SAND-WAVES IN TIDAL CURRENTS 331 



a half hours after. Thus the ridges are shehered 

 from the ebb current which runs to the south-west. 



There were two sizes of large sand-waves super- 

 posed ; three succeeding ridges with wave-lengths 

 of 66, 88, and 64I feet had upon their weather 

 slopes smaller waves of about i 5 feet in length. 

 The larger waves were apparently much flattened, 

 and their height was only about i foot 10 inches. 



On one of the sandbanks in the estuary of the 

 River Dovey there were ridges which reminded 

 me by their size of the large ridges on the 

 Goodwins. I found them to be almost immobile, 

 the sand having become set. There are three con- 

 ditions of the mobility of sand — quicksand (of 

 which I shall have something to say later), ordinary 

 loose sand, and sand which has set. Sub -aqueous 

 sand-waves, even those which are periodically un- 

 covered, are less liable to setting than the sand of 

 dunes. 1 



A process occurs in the obliteration of tidal sand- 

 waves which is of interest in connection with the 

 study of dunes. I refer to the formation of pools 

 by the washing of portions of the crest into the 

 trough during the recession of the tide. The 

 longitudinal section of the pools, with the long, 

 gentle slope on the one side and the short, steep 

 slope on the other, is that of the peculiar hollows 



