• 



is 



to 



A- 



Ca. xxil] 



SILTING UP OF ESTUARIES. 



567 



flowing back upon itself, soon forced a passage to the north- 

 ward along the sand}' plain of Capbreton, till at last it 

 reached tn e sea at Boucau, at the distance of seven leagues 

 from the point where it had formerly entered. It was not 

 till the year 1579 that the celebrated architect Lonis de Foix 

 undertook, at the desire of Henry III., to re-open the ancient 

 channel, which he at last effected with great difficulty .* 



In the estuary of the Thames at London, and in the Gironde, 

 the tide rises only for five hours and ebbs seven, and in all 

 estuaries the water requires a longer time to run down than 

 U p ; so that the preponderating force is always in the direc- 

 tion which tends to keep open a deep and broad passage. But 

 for reasons already explained, there is naturally a tendency 

 in all estuaries to silt up partially, since eddies, and back- 

 waters, and points where opposing streams meet, are very 

 numerous, and constantly change their position. 



Many wr 



our eastern 



coast, since the earliest periods of history, has more ^ than 

 counterbalanced the loss ; but they have been at no pains to 

 calculate the amount of loss, and have often forgotten that, 

 while the new acquisitions are manifest, there are rarely any 

 natural monuments to attest the former existence of the land 

 that has been carried away. They have also taken into their 

 account those tracts artificially recovered, which are often of 

 great agricultural importance, and may remain secure, per- 

 haps, for thousands of years, but which are only a 



level of the sea, and are therefore exposed 

 again by a small proportion of the force re- 

 cliffs of considerable height on our shores. 



few feet 



mean 



move 



If it were true that the area of land annually abandoned by 

 the sea in estuaries were equal to that invaded by it, there 



com 



The tidal current which flows out from the north-west, and 

 bears against the eastern coast of England, transports, as 

 we have seen, materials of various kinds. Aided by the 

 waves, it undermines and sweeps away the granite, gneiss, 

 trap rocks, and sand-stone of Shetland, and removes the 



gravel and loam of the cliffs 



Holderness, Norfolk 



* Nouvelle Chronique de la Ville de Bayonne, pp. 113, 139. 1827. 



