expansion of sea water due to a warming of the sea as