CHAP. XIV. 1 SUMMARY OP GEOGRAPHIC 



completely masked and buried beneath the Grlacial de- 

 tritus, so that the Pleistocene rivers were compelled to 

 make fresh channels for themselves, and it was not there- 

 fore till this time that our present river-system was per- 

 fected. So long as the British coast-line lay near the con- 

 tour of 80 fathoms our modern streams were but tribu- 

 taries of the three great rivers which ran outside the present 

 limits of Great Britain, that to the east being a continua- 

 tion of the Rhine, that to the south a prolongation of the 

 Seine, and that to the west a river to which no modern 

 name could be applied, but of which all the rivers of 

 western England, southern and eastern Ireland, were the 

 tributaries. 



It was not long, however, before a reverse movement set 

 in, and subsidence caused a rapid recession of the coast- 

 line ; the North Sea crept southward over the plains be- 

 tween England and Holland, while the Atlantic crept 

 round Ireland and up the valley of the English Channel 

 till only a narrow isthmus separated England from France. 

 The Straits of Dover doubtless mark the position of the 

 lowest part of the watershed between the river-systems of 

 the Rhine and the Seine during the progress of this sub- 

 sidence. It is not unlikely that the actual straits are the 

 site of a valley formed by a river which ran northward, 

 nearer France than England, and cut through the northern 

 escarpment of the Chalk, as the Stour and Medway do at 

 the present day ; while a second stream ran south-west- 

 ward, nearer the British coast, and cut through the con- 

 tinuation of the South Downs east of Beachy Head. The 

 watershed between these two rivers is possibly marked by 

 the line of sandbanks which runs southward from the 

 Varne Bank and Colbart Ridge ; it would at any rate be 

 of no great elevation, and probably consisted of Weald 

 clay, like that between the rivers Wey and Arun at the 

 western end of the Wealden district. Subsidence under 



