26 GEOLOGY. 



land, and its movement is therefore more generally interfered with 

 by surface obstacles than is the movement of dust. A shrub, a tree, 

 a fence, a building, or even a stone may occasion the lodgment of sand 

 in considerable quantity, though it has little effect on the lodgment 

 of dust. The effect of obstacles is illustrated by Fig. 7 (see also Fig. 5). 

 If the obstacle which occasions the lodgment of sand presents a sur- 

 face which the wind cannot penetrate, such as a wall, sand is dropped 

 abimdantly on its windward side as well as on the leeward; but if it 

 be penetrable, like an open fence, the lodgment takes place chiefly on 

 its leeward side. In cultivated regions cases are known where, in a 

 few weeks of dry weather, sand has been drifted into lanes in the lee 

 of hedges to the depth of two or three feet, making them nearly impas- 

 sable to vehicles. 



Formation of dunes. — In contrast with dust deposited from the 

 atmosphere, wind-blown sand is commonly aggregated into mounds and 

 ridges in the process of lodgment. These mounds and ridges are dunes. 

 Once a dune is started, it occasions the further lodgment of sand, and 

 is a cause of its own growth. Dunes sometimes reach heights of 200 

 or 300 feet, but they are much more commonly no more than 10 or 20 

 feet in height. On plane surfaces, there is a limit in height above 

 which they do not rise, though the limit is different under different 

 conditions. The velocity of wind at the bottom of the air is not so 

 great as that higher up, and as a dune is built up, a level is presently 

 reached where the stronger upper winds sweep away as much sand 

 as is brought to the top. The very even crests of many dune ridges 

 are probably to be accounted for in this way. Wind-blown or eolian 

 sand, not piled up in heaps or ridges, is somewhat widespread, but 

 does not constitute dunes. 



Shapes of duncs.^ — Dunes may assume the form of ridges or of 

 hillocks. The ridges may be transverse to the direction of the pre- 

 vailing wind or parallel with it. Where dunes assume the form of 

 hillocks rather than ridges, a group of them may be elongate in a direc- 

 tion parallel to the dominant wind, or at right angles thereto. The 

 shape assumed by a dune or a group of dunes depends on the abun- 

 dance of the sand, the strength and direction of the wind, and the 

 shape of the obstacle which occasions the lodgment. 



^ A thoroughgoing study of the Formation of Sand Dunes (by V. Cornish) is to be 

 found in the Geog. Jour., Vol. IX, 1897, pp. 278-309. 



