250 MESSES. E. H. EASIALL AND J. EOMANES ON [Aug. I909, 



exceeding 2 feet in diameter. There occur, however, in the Drift 

 huge masses of Chalk or Jurassic clay usually spoken of as ' erratics.' 

 These must also be regarded as true boulders, although they may 

 measure hundreds of yards in length. The best known example of 

 this occurs in the Roslyn Pit, near Ely, where a huge mass of Chalk, 

 Cambridge Greensand, and Gault has been transported by ice aud 

 laid down in a hollow scooped out of the Kimmeridge Clay. 



Besides the boulders of obvious local origin there occur scattered 

 through the clay a considerable number of rocks, both sedimentary 

 and igneous, which cannot be matched from any known locality 

 in the Cambridge area. The presence of these we are forced to 

 attribute to transport by ice originating in distant and widely- 

 separated districts. 



•The High-Level Gravels. 



In some places patches of coarse gravel are found which seem to 

 overlie the Chalky Boulder-Clay, sometimes overlapping it and 

 resting directly upon the Chalk. These gravels occur in most 

 instances on the higher ground : for instance, on Barrington Hill 

 patches of coarse gravel are fouud at about 350 feet above sea-level. 

 These gravels are distinguished from the normal Boulder-Clay by a 

 difference in the proportion of coarse and fine material rather than 

 by any difference in the materials themselves ; in some cases there 

 are patches and seams of sand : in fact, they represent exactly just 

 such a deposit as might be expected to result from the washing- 

 away of much of the finer material of the normal Boulder-Clay. 

 The gravels are generally unstratified, and the rocks of local origin 

 show little if any more rounding than those of the Boulder-Clay. 

 All the evidence available points to the fact that these high-level 

 gravels are simply the heavier residue of Boulder-Clay which has 

 been weathered out in situ. 



Those who maintain that the Boulder-Clay is a marine deposit 

 attribute the coarse gravels to re-sorting by wave-action as the land 

 gradually emerged. 



Post-Glacial Deposits. 



Of the recent deposits we may distinguish for our purpose : — 



(1) The gravels of the ancient river-system ; and 



(2) The gravels of the present river-system. 



Apart from their palaeontological contents, these are all very 

 similar, varying from coarse gravels to sand and loam. They 

 represent the residue of material which has been derived by 

 denudation from the steeper slopes ; and a large part of this must 

 represent the Glacial deposits known to be absent from the sides of 

 the valleys. Although such material has suffered a certain amount 

 of transport since its deposition by the ice, it cannot have been 

 brought from any locality outside the drainage-area of the river- 

 system by which it was deposited. In any case, the fact of real 



