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THE FIXATION OF SAND DUNES
obtaining between a bust modelled out of plaster and one carved by the chisel from a block of marble. It is sufficiently important to receive recognition by a special terminology.
  The Littoral Dune: Details of Methods. — The initial mound, the nucleus of the littoral dune, is created by placing two permeable fences, 6 feet apart, parallel to the shore and at a distance from it corresponding to the position at which the crest of the littoral dune is required. The dune crest will ultimately form over the hinder of the two fences. The sand-catching fences consist of untrimmed pine branches, or other brushwood, standing about 2 feet 4 inches in height and bedded in the ground about 12 inches. The density of the fence should
be such that the ratio brushwood = -. It is important that the interstices 1	r
top of the fences be level. The same method is employed to close any wind-cut gully or passage that may be present on a dune, except that more than two fences should be used with greater intervals between. Wattle hurdles or rude plank fences will produce the same result, but it is not necessary to use expensive appliances where simpler means are available, especially as the hurdles become buried and are non-recoverable. In Gascony this drawback is partially met by making the front fence of 6-inch planks stood vertically to form a palisading with i-inch intervals between. When nearly buried these planks are raised to the necessary height and continue to serve. The back fence, which catches the sand that blows through, and gives a better profile to the dune, consists of hurdling; when buried it must be replaced.
   If the fences are planted in early spring they should be buried by the summer, when new brushwood must be installed. It is quite remarkable how quickly a brushwood fence becomes buried under favourable conditions. We have seen a mound
3	feet high collect in less than a week. By autumn the upper tier of fences should be buried (fig. 15, profile d) and the first planting of Psamma can be put through.
   To understand the method of planting adopted it is well to remember that the object in view is the equal collection of sand everywhere as it drifts and the holding of it in place.