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). 30
