AND OTHER WATER WAVES 183 



Channel. Now, wherever there is much shingle 

 it accumulates mostly near the highest level reached 

 by the wash from the breakers . In such situations, 

 therefore, most of the shingle is out of reach of 

 breakers during the outflowing current of the tide. 

 Moreover, 'for the same roughness in the offing, 

 the breakers are very much smaller when the water 

 is below mean sea -level, on account of the gentler 

 slope of this part of the beach. Thus it is evident 

 that for straight coast and for headlands the effect 

 of the tides (apart altogether from the wind-currents 

 and from oblique waves) is to cause the waves to 

 transport shingle predominantly in the direction of 

 the inflowing current, or up-channel. The pre- 

 dominance should be most marked in the case of 

 the larger shingle. This predominance is inde- 

 pendent of any excess of speed of the inflow 

 current. Where the tide is impeded, the course 

 of affairs is somewhat different. Thus, at the head 

 of a bay or inlet, or where, at a nodal point, tidal 

 currents from opposite directions have met, a 

 " head " of water accumulates which stems the in- 

 flowing tidal current and causes the outflow to com- 

 mence sooner than it otherwise would. The 

 extreme case is that in which the outflow com- 

 mences immediately the highest level is reached. 

 In this case it is possible for the average water- 



