Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/269

Rh 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 there 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. For 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 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, 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