292 WAVES OF SAND AND SNOW 



Suggested effect of greater heterogeneity of material to 

 increase the Hmiting size of the sand-wave — On the relation 

 of velocity of current to size of sand-wave in water of 

 sufficient depth. 



When visiting Barmouth, North Wales, in Decem- 

 ber, 1899, I was struck by the appearance of the 

 sandbanks left exposed at low water in the lower 

 portion of the Mawdach estuary. Their surface 

 was in waves, the length of which was generally 

 about 20 feet from' crest to crest. The rid'ges, 

 which were sometimes nearly straight and some- 

 times sinuous, extended in broad wave -fronts, and 

 the number which followed one another in un- 

 broken series was often large. The variation of 

 length from one wave to the next was generally 

 about one-quarter of the average wave-length. An 

 unbroken series of forty-five waves was measured 

 on January 9, 1900, on the shoal above the railway, 

 bridge. The waves had their steeper face in the 

 direction in which the flood tide runs . The greatest 

 wave-length was 29 feet, and the least 8 feet. 

 On January i ith eleven consecutive waves were 

 measured on the same shoal below the railway 

 bridge. These also faced in the direction of the 

 flood tide. They varied from 21 to 34 feet in 

 length, and the greatest height was 2 feet 2 inches. 

 This part of the shoal is 2 feet lower than the sand 



