506 



DEPOSITING POWEE OF TIDAL CURRENTS. 



[Ch. XXII. 



other's force. 



For the current, as we have seen, may, like 

 the river, hold in suspension a large quantity of sediment, 

 or, co-operating with the waves, may cause the progressive 



motion of a shingle beach in one direction, I have already 

 alluded to the erection of piers and groins at certain places 

 on our southern coast, to arrest the course of the shingle and 

 sand (see p. 531). The immediate effect of these temporary 

 obstacles is to cause a great accumulation of pebbles on one 

 side of the barrier, after which the beach still moves on 

 round the end of the pier at a 

 land. This interference, however, with the natural course 

 or movement of the materials of the beach is often attended 

 with a serious evil, for during storms the waves throw sud- 



s 



om 



denly into the harbour the vast heap of pebbles which have 

 collected for years behind the groin or pier, as happened 

 during a great gale (Jan. 1839) at Dover. 



The formation and keeping open of large estuaries are due 



# 



tidal currents and rivers : for 



when the tide rises, a large body of water suddenly enters 



the mouth of the river 



where, becoming confined within 



narrow bounds, while its momentum is not destroyed, it is 

 urged on, and, having to pass through a contracted channel, 

 rises and runs with increased velocity, just as a stream when 



7 e 



it reaches the arch of a bridge scarcely large enough to gh 

 passage to its waters, rushes with a steep fall through the 

 arch. During the ascent of the tide, a body of fresh water, 

 flowing down from the inland country, is arrested in its 

 course for several hours ; and thus a large lake of fresh and 

 brackish water is accumulated, which, when the sea ebbs, is 

 let loose, as on the removal of an artificial sluice or dam. 

 By the force of this retiring water, the alluvial sediment both 

 of the river and of the sea is swept away, and transported 



of the estuary, that a 



to such a distance 



mouth 



small part only can return with the next tide. 



sometimes 



bar of sand is suddenly made to shift its position, so as to 



efflux 



Thus about the year 1500 the sands at Bayonne were sud- 

 denly thrown across the mouth of the Adour. The river, 





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