556 



PRINCIPLES OF STRATIGRAPHY 



ground, or only a few centimeters above it. Since the sand grains 

 of the shore dtmes have in large part an irregular, more or less 

 flattened form, their movement is not a rolling, but a gliding one. 

 The larger grains move only by jerks, and only under the influence 

 of strong gusts of wind. A greater force is needed to start the 

 movement of a grain, which then will continue its progress even un- 

 der a diminished wind force. 



While a steady and continuous wind will cause a uniform and 

 constant onward movement of sand grains, with the result that 

 eventually all the finer particles are removed by the wind, a more 

 or less interrupted series of wind gusts or variable winds will re- 

 sult in the irregular movement of the particles and their temporary 

 arrangement into wave-like forms. On a small scale these consti- 

 tute wind ripples, while the larger ones are the dunes. 



Wind ripples are common on sandy surfaces where the irregu- 

 lar motion of the wind creates eddies and cross currents. (See 

 Chapter XVH for formation of ripples.) With a given wind they 

 vary in height and distance apart, according to the size of the sand 

 grains composing them. Sokolow gives the following table of such 

 relationship : 



The form of the ripples is symmetrical, with a gentler windward 

 and a steeper leeward side. Up the windward side the sand grains 

 are rolled or j)tished until they reach the crest and roll down the 

 leeward side. In this manner the ripples advance in the direction 

 of wind motion, this advance being at a variable rate dependent 

 on the size of the grain and the strength of the wind. Helman 

 (21 '.^84) obtained the following average rates of advance for sand 

 ripples on continental dunes or barchans in the sandy desert of 

 the Khanat of Chiwa in Turkestan : 



Strength of Wind 



(meters per second) 



6 



4 



3 



Average advance of ripples 



in millimeters 



9.0 



5-2 



3-3 



Similar results have been obtained by Sokolow on shore dunes. 



