296 WAVES OF SAND AND SNOW 



when the shoals are covered by a considerable 

 depth of water. 



Shortly after my visit to Barmouth I went to 

 Scotland in order to take advantage of a spell 

 of severe weather for the study of snowdrifts. 

 When the thaw came I sought upon the map of 

 Scotland for estuaries of a form similar to that of 

 the Mawdach, and visited those of the Findhorn 

 River^ and of the South Esk, at Montrose, which 

 were shown to have the required shape. In both 

 estuaries I found drying sandbanks covered with 

 the large kind of sand-waves. In the wide in- 

 terior basin of the estuary at Montrose some of 

 the drying banks were covered with mussels, whilst 

 others were of clean sand. Of the latter, one, 

 near the main channel by which the tide ebbs, was 

 in large waves, facing with the ebb — i.e., seawards. 

 I measured here on March 6, 1907, a series 

 of fifteen waves, which had an average length of 

 13 feet 2*4 inches and an average height of 

 1 1" 8 3 inches. The variation of length from one 

 wave to the next was 22*3 per cent, of the 

 average wave-length, so that their departure from 

 perfect regularity was about the same as that of 

 the sand-waves upon the shoals of the Mawdach 

 estuary at Barmouth. 



The mean difference between the heights of con- 



