202 W. H. PENNING ON THE PHYSICAL GEOLOGY OF 



entirely by postglacial erosion ; and the Chalk has been cut back 

 beyond the old line, except where such patches still remain. In 

 the early Postglacial times, when the valley began to be reexcavated, 

 a main stream must have run approximately in the same direction 

 as the present river, but then, of course, at a height considerably 

 above it — as much higher really as the thickness of the Boulder- 

 clay that has since been removed. The escarpment covered by 

 Boulder-clay was cut back then in the same way as it is being cut 

 back now, by minor streams running across the strike (p. 196) into 

 the main river parallel thereto. This operation began immediately 

 upon the reelevation of the land after the great submergence, and 

 has continued up to the present time. 



There must have been very considerable accumulation of detritus 

 from the removal of such a mass of Boulder- clay ; and that removal 

 would be the more rapid in proportion to the greater steepness 

 of the area subject to denudation. These accumulations would, 

 in turn, be removed ; but it is suggested that in the mass of Boulder- 

 clay and overlying patches of gravel leading down from the Bivey 

 to the valley below we have evidence of the later stages of that 

 clay's denudation, and small remnants of the resulting accumulation. 



The gravels are made up of exactly such material as would be 

 derived from the waste of Boulder-clay — rolled lumps of hard 

 and soft chalk, flint pebbles, with some fragments of quartz, lime- 

 stone, and other derived rocks, enclosing broken pieces of derived 

 fossils. They constitute also a gradually descending series, with- 

 out bjij very great break in their continuity, either in regard to 

 level or position. All the patches appear to occupy hollows in the 

 surface of the clay, by which circumstance they have, indeed, been 

 preserved — that is, down to the level of the Preglacial valley- 

 system ; below it the gravels rest on the excavated chalk. In many 

 instances they occur in hollows along the top of a ridge — the old 

 stream- channels in which they were left now forming synclinals, 

 and thus preserving the gravels and the clay or chalk immediately 

 beneath, while that on either side has been worn away. 



The marine Middle-glacial currents could not have formed these 

 gravels, even if they had access to the valley, as some of the ridges 

 run up into coombe-like valleys where such currents would have 

 been impossible. The streams that deposited them were doubtless 

 rapid, and occupied gorges in the clay ; this would account for the 

 false-bedding sometimes exhibited. 



The series described as indicating the ancient course of the Cam 

 represents the latest stage of all, previous to the present system of 

 drainage ; in this series and in patches at or slightly above the samo 

 level occur the recent land and freshwater shells. At one place in 

 this series shells of Cardium edule have been found ; and their occur- 

 rence would seem to bear out the suggestion made, that during the 

 Glacial period the Cam valley formed a land-locked bay, on the sandy 

 shores of which this mollusk nourished (p. 194). It occurs, but not 

 plentifully, at the base of the deposit, and probably indicates the 

 position of the old shore-line. 



