200 W. H. PENNING ON THE PHYSICAL GEOLOGY OF 



masses of wash from Boulder-clay are easily accounted for, if we 

 assume that the gravels were formed during the removal by denuda- 

 tion of the Boulder- clay which once filled the valley. The uniformity 

 of level at which they occur would also seem to indicate a fluviatile 

 rather than a glacial origin. But the strongest argument in support 

 of this view is the fact that the gravels in places contain recent land 

 and freshwater shells (Helix, Pupa, Succinea) ; this would be con- 

 clusive evidence if the shells could be more generally found. But 

 it is just possible, although far from probable, that in the few in- 

 stances only in which (so far as I know) they have been discovered, 

 a newer gravel may have been deposited in channels cut by streams 

 through that which is of older date. In my opinion the parts of 

 the gravel where the shells do occur so exactly resemble the other 

 portions where they do not, that such a proposition cannot be enter- 

 tained. Therefore without hesitation I refer these deposits to the 

 existence of an ancient river running, as does the present one, along 

 the foot of the escarpment, although, of course, not on the ground 

 occupied by the Cam of our own time. The Chalk 'scarp was then 

 somewhat different from what it is now ; it had been cut down to 

 such a level only that its foot was higher by as much as these gravels 

 are now above the river. The line of this old river is now indicated 

 by an elongated series of patches of the gravel in question which 

 formerly occupied its channel. The remaining portions of these 

 gravels now form long, low, and rounded ridges, occupying, pro- 

 bably, slight depressions in the Chalk, which depressions (formerly 

 a part of the old river-course) are now somewhat elevated above the 

 surrounding area, the gravels having partly preserved, as usual, the 

 lines of hollow from denudation. This series commences near 

 Eoyston, in a hollow formed by the flexure in the Chalk previously 

 mentioned*, runs in an easterly direction about parallel to the 

 present course of the river, crosses the minor valley of the Cam or 

 Granta, is joined by another line of similar patches from the east, 

 and then sweeps round to the north of Cambridge, being lost in 

 the fens beyond (fig. 1, PI. XV.). 



I am well aware that this solution may not explain the origin of 

 all the patches of gravel that are found in the valley and cannot 

 with certainty be referred either to the Glacial, Postglacial, or Eecent 

 period. But I do not think that any of them have been actually 

 traced under the Boulder-clay, as they would have been if of Lower- 

 or Middle-glacial origin. It is possible that during the period in 

 which the drifts were being formed by the Middle-glacial currents 

 traversing the east coast, a stray iceberg may have occasionally 

 found its way into the land-locked inlet of the Wash and its exten- 

 sion the Ouse and Cam valleys, at that time so much broader 

 through greater submergence. But this would be, I apprehend, a 

 very exceptional occurrence ; and any deposits dropped from such 

 bergs are not to be classed with the current-formed gravels that occur 

 in such force on the other side of the escarpment. 



Taking all the facts into consideration, I feel justified in drawing 

 * Ante, p. 197. 



