196 W. H. PENNING ON THE PHYSICAL GEOLOGY OE 



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 power- 

 fully 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. 



Paet 2. — The Drift-deposits in the Cambridge Valley. 



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 Ehee, a stream that rises on the Chalk a few miles from 

 the town of Eoyston (fig. 1, PI. 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 J^ar, 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, suc- 

 ceeded 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 

 JNeocomian crop out ; but by these and the Gault the Een is skirted 

 all the way to the sea. 



The Chalk escarpment receded to its present position or there- 

 abouts 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 



