Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/268

196 and the whole of the uneven surface of the clay would be more or less covered by such material, although on the flats, in the hollows, and in the channels the larger portions of it would be accumulated.

As it invariably happens that subaerial erosion acts most powerfully along an anticlinal line, the gravels in the hollows and channels would be the longest-preserved from destruction. And we find here and there on the surface of the Boulder-clay patches of this clayey gravel or loam generally capping the higher grounds, frequently many of them in a curved or straight line, showing the direction of the old channel in which they were originally formed. These so-called "Denudation gravels" are never of any great extent or thickness, nearly all having been removed by erosion. Where they exist it may be inferred that there the Boulder-clay still retains its original thickness; and a line drawn from point to point where they occur would give a rough measure of the minimum amount of Postglacial denudation.

The title of "Cambridge Valley" I consider to be most applicable to that main branch of the valley which is occupied by the river Cam, or Rhee, a stream that rises on the Chalk a few miles from the town of Royston (fig. 1, Pl. XV.) This stream runs in a N.E. direction along a line roughly parallel to the Chalk escarpment, until, south of Cambridge, it is joined by the Cam, or Granta. Thence the united streams, under the name of the river Cam, run nearly due north, still parallel to the escarpment. By Ely their waters flow into the Ouse, a river which continues the course hitherto taken by the Cam, and falls into the "Wash" at King's Lynn. The valley occupied by the Cam and its extension the Ouse thus lies parallel to and near the base of the Chalk; it has been in fact cut back, and is still being cut back into the escarpment. In this operation the river is aided by several streams having their source well up in the Chalk district and running across the strike into the Cambridge valley. These are:—the Cam, or Granta, which rises near Saffron Walden, and has a branch from Bartlow; the Lark, from Bury St. Edmunds and Mildenhall; the Little Ouse, from Brandon; the Wissey, from Watton; and the Nar, from the west of Swaffham.

The valley has been, along the greater part of its course, cut down to the horizon of the Gault (fig. 2, Pl. XV.)—its upper portion being enclosed on either side by gentle slopes of Chalk-marl, succeeded by hills of the Lower Chalk. North of Cambridge it opens out into a broad expanse of fen-land, overlooked from the east side only by the high chalk range. Rarely do any beds lower than the Neocomian crop out; but by these and the Gault the Fen is skirted all the way to the sea.

The Chalk escarpment receded to its present position or thereabouts in the Pliocene period: during the progress of its submergence for, and reelevation after, the Eocene deposits it had been subjected to some disturbance, many small faults, flexures, and contortions