DETAILS OF PLANTING
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the transverse Psamma lines are produced a distance of 6 metres. The lines parallel to the fences are omitted, so that no obstacle may be placed in the way of the sand which reaches the crest falling beyond it as a lee talus. The actual crest ultimately forms above the rearward fence — some 2 metres behind the point at which the Psamma network stops. Longitudinal lines of Psamma are undesirable on the crest, as they tend to hold the sand with liability to the formation of projecting ridges or spurs overhanging the lee talus. Such spurs catch the wind and lead to trouble.
   By the second spring, when the surface of the dune will have risen well above the buried fences, Psamma planting may be continued: four more rows at the foot of the dune—two of density type c and two of density D. The transverse lines are also continued an additional 2 metres beyond the lee talus (fig. 15, profile f). By the end of the year the littoral dune has assumed its final form of profile, the slope up to the crest being an even slope all the way (fig. 15, profile g). Had the Psamma been planted in equal density everywhere the outer dune slope would have developed a convex profile with a steep foot towards the sea, inviting undercutting by the waves.
   It is almost unnecessary to add that the upkeep of the littoral dune is of the first importance. All injuries to the surface or edge should be repaired at once by the planting of Psamma alone, or, in serious cases, with the assistance of brushwood or wicker hurdles.
   Crater-like depressions, or “blow-outs”, are very likely to appear on littoral or other dunes, following injury by rabbits, the digging of holes, or wind excavation. Neglected, these blow-outs undergo extension and also tend to migrate, and may prove obstinate to treatment. Here, as in other cases, it is a question of sheltering the surface by adequate cover and planting Psamma.
   The Wandering Dune. — The first step towards fixing wandering dunes is the prevention of movement of the bare sand by the use of cover. As compared with the construction of a littoral dune the problem is relatively simple, as it is not required as well to collect wind-driven sand. In principle all