262 WAVES OF SAND AND SNOW 



considerably greater than those of the waves of 

 the sea. When the model estuary was symmetrical, 

 the oscillating currents, which represented natural 

 tides, followed the same course during ebb and 

 flow. In channels wholly submerged even at low 

 water they produced ripples in the sand which were 

 similar to the ripple -mark of oscillating sea -waves 

 in being symmetrical — i.e., having equal slopes on 

 either side ; but they were not so steep, the length 

 being twelve times as great as the height. 



Current-mark. 



The miniature sand-waves seen at the bottom of 

 clear streams have been called " current -mark." 



I have made numerous observations of the sand- 

 ripples in a little stream which flows across a sandy 

 beach close to where I used to live on the Dorset 

 coast, between Bournemouth and Poole Haven, and 

 have from time to time verified these observations 

 in other places. 



The stream issued from a culvert, and flowed 

 at first at a speed of only 0'63 foot per second, with 

 a depth nowhere greater than two -thirds of an 

 inch. The surface of the water here was smooth 

 and unrippled. The sandy bottom was irregular, 

 having pools in places with little cliffs on their up- 

 stream side one -third of an inch in height. The 



