Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/272

200 masses of wash from Boulder-clay are easily accounted for, if we assume that the gravels were formed during the removal by denudation 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 conclusive evidence if the shells could be more generally found. But it is just possible, although far from probable, that in the few instances 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 entertained. 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, probably, 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 Royston, 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, Pl. 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 Recent 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 extension 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