SAND-WAVES IN TIDAL CURRENTS 315 



the tidal entrance being up the straight stroke, 

 while the space enclosed by the bow of the letter 

 is encumbered by sandbanks which are covered 

 and uncovered during the rise and fall of the tide. 



From the high hills on the north shore a good 

 view is obtained of the whole estuary, and at low 

 water the arrangement and direction of the sand- 

 waves on the surface of the shoals is so clearly seen 

 that they form an excellent map of the strength 

 and direction of the tidal currents above low-water 

 mark. The sand-waves which face with the flood 

 have generally broad, straight fronts, those facing 

 with the ebb are more sinuous. 



On a sandbank called Traeth Malgwyn, in a 

 position bearing south from the north end of the 

 Pier and north-east-by-east from' the Refuge, I 

 pegged out a plot 60 feet square, putting in five 

 rows of stakes parallel to the ridges, four stakes 

 in each row, driving them 3 feet into the sand. 

 Threads, stretched from stake to stake at the two 

 sides of the plot, were used for the measurement of 

 the height of the waves from trough to crest, and 

 also served as a datum by which to ascertain 

 whether there were any change in the mean level 

 of the sand surface.- The height and length of 

 the waves were measured when the sands were 

 uncovered from June ist to 17th inclusive, a period 



