Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/274

202 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 Rivey 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, limestone, and other derived rocks, enclosing broken pieces of derived fossils. They constitute also a gradually descending series, without any 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 occurrence 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.