RIVERS AND LAKES 147 



The river, however, having fixed its course in the 

 Chalk, has cut its way down into it, and now seems 

 as if it had broken a path for itself across the escarp- 

 ment. As all the escarpments are creeping eastward, 

 the length and drainage area of the Thames are neces- 

 sarily slowly diminishing. 



The Severn presents a much more complex course ; 

 but its windings across the most varied geological 

 structure are to be explained by its having found a 

 channel on the rising floor of Secondary rocks between 

 the base of the Welsh hills and the nascent Jurassic 

 escarpments. The Wye and Usk afford remarkable 

 examples of the trenching of a tableland. The Tay and 

 Nith are more intricate in their history. The Shannon 

 began to flow over the central Irish plain when it 

 was covered with several thousand feet of strata now 

 removed. In deepening its channel it has cut down 

 into the range of hills north of Limerick, and has 

 actually sawn it into two. 1 



THE LAKES. 2 



The Lakes of Britain present us with some of the 



most interesting problems in our topography. It is 



obvious that the existence of abundant lakes in the 



more northern and more rocky parts of the country 



1 1 have shown that some of the wildest glens of the Highlands 

 and the deepest dales of the southern Uplands of Scotland have been 

 hollowed out since early Tertiary time by the various streams that 

 still flow in them. Scenery of Scotland, 3rd ed., pp. 162, 339, 361. 



2 The Scottish Lakes have in recent years been made the subject 

 of detailed study by Sir John Murray and Mr. F. Pullar. The 

 results of their investigations have appeared in the Geographical 

 Journal. 



