77 



high ground between the valleys of the Cam and the Ouse is 

 proved to be of Upper Glacial age not only from the similarity 

 of its constitution and contents, but from general physical 

 considerations deduced from a study of the whole county. 



An examination of the heights marked on the ordnance 

 map shows that the basement-level of the clay in this district 

 gradually descends from a height of 250 feet on the south, to 

 that of only 120 on the north. Prolonging this gradual slope, 

 we are not surprised to find that the base of the same clay on 

 the Isle of Ely is only 60 or 70 feet above the sea, and that 

 northward it sinks down below the level of the fens and 

 ultimately below the level of the sea. 



This gradual declination of the Upper Boulder Clay at once 

 disposes of the erroneous correlations Avhich Mr Seeley made in 

 1866 ; it is now seen that neither the Lower Boulder Clay nor 

 the Contorted Drift are to be found in Cambridgeshire ; and not 

 only so, but it is a question whether anything that can be truly 

 called Middle Glacial occurs within the limits of the county. 



Mr Searles Wood, Junr., was the fir.st to remark that the 

 Mid-Glacial gravels do not occur above a height of about 300 

 feet, but he has never spoken of them as limited to the eastern 

 side of the Chalk escarpment, indeed he has mapped them as 

 occurring in Bedfordshire and Huntingdon, as well as beyond 

 the western boundary of the Boulder Clay at Saffron Walden, 

 DuUingham, and Newmarket. 



Mr Penning, on the other hand, has seen reason to conclude 

 that they are so limited, and that no such sands or gravels are 

 to be found in the Cambridge valley ; it is certain at any rate 

 that most of those so mapped westward of the chalk escarpment 

 are Post-glacial, that is to say, posterior in age to the Upper 

 Glacial of Mr S. V. Wood. 



Those which are actually seen to lie beneath the Boulder 

 Clay are, I believe, merely local lenticular patches at or near 

 the base of this clay; such intercalated beds of gravel are by no 

 means rare in Suffolk and Essex, and may be seen for instance 

 near Sudbury. This I believe to be the explanation of the 

 series visible near Audley End and described in the early part 

 of this Essay ; the sands and loams which here intervene 

 between two similar clays being probably the result of con- 



