74 H. Bury — The Chines and Cliffs of Bournemouth. 



But we have still to account for the difference in level between 

 the old and the new valleys. If this feature had been common 

 to all the valleys of the district we should have been justified in 

 postulating a change in relative sea-level, and perhaps connecting 

 the old valleys with the well-known raised beaches, rather than with 

 the later period of the Coombe Rock, when the sea had sunk some- 

 what below its present level. But we have seen that the difference 

 in level is confined to those valleys which run down to the sea, 

 and a little consideration will convince us that, though a change 

 in sea-level is not in itself improbable, the mere encroachment of 

 the sea on the land is quite sufficient to produce the observed 

 phenomena. 



There are strong reasons for believing that in former times a 

 river (Solent River) ran across what is now Bournemouth Bay, 

 being cut oft' from the English Channel by a line of hills connecting 

 the Isles of Wight and Purbeck ; and although opinions differ as to the 

 exact time at which its southern boundary was breached by the sea, 

 yet the balance of evidence seems in favour of a fairly late date.' 



Fig. 4. — Longitudinal section sliowing the relation of the old and new valleys 

 to the changing shore-line. A, Solent Eiver. B, Floor of old valley. 

 C and D, Successive positions of the cliff, with the floor of the new valley 

 (dotted lines) adjusted to them. 



The lateral streams from the Bournemouth district would probably 

 meet this river about one or two miles from the present coast, but 

 the destruction of the river by the sea and the subsequent recession 

 of the coastline have led to a shortening of their valleys and con- 

 sequent readjustment of the thalweg to new gradients. Owing, 

 however, to the greatly reduced size of the streams, only a short 

 and narrow portion of the old floor was affected, thus giving rise 

 to the chines, as shown diagrammatically in Fig. i. The same 

 figure also explains how, without any change in sea-level, the 

 remnants of the old valley floor appear at higher and higher levels 

 in the cliffs as the latter recede. 



There is, it is true, no direct evidence that the Solent River 

 survived to quite so late a date as is here suggested, but the wide 

 floors of the old valleys must have required time for their formation, 

 and appear to me more consonant with a fixed outfall into a main 

 river than with a shifting one into a constantly encroaching sea — ■ 

 and the conditions are such as to render a stationary coastline 

 highly improbable. Moreover, while it seems certain that chines, 



^ Mem. Geol. Surv., Bournemoiith, 2nd ed., 1917, p. 48. 



