XI.] THE SEA AND ITS WORK. i8i 



seasons the head of water attains to as great a height as 

 forty feet. 



In the estuary of a tidal river, the tide periodically agitates 

 the water; and thus hinders deposition of sediment. The 

 flow of the river seawards is, however, checked every time 

 the tide comes in, and sediment is then deposited ; hence, 

 bars, or banks of sand, are common at the mouths of rivers ; 

 and, even in the estuary of the Thames, the shifting shoals 

 indicate similar depositions. But, it has been shown in a 

 former chapter, that the ebb-tide, by scouring out the 

 estuary, prevents the formation of a true delta. The posi- 

 tion of the principal sands in the estuary of the Thames 

 between the Nore and Margate, is shown in Fig. 48, which 

 is reduced from a part of the Admiralty chart. 



The sediment which the tidal water carries away from 

 the mouth of a river at one part of the coast may be 

 deposited at another point, and thus the sea may become 

 a constructive agent charged with the formation of new 

 land. Usually, however, the suspended matter swept away 

 by the ebb-tide is carried out to sea, where it maybe caught 

 up by currents and thus drifted to a great distance. Hence 

 the tides and currents assist greatly in distributing the solid 

 matter derived from the waste of the land. 



Putting together what has been said in this chapter with 

 reference to the action of the sea upon the land, it may be 

 concluded that its work, on the whole, is a work of destruc- 

 tion, yet not exactly like that of rain and rivers. To 

 observe this difference, it must be borne in mind that marine 

 denudation is not equally active at all depths of the sea. 

 The waves, as explained above, indicate only superficial 

 agitation, and have no effect on deep water. Most of the 

 destruction wrought by the sea is consequently confined 



