202
PLANT WINNING OF TIDAL LANDS
than similar ground close by carrying Salicornia annua only. This difference is roughly equivalent to a vertical rise of i foot in about 60 years.
    The method by which these results were obtained was not that of placing the levelling staff on the same spot at convenient intervals of time and comparing the readings with a fixed bench mark. The expansion and contraction to which tidal soils are liable and the varying state of muddiness of the surface render such a method quite unreliable for the determination of small increments. Driving a peg and measuring the length projecting from the surface is also open to objection, as the peg may be trodden on, moved, or pulled out of the ground, and in any event is liable to promote scour and erosion in its immediate neighbourhood, or to collect seaweed and other drifting matter.
    We lay a new surface closely similar in texture to the actual ground, harmless to vegetation, and of a distinctive and permanent colour. For this purpose we experimented with a series of the well-known coloured Alum Bay sands, from which the plum-coloured sand was finally selected as being quite unlike any silt occurring in the Blakeney area.
    The method of procedure was as follows:—
    Places or stations for accretion observations having been selected, e.g. at measured distances along a straight line joining two known points, the coloured sand, previously pulverized in a mortar and passed through a sieve of about 50-60 meshes to the inch, is laid on the ground in circular areas, 6 inches in diameter, to a thickness of 1 millimetre. As a rule at each point these areas are laid in duplicate, right and left of the line and touching one another. That the coloured sand may lie evenly on the ground, and that losses by wind may be avoided, the sand is applied from a conical Cerebos salt-cellar of the ordinary pattern on to a sieve (6 inches in diameter and having 30 meshes to the inch) lying on the ground. The frame of the sieve should have at least 2 inches of freeboard, under cover of which shelter from the wind is obtained during application. From the sieve the sand falls on to the ground.
    For additional assistance in rediscovering the area after lapse of time three little pegs in the circumference of each area should be driven into the ground till almost flush with the surface. Even if entirely bedded over by accretion when the time comes for re-examination the pegs can generally be located by probing. For the recovery of the stations it is of great importance to mark the ends of the line by visible posts, and to measure and book accurately the distances.
    To determine the amount of accretion a vertical slice is cut out from the area and measurement made from the upper surface of the coloured layer to the new surface of the ground. It is essential that the slices so measured be cut vertically, otherwise the results are exaggerated. If the holes made in removing these slices be filled in with mud the same area can be used on future occasions. It is well to avoid replacing the actual slices cut out, as the pigmented zone is difficult of adjustment.