XXIV. 



"WATER-LEVELS IX THE CHALK NEAR ROYSTON. 



By Harold Warren-, Assoc. M. Inst. C.E. 



Communicated by II. G. Fordham. 



Read at St. Albans, 6th April, 1897. 



PLATE VI. 



I HATE been asked to bring- before the Society the results I have 

 obtained by taking the water-levels in several of the wells and 

 springs in the north-east of Hertfordshire and south-west of 

 Cambridgeshire. The district embraced, as shown on the map, is 

 about sixty square miles in extent, with lloyston nearly at its 

 centre, and Ashwell at its extreme west. This includes the whole 

 breadth of the Chalk escarpment, which here reaches its highest 

 point at Therfield, and throws out at its base the magnificent line 

 of springs which feed the Eiver Rhee or western branch of the Cam. 



This being a bare chalk area, the plane of saturation almost 

 invariably rises beneath high ground, the springs below increasing 

 or decreasing according to the hydi'ostatic pressure behind them, 

 thus maintaining a general balance. The variation of level due to 

 rainfall is, of coiu-se, felt in the wells on the higher grounds more 

 than in the shallower wells and streams in the valley. 



The wells in which these water-levels were taken all derive 

 their water-supply fi-om the Middle and Lower Chalk, only those at 

 Therfield and Great Chishall being sunk through a few feet of 

 Upper Chalk, while the springs in the valley, as is well known, 

 are thrown out by the Totternhoe Limestone, which everywhere 

 overlies the Chalk Marl in this district. 



For the sake of comparison with other years I show on a table 

 the monthly rainfall at Odsey and Therfield for 1895, and also 



a somewhat broken record of the depth of water in the well at 

 Therfield Rectory for the same period, taken at the end of each 

 month . 



VOL. IX. PART VI. 



15 



