SPRINGS. 79 



An important series of springs issues from the Chalk, at its junction 

 with the Thanet Sand, between Ewell and Croydon ; from Carshalton to 

 Croydon the various sources of the River Wandle arise from springs, 

 which have a marked effect on the water-level in neighbouring wells. 1 



Other Chalk springs are found in the Colne valley north of Watford, 8 

 at Erith and Greenhithe in Kent, and Purfleet in Essex. 



From the Woolwich and Reading Beds and the Blackheath Beds, 

 which consist of frequent alternations of pervious and impervious strata, 

 numerous small springs are thrown out; these are frequently of use to 

 the field geologist in mapping the various beds, but are of little importance, 

 except, perhaps, on the east side of Croydon. 8 



Though on the whole an impervious stratum, the London Clay gives 

 rise to a number of small springs, important because of their mineral 

 qualities ; these include the famous ' Epsom Waters ' and nearly a)l the 

 ' Spas ' of London. The water is derived, as a rule, from gravels overlying 

 the clay ; by means of cracks, it finds its way into the mass of the clay 

 and travels along a sandy parting or a layer of septaria; in its passage 

 it becomes charged with salts, mainly the sulphates of magnesia and lime, 

 sulphuric acid being the result of the decomposition of pyrites in the clay. 

 The Epsom Springs are on the Common between Epsom and Ashtead, 

 near a modern house called ' The Wells ' ; their medicinal quality was 

 discovered in 1618; in the 17th and 18th centuries they were much 

 resorted to and gave the name ' Epsom Salts ' to the sulphate of 

 magnesia. 4 Of the London Spas obtaining their water from the London 

 Clay the more important were Kilburn, Streatham, Beulah and Sydenham, 

 while in Essex genuine mineral springs were known at South Weald and 

 Upminster. 5 



From the Bagshot Sand many springs are thrown out by the under- 

 lying clays. At Hampstead such springs form the sources of the West- 

 bourne, Tyburn, Fleet and other small streams (p. 77). 



The base of the sands is here marked by a band of ironstone (p. 34), 

 and the water is often chalybeate, as, for instance, at the famous Spa in 

 Well Walk, which flourished in the 18th century.' In Essex notable 

 springs from the Bagshot Sand are found at High Beech and in the 

 neighbourhood of Brentwood. A spring from the Barton Sands on 

 St. George's Hill is interesting, as it formed the water supply for the 

 ancient camp, and a special earthwork was constructed for its protection. 7 



At the junction of the various Pleistocene gravels with the London 

 Clay, springs are of frequent occurrence ; such springs formed the main 

 supply for London and the neighbouring hamlets for centuries (p. 89), 

 but few of them are now visible; the Glacial Gravel of Wimbledon 

 Common gives rise to several, of which the largest is known as ' Caesar's 

 Well ' ; springs from similar gravel at Coombe were piped to Hampton 

 Court in Tudor times. 



1 ' Geology of South London ' (Mem. Geol. Surv.), 1921, pp. 72, 75. 



2 Proc. Geol. Assoc., vol. iv, 1875, p. 284. 



8 ' Water Supply of Surrey ' (Mem. Geol. Surv.), 1912, p. 485; Fagg, C C.; 

 Proc. Croydon Nat. Hist. Soc., 1911, p. Hi. 



4 ' Geology of South London ' (Mem. Geol. Surv.), 1921, p. 75. 



5 Christy, M., and M. Thresh, ' Mineral Waters and Medicinal Springs of 

 Essex,' Essex Field Club, 1910. 



Foord, A. S., op. cit., pp. 137-152; Potter, G.W., ' Hampstead Wells..' 1904. 

 T Gardner, E., Surrey Archceol. Coll., vol. xxiv, 1911. 



