EROSION IN ESSEX AND SI FFOLK. 225 



given very little information as to the varying depths at which 

 the London-clay was reached beneath the river deposits.' It is 

 true that here and there at the bottom of the excavations the top 

 of the Loadon-clay was either visible, or the presence of frag- 

 ments of septaria nodules in the gravel suggested the nearness of 

 the clay. But, as we shall see, the general impression derived from 

 visits to the reservoir excavations, as to the depth of the London- 

 clay, needs correction from the additional evidence obtained 

 during the construction of the puddle-trenches in the centre of 

 the boundary banks. 



I am greatly indebted to Messrs. Sharrock and Spencer, the 

 engineers representing Messrs. Pearson, the contractors, for 

 information as to the strata met with in the puddle-trenches, of 

 which they have preserved a most careful account. But I do 

 not propose here to touch upon the details of the river-deposits, 

 or even to note minutely the varying depths at which the 

 London-clay, was found in the puddle-trenches, my object being 

 simply to give the distribution of the depths which are within 

 normal limits and of those which are beyond them. 



In the first place, it seems desirable for the sake of com- 

 parison, to note at what depths the London-clay has been 

 touched beneath the river deposits of the Lea Valley outside the 

 new reservoirs but within a short distance of them. On con- 

 sulting Mr. Whitaker's Geological Survey Memoir on The Geology 

 of London and of Part of tlie Thames Valley (vol.2) we learn that 

 at Waltham Abbey the East London Water Company in their 

 well close to Waltham Lock, found 18 ft. of river deposits above 

 the London-clay, at Walthamstow Marsh 17^ ft., at Chingford 

 Mill 10 ft., and at Longwater Pumping Station, Tottenham, 

 20 ft. at one spot and 16 ft. 100 yards away. And the records 

 of borings and sinkings through the somewhat older river 

 deposits of Tottenham and Edmonton nowhere show them to be 

 more than 21 ft. thick. But as they are few and scattered over 

 a large area, they do not necessarily preclude the existence of 

 spots between them which might give a different result. 



Dr. Henry Woodward, P\R.S., in his paper on " The 

 Ancient Fauna of Essex " (Trans. Ess'X Field Club, vol. Hi., 

 pp. i-2g, 1882-3) gives an account of the sections exposed during 

 the construction of the reservoirs of the East London Water 



I T. v. Holmes " Geological Notes on the New Reservoirs in the valley of the Lea, near 

 Walthamstow," an^e pp. 1-16. 



