68
SAND DUNES
of excessive rainfall over dune areas in Northern Europe. As the velocity of wind blowing over dunes increases, its relative sand-transporting power advances rapidly in a progressive ratio.1
   Immediately east of the town of Rhyl dunes are in regular course of formation (fig. 13). In 1899 portions of the dunes then forming were levelled, but they have sprung up again, and are a continual source of expense to the town. Between 1901 and 1905 this expenditure averaged ^176 per annum; during the next four years it increased to ^277 per annum, and has since remained at about this figure.2 The expedient hitherto adopted has been that of removal of the sand from the land side of the dunes. The constant encroachment of the drifting sand is a serious nuisance, apart from the expense of its removal.
   A characteristic form often assumed by wandering dunes is the crescent dune or “ barchan In this the concave side of the crescent lies to leeward, the sand is blown up the convex windward face, and falls into the interior of the half-crater as a steep talus. The horns are directed forward because they travel faster than the main body of the dune in consequence of their lower altitude. Barchans are particularly well shown by desert dunes. Indeed, the freaks played by the travel of sand in Arabia are almost incredible. Mr. W. S. Blunt took the measurement of some of the desert “fuljes”. These resemble gigantic pits sunk below the level of the sandy plain. They are horseshoe-shaped excavations, having a maximum depth of 280 feet, rising up to and above the level of the desert in a flat gradient. A shift of wind may quickly obliterate them or cause them to forge ahead.3
   It is a curious fact that records of the wandering of dunes in Europe are for the most part modern, though the phenomenon of wandering, had it been known to them, must have been described by ancient writers. In New Zealand, also, in the early
    1 "The Nature and Formation of Sand Ripples and Dunes” (W. J. Harding Cox), Geographical Journal, Vol. XLVIII, p. 207.
    3	Data furnished by Mr. A. A. Goodall, engineer to the Rhyl U.D.C.
    * For a detailed study of the movements of blown sand, including many of the phenomena of desert dunes, the reader is referred to Vaughan Cornish’s Waves of Sand and Snow (Fisher Unwin, 1914).