OF SAND FOEMATIONS ON MARINE COASTS. 27 



of the dominant wind .does not take place. It is the gravity which here 

 exercises its force and reduces to the angle of rest any steeper slope 

 caused by the air currents or other factors. The development of a dune 

 is similar to that of a ripple, although it takes place on a larger scale. 

 In lee of the dune crest there is an eddy, the upward motion of which 

 lifts the fine sand particles, and in cooperation with the wind sends 

 them flying from the summit. Gravity acts upon these particles, 

 causing the fall across the stream lines of the air. The coarser sand falls 

 more steeply, and this pitch is further increased by the backward 

 motion of the eddy. 



There are thus several factors which influence the formation of 

 dunes. Of these operating factors the force and direction of the wind, 

 the sand shower, the eddy in the lee of any obstacle, the gravity, the 

 configuration of the surface, and the moisture are the principal ones. 

 If the dune is formed at a certain constant sum-total of these factors, 

 it retains its form as long as these factors are constant. 



THE SCULPTURAL FORM OF DUNES. 



The forms of dunes have a greater variety than those of ripples, 

 because a dune is the result of many changing winds. While the dunes 

 do not owe their origin to the action of the sand grains, like the ripples, 

 still rippling plays a part in the shaping of every dune. Eeversible 

 winds produce dunes having both front and back slope very steep. The 

 first effect of reverse of winds is to turn the top of the dune. !) During 

 strong winds the troughs are always deeper. 2 ) On a hard ground, the 

 windward slope can be as steep as the angle of rest, in case the sand 

 supply fails and tne wind is strong. If such is the case the dunes are 

 widely separated. 3 ) This form differs from that of dunes produced 

 in deep sand by dominant wind. The angle of the windward slope in 

 this case decreases with the density of the sand shower, and increases 

 with the power of the wind. 



The amplitude of the dune does not exceed one third of the wave- 

 length, and this limit is most nearly approached when the wind blows 



1) Sven Hedin: A journey through the Takla-Makan Desert, Chinese 

 Turkestan. Geogr. Journ., VIII: 264278, 356372. 1896. 



2) G. Grand jean: Les Landes et les dunes de Gascogne. Bull. Soc. G6ogr. 

 Coml. de Bordeaux, March 1896. 



3) V. Cornish: On the formation of sand dunes. Geogr. Journ. IX., p. 

 286. 1897. 



