in East Anglia in Tertiary Times. 205 



not appear to be present in the Thames, Lea, Roding, Waveney, 

 Norfolk rivers, and "Wash rivers. Such rejuvenation may be due to 

 many causes, but in the present instance most may be eliminated. 

 The case is simplified by the fact that the whole district was 

 subjected in post-Glacial times to a depression which turned the lower 

 portions of the valleys into estuaries and caused them to be partly 

 silted up. This depression, of at least 25 feet in the critical area, and 

 considered to be 60 to 80 feet over the area from the Thames to the 

 Wash, etc., by Mr. Clement Reid, successfully arrested the develop- 

 ment of the rivers, the valleys of which were already overloaded with 

 glacial debris. 1 Thus no release from damming has occurred ; also 

 rainfall has diminished, and there has been no increase in power or 

 drainage area of the rivers, since late-Glacial times. Such capture 

 as appears to have taken place is related only to the pre- 

 Pliocene grain of the country and is certainly pre-Upper Glacial. 2 

 Moreover, the Colne is a beheaded stream, and yet shows the same 

 amount of rejuvenation as the Stour, which has robbed it. The 

 Gipping, with similar rejuvenation, is purely a dip stream. The 

 Deben and Chelmer have largely increased their drainage area by 

 piracy, but exhibit less change in gradient. It is submitted, there- 

 fore, that a strong probability exists that the rejuvenation is due to 

 uplift. 3 When the results are plotted diagrammatically, Fig. 3 is 

 obtained, and the evidence points to differential uplift in the area 

 previously proved to be unstable. 



SUFFOJLH A/OXFOLK 



Essex 



■^ — Distance in miles from the area of maximum uplift. — ^> 

 FlG. 3. — Diagrammatic representation of the variations in amount of the 

 rejuvenation of East Anglian rivers. The heights to which rejuvenescence 

 occurs are marked off vertically. 



Summarizing, we may say that the outstanding feature is the 

 manner in which the points of deflexion where change of strike 

 occurs, and the axes producing them, moved north-eastwards. The 

 Chalk zones are the first to change strike, then the sub-Eoeene surface 

 contours, next the Lower London Tertiaries, and finally the London 

 Clay. The ripple then seems to have reached its farthest extent 

 north-eastwards, and was subsequently reflected from the area under 

 which the Palaeozoic floor rises. 



The river capture which has taken place' has curiously had the 

 effect in all important cases of throwing the drainage in towards the 



1 It was possibly this depression which buried the Neolithic skeleton and 

 deposits found below tide-level near Walton-on-Naze by Mr. Hazzledine Warren. 



- Q.J.G.S., vol. lxix, p. 612, etc., 1913. 



3 Rejuvenation up to the 50 ft. contour does not mean an uplift of 50 feet. 

 A much smaller elevation would yield the result. 



