62 LONDON DISTRICT. 



Holmes to the effects of river -ice. 1 The Trail near Crayford has 

 been described and discussed by Mr. Chandler 2 ; that in the 

 Darent Valley by Prestwich. 3 



The Buried Channel. The date of the Buried Channel is not- 

 far distant from that of the Trail and Coombe deposits, though 

 whether it is slightly earlier, contemporary, or slightly later is 

 difficult to determine. Possibly the melting of the snows that 

 had accumulated during the arctic period contributed to its 

 formation, which was evidently rapid, since it is narrow, 

 straight, comparatively steeply graded, and can be traced only 

 a short distance up the main stream or the tributaries. As 

 already mentioned, it first becomes noticeable near Brentford 

 and thence follows a fairly straight course through Fulham and 

 Battersea, where depths of 35 ft. are known, to Nine Elms, where 

 as much as 72 ft. of river deposits have been proved, giving the 

 level of the bottom as 60 ft. below Ordnance Datum. Through 

 the remaining part of London it appears to keep near the 

 Tertiary outcrop which forms the southern bank of the Thames 

 valley. The Bakerloo Tunnel beneath the river showed 80 ft. 

 of superficial deposits at one spot, but records in the immediate 

 neighbourhood show that this depth is not due to the Buried 

 Channel, but to the encountering of an old shaft sunk in the 

 river-bed many years ago for a proposed ' pneumatic railway ' ; 

 this shaft collapsed and became filled up with river ballast. 



Traces of the Buried Channel have been found in the valley 

 of the Lea, Mr. T. V. Holmes recording that at one spot the 

 section opened for the construction of the reservoirs near 

 Walthamstow showed the surface of the London Clay to be 

 57 ft. below the river level. 4 Other records in the main valley 

 are 50 ft. at Greenwich, 60 at Albert Dock, 70 at Beckton, while 

 at Erith an unbottomed depth of 70 ft. has been recorded in the 

 valley of a little tributary stream. 5 



RIVER DEVELOPMENT AND TOPOGRAPHICAL EVOLUTION. 



From the nature and distribution of the deposits described 

 above, it is possible to deduce certain changes that have taken 

 place in the courses of the Thames and its tributaries. The 

 evolution of the Thames has been a favourite subject for specu- 

 lation amongst geologists and geographers for nearly a century, 

 but a discussion of the widely differing hypotheses put forward 

 would necessarily take us far beyond the limits of our map and 

 the scope of this memoir. 



An attempt has been made to trace the pre-Glacial course of 

 the Thames through a part of our area. 6 It is thought to have 

 passed through Rickmansworth and Watford, but at the period 



1 Proc. Geol. Assoc., vol. xii, 1891, p. 197. 



2 Ibid., vol. xxv, 1914, pp. 66, 67, and Plate 10. 



3 Quart. Journ. Geol. Soc., vol. xlvii, 1891, pp. 153, 154. 

 Essex Nat., vol. xii, 1902, p. 226. 



6 Proc. Geol. Assoc., vol. xxiii, 1912, p 186, and vol. xxv, 1914, p. 64. 

 Sherlock, R. L., and A. H. Noble, Quart. Journ. Geol. Soc., vol. Ixviii, 1912, 

 pp. 199-212. 



