THE UNDULATIONS OF THE CHALK IM 

 ESSEX. 



\W \V. H. DAl.TON, F.G.S , /<iU of H.M. Ceolosical Sun'cy. 

 [Read May lyth, iSgo.] 



WITH MAT, ri.ATE III 



A S the surflice of the greater part of Essex consists of clay, the 

 water-supply is almost everywhere derived from wells, and a 

 considerable proportion of these have been carried down to the 

 Chalk, and derive their value from the copious stores of water yielded 

 by that formation. There are, of course, hundreds of shallow wells 

 in the gravel areas, and a great many artesian borings that go no 

 further than the sands in or under the London Clay. But the water 

 in gravel is always liable to contamination by infiltration of impurities 

 from the surface, whilst the yield from the Tertiary beds, besides 

 being often charged with an obiectionable amount of mineral matter 

 in solution,^ is very apt to be diminished, if not altogether stopped, 

 by the influx of sand carried up by the water into the bore-hole. In 

 fact, the utility of any such wells is but a question of time, and in 

 view of their cost, it is often found that the larger primary expenditure 

 is eventually the more economical procedure. Accordingly, scarcely 

 a month passes but we hear of some new boring being made, or an 

 old one deepened, to the Chalk. 



This being the case, it becomes not infrequently a matter of much 

 importance to know the depth at which the Chalk lies from point to 

 point, so as to estimate the approximate cost of getting water from 

 that source in spots hitherto supplied from higher beds. 



If the surface of Chalk were a uniform plane the determination 

 of its position with regard to sea-level of any desired point would be 

 one of the most simple geometrical problems — scarcely more than a 

 rule-of-three calculation, but the case is very much otherwise. Instead 

 of a plane we have an elaborately-puckered surface, which I have 

 tried to illustrate by the accompanying map, a reproduction, with 

 geological additions, of a part of the index of the old Ordnance 

 Survey. The scale is ten miles to an inch. Photography does not 

 admit correction of names misspelt in the original. The curved 

 lines with figures annexed indicate api)roximately where the surface 



I See Y)T. ]. C. Thresh's Report on the Water Supplies of the Chelmsford and Maldon Rural 

 Sani;ar>' Uislricts, 8vo, C/te/ms/orJ [i8gi]. 



