io6 



NATURAL SCIENCE. 



August, 



are essential members of the river systems of England to the south 

 and east of the Humber and the Severn, and that the main outlines of 

 this system were determined long before Pleistocene times. If we 

 examine a map of this portion of England (Fig. 3), we see that all 

 the rivers rise in a central area included within a radius of about 

 thirty miles from Rugby. Hence rivers flow north-east into the 

 Wash and the Humber, south-west into the Severn, and south-east 

 into the Thames. If we examine a sketch-map showing these rivers, 

 two facts are clearly brought out : first, that the drainage lines are 

 dependent on the ridges of the Oolites ; and, secondly, that they appear 

 to be independent of the arrangement of the highlands of the chalk 

 in the Chilterns. Thus, while the Thames and the valleys of 

 the Loudwater, Miss, Chess, and Colne run right across the Chalk 

 at right angles to the ridge, and in a south-easterly direction, the 

 rivers of the Oolitic country, on the other hand, flow in the main to 



Ft. 

 Soa 



700 



600 



500 



.400 



300 



soo 



)00 



Burbur/ 

 H.ll ' 



Liddinoton Lamby UfOnoron 

 . ■• Down CoStle 



castle'^ 



R.Thomes 



^4.0 



Fig. I. — Outline of the Berkshire Chalk Escarpment. 



R.Thames Gre^nmoor Cooklfv 

 Hill Oreen' 



ft. 

 500 

 700 

 6qo 

 soo 



tfOO 

 JOO 



;2oo 



CrowelJ Saunderton Wendover 



Hill Gap Hampden Ga|3 



Gap 



Tn"n^ Whips - 



Fig 2. — Outline of, the Chiltern Escarpment, showing the valleys of the Thames, 



Loudwater, Miss, and Colne. 



north-east or south-west, parallel to the ridges. That is to say, the 

 river systems of the Chalk country were antecedent to the hills, while 

 in the Oolitic country they were subsequent or simultaneous. This 

 admits of the simple and probable explanation that the rivers rose on 

 a central plateau and radiated thence in every direction, flowing 

 along the lines of least resistance. Those rivers whose course was 

 parallel to the strike of the beds cut valleys through the soft clays, 

 and left the harder limestones standing out as ridges between them. 

 Those which drained the south-eastern slope had to cut their way 

 at right angles to the strike, and thus formed deep narrow valleys 

 through the Chalk ridge. At first these were probably all good sized 

 streams, but the Thame and the Ouse which flowed in the valley 

 between the Chalk and the Oolites cut the escarpment further and 

 further to the south, and thus drained off the headwaters of the 

 Buckinghamshire tributaries to the Thames. 



