GEOLOGY OF THE DISTRICT AROUND DAGENHAM BREACH. H5 



rated by the marshes of Erith and Plumstead, and to this extent its 

 course is now, locally, more northerly than it was at no very distant 

 date, speaking geologically. But while the Thames never ran south 

 of the foot of the Lower Tertiary escarpment, it was during many 

 ages slowly cutting its way from the northern edge of the broad river 

 flats of Essex, and from a height of one hundred feet above its 

 present level, to the position it now occupies. Were the rock under- 

 lying the river-gravel between Romford and the Thames Chalk, we 

 should, doubtless, be able to trace distinct terraces at various levels, 

 marking stages in the progress of the river southward and downward 

 to its present position. Indications of these terraces are indeed 

 visible here and there; but, owing to the softness of the London Clay 

 underlying the river-gravel, they cease to be traceable in the course 

 of a few yards, becoming merged in a vague slope. I have noticed 

 a precisely similar state of things in Cumberland at a spot where the 

 river Eden had cut terraces in soft, incoherent Glacial Drift. During 

 its lateral progress southward, the Thames may have destroyed con- 

 siderable areas of high ground. Mr. Whitaker (Geol. Lond., vol. ii., 

 p. 14) gives the full thickness of London Clay in this district as 

 about 450 feet. Of course we are only likely to meet with this 

 thickness where the London Clay is capped by the conformable, or 

 nearly conformable, Bagshot Beds ; where it forms the surface, or is 

 covered by unconformable gravels or other deposits, it may be 

 greatly attenuated, and is sure to be so to some extent. It is there- 

 fore somewhat remarkable that at Dagenham Hall there are 400 feet 

 of London Clay beneath the old river-gravel, or almost as much as 

 in the well of the Essex Lunatic Asylum at Brentwood, close to the 

 outcrop of the Bagshot Beds, which cap the hill. Coming nearer to 

 Dagenham, I note that at Upminster Rectory the London Clay, 

 beneath old river deposits, was only 150 feet thick, while at Ilford, 

 beneath similar beds, the greatest thickness recorded is 93 feet. 

 Now at first sight it would seem probable that there would be more 

 London Clay beneath the river deposits at either Upminster or 

 Ilford than at Dagenham ; and certainly the existence of 400 feet at 

 Dagenham seems to need some explanation. Last year, during the 

 voyage of the Essex Field Club from Maldon to Chelmsford, I 

 suggested (Essex Naturalist, vol. v., p. 201) that the Bagshot out- 

 liers of Warley, Stock, and Billericay might owe their preservation, 

 to a considerable extent, to their position on a long line of synclinal 

 fold, the continuation of which, on the Kentish shore, appeared at 



