CHAPTER V
Sand Dunes
   Whilst the term Dune is applied generally to hills or other topographical features of which the materials have been assembled by the winds, and which are still, or were till a comparatively recent period, subject to modification of relief by the same agency, consideration will here be restricted to dunes composed of sand, and in the main to those of the littoral zone. If the estimate by a competent authority be accepted that as much as seven per cent of the earth’s surface is dune-encumbered, it is evident that the coastal sand dunes can form but a fraction of the whole area. Though not strictly coming within the category of “ tidal lands”, the sand dunes of the shore naturally fall within the subject of this book. They abut on and delimit many tidal lands, and are important in relation to defence from the sea. Moreover, as they readily lend themselves to stabilization and reclamation by planting—in connection with which a highly elaborate technique has sprung up—it is evident that the subject must receive the attention of the maritime engineer.
   In effect, the sand dune is reproduced in miniature in the ripple marks of every seashore, and it can be simulated by the action of the sand blast. If the particles of sand on a shore were all exactly the same size and weight, no ripple marks would be produced. These are due to slight variations in the weight of sand grains which are whirled into suspension by the wind and heaped up by reflex action. The wind, flowing in lines parallel to the surface, on passing the crest of any irregularity or obstruction sets up a return eddy, lifting the lighter particles to the top of the ridge so produced. Its action is
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