Masterpieces of Science 



narrow seas, it becomes converted into an actual 

 wave of translation. Where the channel is 

 contracted, as in a narrow strait, the tide may 

 produce a rapid rush of water, or a race. If, 

 again, the tidal wave rolls into a narrow estuary, 

 the water becomes heaped up, and produces a 

 sudden rush into the channel of the river: such a 

 wave is called a bore, and is well seen in the 

 Bristol Channel, at the mouth of the Severn, 

 where at certain seasons the head of water attains 

 to as great a height as forty feet. 



In the estuary of a tidal river, the tide periodi- 

 cally agitates the water; and thus hinders deposi- 

 tion 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 riv- 

 ers; and, even in the estuary of the Thames, the 

 shifting shoals indicate similar depositions. But 

 the ebb-tide, by scouring out the estuary, pre- 

 vents the formation of a true delta. 



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 may 

 be 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 land. 

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