74
THE FIXATION OF SAND DUNES
   The general method of procedure is to erect two brushwood fences parallel to the shore, from 20-100 yards distant therefrom (fig. 15, a). These fences soon collect a bank of sand, and when they are buried two more fences are placed above them (fig. 15, profiles B, c). When these in their turn have been buried the resulting bank of sand is planted with Psamma (d, e). The fences determine the position of the crest; all further catching of sand and growth of the dune is carried out by the Psamma alone.
   It is generally possible to plant the dune with Psamma by the end of the first or beginning of the second year. By the end of the second year a functional littoral dune should be in existence. Great importance attaches to the form of the profile or section of the dune; the slope up from the shore must be at a gentle angle and quite uniform, the crest broad and flat. The equal growth of the slope is determined by the density of planting, the spacing of the Psamma being more open towards the shore and closer as the crest is approached (for stages see fig. 15, E, F, g). By this device the natural tendency of the foot of the slope to hump is eliminated. Local irregularities are corrected by removal of Psamma; the wind does the rest.
   The upkeep of the littoral dune is of the first importance; injuries to its surface from storms, rabbits, or other causes must be repaired at once by the planting of Psamma. When a littoral dune grows so high that its maintenance becomes difficult it is probably time to see about the building of a new one on the seaward side.
   The littoral dune is thus an artificial product, designed and regulated by man with the aid of plants and the play of physical forces. It is one of the objects of this book to draw attention to this inherent plasticity of all tidal lands (sand, shingle, mud), so that with further study the possibility of far-reaching control in their relief may be attained. The relief of tidal lands differs from that of the mainland, in that it depends primarily on the circumstances under which the unit particles are assembled. The relief of the mainland, on the other hand, is determined by the agents of erosion; it is what survives when these agents have taken their toll. The difference is comparable to that