448 MK. T. V. HOLMES SECTIONS ON THE NEW [Aug. 1 894, 



The denudation which has separated this once hroad, simple valley 

 into three parts, drained respectively by the Mardyke, the Crouch, 

 and the Blackwater, is of very much more recent date. Let us take 

 the case of the Mardyke. At North and South Ockendon, west of 

 Bulphan Fen, there is a broad expanse of Thames Valley gravel, 

 the eastern boundary of which is close to those villages. This 

 gravel must at one time have ended against higher ground, a little 

 eastward of its present limits. But it now forms a plateau, the 

 highest point of which is 110 feet at North Ockendon, and 79 feet at 

 South Ockendon, while eastward we see a tract of ground about 3 

 miles in breadth which seldom attains a height of 30 feet. A similar 

 examination would show that the separation of the valley of the 

 Crouch from those of the Mardyke and the Blackwater was of 

 equally recent date. 



At length, on my hypothesis, the Thames, locally eating its way 

 northward, eroded away the high ground intervening between itself 

 and the Romford stream, and thus tapped the latter's water-supply 

 and altered the course of the drainage. Fortunately a fragment of 

 the silted-up and superseded stream-course was preserved. The 

 course taken by the Thames in its most ancient days, east of Horn- 

 church, is clearly indicated on the Geological Survey map, in the 

 deposits of gravel and loam shown at North and South Ockendon, 

 Chadwell, Mucking, and Corringham. And I think there can be 

 little doubt as to the correctness of Mr. Whitaker's view that in the 

 patches of gravel and loam, stretching from Leigh and Southend, 

 through Canewdon, Burnham, and Southminster to Bradwell, we 

 have deposits formed on the western bank of the ancient valley of 

 the Thames. 1 



In the discussion on my former paper, Mr. Hudleston pointed out 

 that the discovery of Boulder Clay at so comparatively low a level 

 as that of the Hornchurch cutting raised a question as to the possibly 

 pre-Glacial age of the Thames Valley. On the other hand, the 

 position in which the Boulder Clay was found, beneath gravel 

 belonging to the highest, and presumably oldest terrace of the 

 present Thames Valley system, seemed to show that the valley into 

 which it had descended was hardly that of the present Thames. 

 Consequently, the discovery of Boulder Clay near Romford on 

 precisely the same level, and covered by gravel of the same age as 

 that at Hornchurch, together with the silted-up fragment of an 

 ancient river-channel of pre-Thamesian age, enables us to reconcile 

 without difficulty these apparently antagonistic considerations. 



It has, however, been doubted whether the valley of the Lower 

 Thames shows any signs of the ordinary terraced arrangement 

 usually found in river- valleys. For my own part I have detected 

 nothing abnormal in that respect, possibly because, when a worker 

 on the Geological Survey, it very frequently became my duty to map 

 river-terraces in rocks of very varying degrees of hardness ; while 



1 Geol. Surv. Mem. ' The Geology of London and of Part of the Thames 

 Valley,' 1889, vol. i. p. 476. 



