28 FISHERIES OF THE NORTH SEA t 



rock and soil are deposited and become 

 the alluvial land at. the mouth of the 

 river. This often extends into and under 

 the sea for many miles and forms such 

 deltas of very fertile land as we see for 

 instance at the mouth of the Nile, Rhone, 

 and Rhine. The bed of the North Sea is 

 partly the result of this action going on 

 for countless centuries. For this reason it 

 is a sea prairie of great fertility and is the 

 prime cause of our wonderful fisheries. 

 The Oder, Vistula, and scores of other 

 rivers descend into the Baltic ; the Rhine, 

 Elbe, Weser flow direct into the North 

 Sea. As these rivers wash down a water- 

 shed of one quarter of Europe, it will be 

 seen how great must be the effect of their 

 sediment on the seabed. The Thames, 

 Trent, Ouse, -and other English streams do 

 the same work on a smaller scale, but 

 sufficiently large to have a considerable 

 effect. In addition to the huge quantity 

 of detritus which these rivers have brought 

 down in past ages, the moraines of the 

 glacial period have left their mark in 

 filling up the bed ; glacial action has also 

 assisted in fertilizing the sea bottom, and 



