A HISTORY OF HERTFORDSHIRE 



tion and to raise this plane to a steeper inclination than that of the 

 bottom of their usually dry valleys, this frequently happening some time 

 after all heavy rain has ceased ; they cease to flow when they have 

 conveyed away a sufficient quantity of water to reduce the level of the 

 plane of saturation to that of their beds. Our Hertfordshire Bourne 1 is 

 a tributary of the Bulbourne, into which it flows, occasionally, at Bourne 

 End, a small hamlet about half-way between Berkhamsted and Boxmoor. 

 It sometimes has its source about four miles up its valley, and it has been 

 known to run in such a powerful stream as to overflow the usually dry 

 culvert under the road at Bourne End, and to flood this road. The 

 Bourne flowed about once in every seven years between 1852 and 1873, 

 and about once every alternate year from 1873 to 1883. It has only 

 flowed since then in 1897, after an interval of quiescence of fourteen 

 years. On each of these occasions the mean rainfall in Hertfordshire for 

 the twelve months ending 3151 March of the year of flow exceeded 30 

 inches. 



The ' bourne,' if such it may be called, which occasionally forms 

 the source of the Colne, is one of a very different kind. For a certain 

 distance it flows over the London Clay and therefore always runs with or 

 after rain, but where it leaves this impervious bed for the Chalk it usually 

 ends, at least on the surface, giving to that place the name of ' Waterend.' 

 It disappears in a ' swallow-hole ' in the Chalk. If this cannot take it 

 all there is another ready a little farther on, and so on as far as the 

 swallow-holes at Potterells near North Mimms. Seldom does any water 

 get beyond these great chasms, down one of which at least a man might 

 be carried ; but sometimes they cannot take it all, not because they have 

 not sufficient capacity, but because they are full owing to the plane of 

 saturation having risen in the Chalk up to their capacious mouths. Then 

 there is a flood, the river forms a lake hiding the swallow-holes from 

 view, and the bed of the Colne, dry for some distance below this point 

 year after year, is unable to carry off all the water, its banks overflowing, 

 submerging the meadows, and rendering some of the roads between 

 Colney Heath and Smallford impassable. The water which sinks into 

 these swallow-holes is probably conveyed in channels in the Chalk into 

 the lower part of the valley of the Lea, for that would be its direction if 

 it follows the dip of the Chalk. There are several interesting questions 

 connected with this phenomenon which have been discussed elsewhere. 2 



We have also many valleys, sometimes several miles in extent, down 

 which rivers have not been known to run in historic times. Such dry 

 valleys are merely elongated Chalk combes. They were probably 

 formed when the impermeable Tertiary beds extended over the permeable 



1 Evans, 'The Hertfordshire Bourne,' Trans. Watford Nat. Hist. Soc., vol. i. p. 137 

 (1877) ; Littleboy, 'The River Bourne,' Trans. Herts Nat. Hist. Soc., vol. ii. p. 237 (1883); 

 Hopkinson, ' The Chadwell Spring and the Hertfordshire Bourne,' op. cit. vol. x. p. 69 (1899). 

 The above explanation of the flowing of the Bourne is from the paper by Sir John Evans. 



8 Hopkinson, 'The River Colne and the Swallow-holes at Potterells,' Trans. Herts. 

 Nat. Hist. Soc., vol. vi. p. xxix. (1892). 



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