76 LONDON DISTBICT. 



At Bermondsey brown sandy loam with land-shells (Marsh 

 Clay) is overlain by carbonaceous silt with freshwater shells 

 and remains of Roman pottery. The Marsh Clay here gives 

 evidence of ' a land surface liable to flooding, each flood adding 

 to the thickness of the deposit.' It was formed by a tributary 

 of the Thames ; by subsequent sinking the influence of the main 

 stream was felt and the carbonaceous silt was deposited. 1 



THE THAMES AND ITS TRIBUTARIES. 



Owing to the windings, the course of the Thames through 

 our area is about 62 miles in length, the distance in a straight 

 line being 36 miles; the lower 42 miles, below Teddington, is 

 tidal ; the ancient name of that hamlet is Totyngton, so that the 

 usual derivation from Tide -end -town must be discarded. At 

 London Bridge the tides ebb seven hours and flow five hours, 

 the average range being 16 or 17 ft., though at times it has been 

 as much as 27. At Twickenham it is only 8 ft., low- water being 

 about 5 ft. above OJD. The fall thence to London Bridge is 

 less than 1 ft. per mile ; Trinity High Water mark, the old 

 standard height, is 12J above O.D., a fact which must be borne 

 in mind in using Mylne's excellent contour and geological map 

 of London, published in 1856. The low-water depth of the 

 river at Crayfordness is about 22 ft., decreasing to 16 or 18 ft. 

 off Greenwich, 14 ft. and less along the Pool, 9 ft. at London 

 Bridge, 5 or 6 ft. at Hammersmith, and 4 to 6 ft. at Brentford 

 and Teddington; dredging operations render the figures liable 

 to variation. 



In the non -tidal part, above the weir at Teddington, the fall 

 is about If ft. per mile, controlled largely by the locks and weirs 

 of the Thames Conservancy. The average daily flow at 

 Teddington for the twelve months ending March 1921 was 

 1,467*5 million gallons, that for the preceding 10 years being 

 1,792-2 and for 37 years 1,3 66 '8. Taking the salts in solution 

 at 19 grains per gallon, Huxley calculated that the river carried 

 down daily about 1,000 tons of carbonate of lime and 238 tons 

 of sulphate of lime, to which must be added the material carried 

 in suspension. The Thames, therefore, although it has practi- 

 cally reached the base-level of erosion, is still an active agent in 

 denudation. 



The maximum and minimum flow of the river is reckoned to 

 be about five months later than the corresponding points in the 

 mean rainfall of the Thames Basin. 2 



The more important of the tributary streams are well known, 

 and are of course shown on the maps. More interesting, perhaps, 

 are the ' buried rivers ' which once flowed through the London 

 area ; as they materially affect the geographical features and 

 geological boundaries, and therefore the history of London 

 (p. 89), a few particulars are necessary. 



1 Kennard, A. S., and S. H. Warren, Geol. Mag., 1903, p. 456. 

 2 Lockyer, W. J. S., Nature, vol. Ixxii, 1905, p. 178. 



