250 WAVES OF THE SEA 



calm, being unaccompanied by any wave or other 

 commotion. 



It remains to explain why the flood tide, at its 

 first rise, preserves alternative channels in the lower 

 part of the steep slope below Severn Bridge and 

 not in the upper, or only to a limited extent, for 

 on the absence of side channels depends the possi- 

 bility of forming a bore where the other circum- 

 stances (shallowness and a fairly steep gradient) 

 are present. 



The sands which encumber the bed of the Severn 

 from Hock Cliff to the Sheperdine Shoal are sub- 

 ject to arrangement and redistribution alternately by 

 the waters which flow seaward and by those which 

 flow landward. In the upper part of this region 

 the sands are arranged so as to form a channel 

 almost like that of a non -tidal river flowing in an 

 alluvial plain. Particularly is this the case when 

 the land water much predominates, as during neap 

 tides and after heavy rains. The ebbing stream, 

 flowing continuously for many hours in its narrow 

 winding channels, continually lowers its bed, scour- 

 ing the sand from beneath and forming a channel 

 which, at the time of first rise, has high, steep 

 banks. The first rise of the flood tide has there- 

 fore, as a rule, to make its way against this ebbing 

 stream up the normal river channel which has been 



