Yol. 65.] THE BOULDERS OF THE CAMBRIDGE DRIFT. 249 



IV. Short Account of the Glacial Drift of 

 Cambridgeshire. 



In the area under consideration there is a complete absence of 

 any pre-Glacial strata younger than the base of the Upper Chalk. 

 Farther south this great stratigraphical break is partly filled up 

 by the occurrence of certain Tertiary deposits, though in no part of 

 the country is the succession complete right up to the Pleistocene. 



In Cambridgeshire there is no evidence as to whether these 

 Tertiary beds were ever deposited ; but, even if they were once 

 present, they must have been completely removed before the 

 glaciation of the district began. We therefore find the Glacial 

 deposits resting upon an old eroded land-surface, on which beds 

 from the Upper Jurassic to the base of the Upper Chalk are exposed. 

 In East Anglia the general succession of the* Glacial beds seems 

 to be as follows : — 



Plateau-gravels ; 



Great Chalky Boulder-Clay ; 



Contorted Drift ; 



Cromer Till. 



As, however, we move westwards from the coast, the lower beds 

 are seen to disappear ; and around Cambridge the two upper 

 members alone can be recognized, and of these two the Chalky 

 Boulder-Clay is by far the thicker and more abundant. As might 

 be expected, the Chalky Boulder-Clay, having been formed on an 

 irregular land-surface, shows great variation in thickness. In many 

 cases it seems to have filled up deep pre-Glacial valleys, and borings 

 have proved thicknesses up to nearly 500 feet ; a good example 

 of this occurs at Newport just outside the area here dealt with : 

 a boring there failed to reach the base of the Drift at a depth of 

 340 feet. In such cases the base of the Drift is far below sea- 

 level, and consequently these valleys must, have been formed at a 

 time when the land stood considerably higher than now. 



At the present time the Boulder-Clay is found principally on the 

 higher ground, as much of it has been removed by post-Glacial 

 denudation from the recent valleys. In composition the Boulder- 

 Clay varies considerably, according to the nature of the bed on which 

 it is resting ; where this is Chalk, the matrix is of a greyish colour, 

 and evidently consists largely of finely ground-up chalk. "Where it 

 rests upon Gault or any of the Jurassic clays, it takes on a much 

 bluer colour and a more sticky consistency ; indeed, in such cases 

 it is often hard to draw a sharp line where the Boulder-Clay ends 

 and the other begins, so closely may it approximate in colour and 

 texture to the underlying clays. 



Throughout the Boulder-Clay are scattered innumerable fragments 

 of rock. By far the commonest types are flints together with lumps 

 of Chalk, Gault, etc., which have escaped being ground up ; frag- 

 ments of more rounded sandstone and limestone are also common. 

 These boulders are mostly of quite a small size, not generally 



