EAST ANGLIA DURING THE GLACIAL PERIOD. 197 



being the result. To a considerable flexure of this period is owing 

 the present position of an upper extremity of the valley. 



That the Cambridge valley, as such, is mainly preglacial is 

 evident from the consideration of several reasons, amongst others 

 the position which the Boulder-clay now occupies therein*. It 

 rests on the escarpment at an elevation of more than 500 feet ; it is 

 found here and thero on the flank at lower and lower levels ; and it 

 occurs in the bottom of the valley at but a small height above the 

 sea. It is true that in those instances where Boulder-clay is found 

 on the flank it is in small outlying patches only (fig. 2, PL XV.) ; 

 but this is inevitable from the progress of recent erosion ; and they 

 are quite sufficient to prove the former extension of the mass. 

 These patches now cap small elevations resulting from denudation ; 

 and a line drawn from the escarpment to the low ground would 

 intersect them all. 



Although the escarpment had been cut back to nearly its present 

 position during the Pliocene period, and the land was somewhat 

 submerged during the deposition on the east coast of the Pliocene 

 beds, we find in this valley no signs of their present or former 

 existence. lor the valley was not excavated to its present depth, 

 by perhaps 50 or 100 feet ; consequently any deposits that may 

 have been left in it during the Pliocene era have long been swept 

 away, for the same reason, perhaps, we have no beds of Lower 

 Glacial age, which are, so far as I have seen, confined to areas of 

 comparatively slight elevation. It is not difficult to suppose that 

 these Lower Glacial deposits would be excluded from this valley by 

 its then height relatively to that at which they are now found, or 

 that any which may possibly have once occupied the area have been 

 subsequently removed. 



But it is far more difficult to account for the apparently total 

 absence of the Middle Glacial deposits, which, just over the Chalk 

 escarpment, run up to a height of 300 feet or more above the sea, 

 and to at least 200 feet above what must have been the bottom of 

 this valley at the time of their deposition. The subsequent physical 

 conditions were not favourable to, nor was the time sufficient for, 

 the removal from the valley of beds of any extent (assuming them 

 to have been therein deposited) before the deposition of the Great 

 Chalky Upper Boulder-clay. We should find them still, however 

 great the subsequent denudation, between the older rocks and the 

 Boulder- clay in some at least of the many instances in which it 

 occurs. It cannot be assumed, as in the case of the Pliocene and 

 the Lower Glacial, that the Middle Glacial beds at one time occupied 

 the valley, and were afterwards, and before the Upper Boulder-clay 

 period, removed ; yet in no instance do we find the Boulder-clay 

 resting on any other than preglacial formations. As suggested in 

 the first part of this paper t, the currents from the north that- 

 formed the Middle Glacial gravels and sands were confined to the 

 seaward side of the Chalk range, which was not wholly submerged, 

 and were entirely excluded from the Cambridge valley ; for, as we 

 * Ramsay, ' Physical Geology,' p. 211. t Ante, p. 193. 



