ORIGIN OF SAND DUNES 



49 



ability to drag the stones backward down the slope is less 

 than its forward thrusting power ; thus the pebbl)' sea-walls 

 are much steeper-faced than those of sand. 



The wave-made embankments on the sandy beaches differ 

 in their form and in the conditions of their construction 

 from those which arc made up of pebbles. The sand, 



Carmel Beach, California 



Showing extensive sand-flat with slight dunes above the water level. The foreground also shows traces 

 of blown sand partly imprisoned by herbage. 



owing to the fineness of its grains, is easily blown about by 

 the wind. When the tide retires, a broad expanse of this 

 material is left for some hours exposed to the sun. The sur- 

 face dries, and the gales from the sea sweep the particles up 

 the slope until they arrive at the crest-wall ; here they are 

 often caught in the tangle of beach grasses, and other plants 

 4 



