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The formation underlying the greater portion of the fen- 

 country is the Oxford Clay ; the Isle of Ely is formed of Kim- 

 meridge Clay capped by outliers of Lower Greensand and 

 Boulder Clay ; and the other low islands are caused by banks of 

 the older estuarine Gravels. 



Having thus given an outline of the general physical and 

 geological features of the country, I will proceed to describe in 

 greater detail the Post-tertiary or Pleistocene deposits, treating 

 first of the Glacial beds exhibited in each of the above named 

 districts, and secondly considering the Post-glacial Gravels 

 which for the most part occupy the intermediate valleys and low 

 grounds. 



The Glacial Beds. 



§ a. The Southern District. This area presents an undu- 

 lating surface of Lower Chalk covered here and there by a 

 patch of Boulder Clay or early River-Gravel. It is bounded on 

 the south and south-west by the escarpment of the Upper 

 Chalk, which extends from Hitchin and Baldock by the hills 

 south of Royston to Saffron Walden and Linton, and is capped 

 with the Drift deposits which extend thence south-westward 

 over Essex. 



The boundary of the Upper Boulder Clay may be traced 

 along the summit of the hills by Tharfield, Newsells Bury, 

 Chishall, Heydon, Elmdon, stretching out in a spur to Strethall 

 and Littlebury Green, but trending south-westwards from 

 Christhall and Rockell's Farm to Ai'kesden and Wenden. 



There are two remarkable points about the lie of this clay: 

 first that it everywhere rests directly on the chalk, without the 

 intervention of sand or gravel; secondly that it not only runs up 

 to the very crown and brow of the hills above mentioned (about 

 500 feet high), but it appears to have also run down the northern 

 slope of the scarp, and outlying patches occur at lower levels 

 beyond the limits just indicated. Near Newsells Bury, for 

 instance, a tongue runs down into the valley; near Boyston there 

 are one or two small outliers that must be at least 150 feet lower 

 than the top of the scarp ; a larger outlier caps the high ground 



