204 P. G. H. Bosivell — Differential Movement 



the broad V in the Crag boundary is not due to a valley and geometry. 

 Both considerations lead to the same conclusion of instability over 

 the central area. 



The lied Crag is believed to have been deposited in a number of 

 land-locked bays which were being gradually forced northward by 

 a series of earth-ripples, resulting in a general uplift on the south 

 and subsidence on the north. 1 It maybe merely coincidence that the 

 Crag between the Orwell and the Stour shows evidence of much 

 oscillatory movement ; abundant pebbles, an impoverished fauna, and 

 rolled and broken shell-fragments are characteristic. 2 The Crag 

 deposits of Walton and Oakley on the one hand, and of Sutton, 

 Butley, etc., on the other, while being drift-bedded, are richer 

 in species, contain tiny and fragile shells, and show less evidence 

 of disturbing conditions. It should also be remarked that the 

 Boxstone Bed at the base of the Crags is confined to the 

 central area. 



The bearing of the form of the river-systems (which probably had 

 their inception in Miocene times) upon the question will be dis- 

 cussed later. 



The boundary between the two very different types of Upper 

 Boulder-clay (Great Chalky Boulder-clay of S. Y. Wood, jun.) in 

 Suffolk has been inserted on the Map. This is adapted from 

 Mr. F. W. Harmer's maps, with slight corrections, as a result of 

 detailed mapping, by the writer. 3 We know that the general 

 topography of Suffolk and Northern Essex was, in pre-Glacial times, 

 much as it is now, and exercised a strong influence upon the ice- 

 movements. This appears to be indicated by the boundary between 

 the Chalky-jSTeocomian Boulder-clay and the Chalky-Kimmeridgic 

 Boulder-clay in the lower part of the Waveney Valley, the latter 

 occurring at Somerleyton, Burgh Castle, etc., and the former at 

 Haddiscoe, etc. 4 The line of demarcation between the Chalky- 

 Kimmeridgic Boulder-clay and the Chalky (or Chalky-Oxfordic) 

 Boulder-clay in Suffolk is equally abrupt, and its position noteworthy, 

 even if meaningless in the present connexion. 



One of the best methods of registering secular movements taking 

 place at the present day is provided by the behaviour of rivers. The 

 following facts were not sought, but accumulated in the course of work 

 upon the evolution of several East Anglian rivers. 



Gradient-curves of all the East Anglian rivers have been plotted, 

 and are seen to be of the usual logarithmic form. Rejuvenation, 

 however, is revealed up to about the 50 ft. contour in the case of 

 the Gipping, Stour, and Colne, by the introduction of a new and 

 smaller logarithmic curve upon the lower portion of the old curve. 

 In the case of the Chelmer and Deben, this rejuvenation extends to 

 about the 25 ft. contour ; it is hardly shown by the Aide, and does 



1 Mr. F. W. Harmer in numerous papers. 



2 See the writer, Proc. Geol. Assoc, vol. xxiv, p. 330, 1913. 



15 F. W. Harmer, Geology in the Field, p't. i, pi. iii, 1909 ; Trans. Norfolk 

 and Norwich Nat. Soc, vol. ix, plate at p. 132, 1910; see also Proc. Geol. 

 Assoc, vol. xxv, pi. xxiv, 1914. 



4 F. W. Harmer, loc cit., 1910, p. 119. 



