4 8 



PHYSICAL GEOLOGY 



deposit which vary in direction are called, is clear when the conditions 

 of their formation are considered. If the direction and force of the 



wind remain constant, the 

 sand will be carried up the 

 gentle slope and will fall over 

 the steep slope, forming lay- 

 ers of uniform inclination. If, 

 however, as is nearly always 

 the case, the force and direc- 

 tion of the wind vary, the 

 sand will be laid down on 

 difFerent sides of the dune at 

 different times. In either 

 case cross-bedding will be 

 produced, but it will be more 

 irregular in the second case 

 than in the first. 



Shape and Origin of 

 Dunes. — When winds are 

 moderate and the supply of 

 sand is small, crescent-shaped 

 dunes (Fig. 27 A) are formed ; 

 if the wind is moderate but the supply of sand great, dune ridges are 

 often developed which are at right angles to the direction of the wind 

 (Fig. 27 B) ; while in regions where the prevailing winds are strong 

 and the sands abundant, long ridges parallel to the direction of the 

 wind usually result (Fig. 27 C). The crescent shape of dunes results 

 when wind, blowing over a sandy stretch, heaps the sand into small 

 piles. The grains of sand which are subsequently carried to the piles 

 are deviated to the right and left, until crescents are formed. When 

 the direction of the wind changes, the points of the crescents dis- 

 appear and then form on the lee side in a new direction. 



Any obstacle, such as a bush, a rock, a fence, or even a mere rough- 

 ness of the land surface, may cause the beginning of a dune. Dunes 

 are also sometimes formed when the sand is wet by slow springs. 

 These moist heaps serve to anchor additional particles of sand until 

 a mound some feet in height is formed, which may afford lodgment for 

 shrubs. The presence of obstacles is, however, not always essential 

 to the formation of dunes. This can be observed on a small scale on a 

 smooth asphalt street, where the dust is seen to be collected into small 



Fig. 26. — A quarry in eolian limestone, 

 Bermuda Islands. The cross-bedding was 

 formed by the shifting winds which carried the 

 sand. 



