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A regular current may drift suspended earth and seaweed along a coast until they are caught by an eddy and finally deposited out of the reach of further disturbance, or it may scoop out the bed of the sea and undermine promontories and headlands; a powerful river, as the wind changes the direction of its flow at its outlet, may wash away shores and sandbanks at one point to deposit their material at another; the tide or waves, stirred to unusual depths by the wind, may gradually wear down the line of coast, or they may form shoals and coast-dunes by depositing the sand they have rolled up from the bottom of the ocean.
They spaced themselves out, dragged themselves forward one at a time, took advantage of every slightest depression, every wrinkle in the sandy desert-floor, every mummy-like acacia and withered tamarisk-bush, some sparse growth of which began to mingle with the halfa-grass as they passed from the coast-dunes to the desert itself.
These embankments are constructed of sand from the coast-dunes or from sand-banks, and of earth from the mainland or from flats outside the dikes, bound and strengthened by fascines, and provided with sluices, which are generally founded on piles and of very expensive construction, for drainage at low water.
Two forms of sand deposit are specially important in European and American geography. The one is that of dune or shifting hillock upon the coast, the other that of barren plain in the interior. The coast-dunes are composed of sand washed up from the depths of the sea by the waves, and heaped in more or less rounded knolls and undulating ridges by the winds.
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