Geology. 23 



Lower Terrace Gravels, etc. 



Messrs. Hinton and Kennard in their recent paper refer to a 

 fourth terrace at 20 feet above O.D. (21), and draw attention to what 

 they consider to be evidence of a buried channel below the bed of 

 the river. Further local information on this is desirable. Some of 

 the depths given are as follows, in feet : Limehouse, 49 ; Belvedere, 

 50 ; Crossness, 7H ; Greenhithe, 50 ; Northfleet, 72J. 



East Wickham Valley. 



From Erith to the East Wickham Valley the Tertiary escarpment 

 is unbroken, although several very deep coombes have cut their 

 way back some distance, and can be well seen on Bostal Heath and 

 along the road to Belvedere. 



Rain falling on the extensive sheet of Blackheath Pebble Beds is 

 arrested in its downward progress through the earth by the clayey 

 beds of the Woolwich and Reading Series, and a set of springs along 

 their outcrop is the result. 



The East Wickham Valley is really a coombe which has been so 

 well developed, that it has captured the drainage of the London 

 Clay district to the West. A full account of the deposits now to be 

 seen in this valley has been recently written by Mr. A. L. Leach, 

 to which further reference should be made (22). 



Recent excavations show sections of small streams which have 

 become completely blocked up by re-deposited clay, etc., resulting 

 from the denudation of the London Clay country drained by the 

 streams. Good examples of hill-creep and contortions due to 

 pressure are to be seen, and are well worthy of study. 



The Ravensbourne. 



The Darenth cuts through the Chalk escarpment near Otford, 

 and has its head-waters in the Lower Greensand hills beyond. The 

 Ravensbourne, however, does not do this, but rises on the Northern 

 Slopes of the North Downs. . In consequence of this the Drift 

 deposits connected with it contain only debris from the Chalk and 

 Tertiary strata. 



A good idea of the extent to which erosion has acted in this valley 

 can be obtained by standing on the Hilly Fields Recreation Ground, 

 Brockley, and looking across to Blackheath. Up to quite recently 

 a good section in Old Ravensbourne gravel was exposed between 

 Undercliff and Overcliff Road, Lewisham, and it is very probable 



(21) Proc. Geol. Assoc., Vol. XIX., pp. 76, etc. 



(22) Proc. Geol. Assoc., Vol. XIX., p. 341. 



