6o
SAND DUNES
amount of shelter which the obstacle provides. When this shelter is satisfied no further expansion will take place, as there will be equilibrium between the amounts of sand brought and removed by the wind. A brushwood fence planted in a sandladen windway will collect a bank of sand not exceeding the height of the fence. On the other hand, a living plant which continually grows up through and projects beyond the sand heap which it collects will raise a dune of considerable proportions. This result, observable on any system of sand dunes, was very concisely described by the late Sir Francis Galton in speaking of the ’Nara, a prickly gourd (Acanthosicyos) at Wal-fisch Bay. “ It (the ’Nara) is a very useful agent towards fixing the sands; for as fresh sand blows over and covers the plant, it continually pushes on its runners up to the air, until a large hillock is formed, half of the plant, half of sand.”1
   This is the essential principle on which primary dune systems arise. For simplicity, the case may now be considered of dunes arising on a growing coast-line, i.e. a shore advancing seawards by the accretion of successive terraces of shingle or sand which reach above high-water mark. Grasses, such as Marram (Psamma arenaria) and Triticum pungens, or the Sea Sandwort (Arenaria peploides), arise directly upon the shingle from seed left in the tidal drift, and around these sand collects to form embryo dunes (PI. VIII, i, p. 84). The plants push through to the surface and more sand is collected, and the little dunes grow in height. Some plants (Triticum and Arenaria peploid.es) have quite limited powers of vertical extension, and unless some other species comes to their aid such dunes will remain dwarf, but Psamma is endowed with capacity for indefinite growth, growing vertically and spreading horizontally (fig. 12). In this way the embryo dunes expand both vertically and laterally till they coalesce into dune systems, and the growth continues till the dune systems form ranges of Psamma-covered sand-hills. The rate of expansion depends largely on the amount of sand blown up from the foreshore—a vertical rise of a foot a year being of normal occurrence. As the Psamma never forms a close turfy covering, the bare surface remaining
     1 F. Galton, The Narrative of an Explorer in Tropical South Africa, p. 22. 1853.