154 ORIGIN OF SCENERY OF BRITAIN 



and swallowing up the waste. We have only to con- 

 trast the colour of the Atlantic on the west of Ireland 

 or of Scotland with that of the North Sea, to be assured 

 of the wide diffusion of fine mud in the water of 

 the latter. There is practically no outlet for the 

 detritus that is thus poured into the basin of the North 

 Sea. From the north a vast body of tidal water enters 

 between Scotland and Norway, and travelling south- 

 ward, aided by the strong northerly winds, sweeps 

 the detritus in the same direction. On the other hand, 

 another narrower and shallower tidal stream enters 

 from the Strait of Dover, and, aided by the south- 

 west winds, drives the sediment northward. Yet, 

 making every allowance for the banks and shoals 

 which this accumulating deposit has already formed, 

 we can still, without much difficulty, recognise the 

 broader features of the old land-surface that now lies 

 submerged beneath the North Sea. As already men- 

 tioned (p. 131), it presents two plains or platforms, of 

 which the southern has an average level of perhaps 

 a little more than 100 feet below the surface of the 

 water. This upper plain ends northward in a shelving 

 bank, probably the prolongation of the Jurassic escarp- 

 ment of Yorkshire, and is succeeded by the far wider 

 northern plain, which lies from 100 to 150 feet lower, 

 and gradually slopes northward until it is trenched by 

 the great south Scandinavian submerged fjord. The 

 drainage-lines of the united Rhine, Thames, etc., on 

 the one side, and the Elbe, Weser, etc., on the other 

 can still be partially traced on that sea-floor. The 

 site of the Irish Sea was probably once a terrestrial 

 plain dotted with lakes. This land-surface appears to 



