8o
THE FIXATION OF SAND DUNES
methods agree in the employment of materials most readily available, and these in the minimal quantities that will give the required result.
   Generally the cover relied on is dead, i.e. low palisades of trimmed sticks, about i foot high, with 6 inches buried in the ground. These palisades are run in parallel lines 12 feet apart, with similar lines intersecting them at right angles. In this way the ground to be stabilized is covered with a network of squares, the side of each square being 12 feet. One set of lines is orientated parallel to the direction of the prevalent wind, so that the winds may not be liable to blow continually along the diagonals of the squares.
   On steep slopes, such as a dune talus, the sides of the squares may be contracted to 6 feet, or 9 feet squares may be used with additional palisades along the two diagonals. The second of these alternatives requires, however, about 16 per cent more sticks, and, of course, labour in proportion. The
, . , . brushwood 1	1 . ,	,
ratio adopted is -—-r-—• = -, or even - in sheltered places r	interstices 2	4	r
on the lee side of dunes. Pine sticks with the smaller twigs and needles removed are suitable for these palisades, or such other brushwood as may be available. Being of common occurrence on marshy ground, and in old creeks of reclaimed marshes near the sea, the dry stalks of the common reed (Phragmites communis) of the previous season are often employed to make the palisading, and they have the advantage of being easily raised to a higher level in case the sand drifts. They must,
however, be packed closer than the - ratio, as many straws get lost or broken.
   Where seeds are sown direct on the unstable ground, as in Gascony, prostrate cover of pine branches is employed for protection. These are arranged overlapping like the slates on a roof, with the butts to windward.
   Seaweed and sea grass (Zostera) from the drift line are sometimes employed as prostrate cover, as also the small twigs stripped from pine branches in preparing the latter for the palisades. There is, however, no restriction in the possibili-