OP WESTERN HERTFORDSHIRE. O 



continued their course over this ridge and found their way 

 independently to the Thames. It is this possibility that I wish 

 to enlarge upon, and to suggest in connection with it the probable 

 early course of some of our rivers. 



The modern theory of the development of such a river-system, 

 as expounded by Professor Davis, Professor Russell, and others, 

 is admirably summarized in Lord Avebury's book. It may be 

 briefly stated as foUows : — As soon as a sloping plane of drainage 

 is established by elevation or depression taking place over a 

 large area, a series of streams will commence to flow, taking the 

 path of least resistance down the slope in roughly parallel lines. 

 Where, as in the present instance, the slope lies across the 

 outcrops of a series of beds of different hardness, the softer beds 

 will be worn away more quickly than the harder, and the valleys 

 will be widened most in the softer parts, giving rise to lateral 

 streams, especially behind a band of harder material. That 

 stream which has the widest area of drainage will be the largest, 

 and will wear down its valley more quickly than the others, and 

 its tributaries will wear back their head valleys, and gradually 

 encroach on the drainage-area of adjacent smaller streams until 

 they actually cut into the main valleys of those streams and 

 draw them off or " capture " them, as it is termed. The valley of 

 a captured river below the point of encroachment then becomes 

 the bed of a much smaller stream, the head of which retreats 

 down the valley as the encroaching stream still further enlarges 

 its drainage-area. The upper part of the beheaded valley may 

 ':hen become the bed of a small stream, flowing in the contrary 

 direction, and therefore called an " obsequent" stream, which falls 

 into the encroaching river. The lower part of the valley will 

 thus become a wide gap, from the highest point of which small 

 streams will flow in both directions. 



There are two such gaps in the ridge of the Tertiary beds near 

 Watford, at Oxhey and Northwood respectively, which are in the 

 direct lines of the valleys of the Gade and the Chess, and only 

 Pibout sixty feet above the present levels of these rivers where 

 they fall into the Colne. My suggestion is that the Gade and 

 the Chess formerly flowed through these gaps in direct 

 continuation of their present courses. The ridge east of 

 Stanmore Common may also have been traversed by the Ver 

 at a time when that river flowed (as it or its predecessors must 

 have done at some remote period) at a height of nearly 400 feet 

 above sea-level ; but in this case there is no well-defined gap at 

 present existing, though there are several minor gaps. The cap 

 of higher Tertiary gravels which surmounts the ridge referred to 

 would provide a hard band behind which such a development 

 of lateral streams would be likely to take place, especially 

 having regard to the fact that the Lower Tertiary sands 

 immediately overlying the Chalk would offer an area in which 

 such a series of lateral streams could form and wear down 

 their beds. 



