6 Geology and Physiography 



THE MESOZOIC ROCKS OF CAMBRIDGESHIRE' 



The Mesozoic rocks of Cambridge strike roughly north-east and south- 

 west, with a very gentle dip to the south-east.^ The older beds (Jurassic) 

 thus occupy the north and west of the County,^ the younger beds (Cre- 

 taceous) the south-east. A few outliers of Cretaceous beds interrupt the 

 Jurassic outcrops, as at Haddenham and Ely ; and an anticlinal fold produces 

 a Jurassic inlier surrounded by Cretaceous at Upware. The Cretaceous- 

 Jurassic boundary is an unconformity ; the base of the Cretaceous rests on 

 the Kimeridge Clay at Ely, but, when traced to the south-west, this base 

 oversteps the Kimeridge and Corallian in turn on to the Oxford Clay. 

 These Jurassic rocks form the northern limb of an anticline with its 

 axis (in the region of Sandy, Beds) pitching south-east. 



The formations in the County may be summarised as follows: 



rChalk 

 Cretaceous IgjI^'^'^S^ Greensand 



[ Lower Greensand 

 1^ Kimeridge Clay 



Jurassic J CoraUian j^,"^P''^;j! g^^y- f ^-^^^ ^^S' C°"^"^ 0°"^^ 

 •' (Elsworth Rock Series 



V Oxford Clay 



The Jurassic rocks are covered by drift in the Fens, except when they 

 protrude to form the "islands" of Haddenham and Ely. The Cretaceous 

 beds are largely covered by glacial deposits, both in the south-west and 

 in the south-east (see Figs. 4 and 29). 



OXFORD CLAY 



Oxford Clay is found in the west of the County and beyond, but it is 

 badly exposed. A little beyond the County boundary at Forty Feet Bridge, 

 north-east of Ramsey (Hunts), the Geological Survey have recently 

 collected ammonites identified by Dr Spath as Scarburgiceras scarhurgense 

 (Young and Bird) ; but the best exposure is to the south-east of Ramsey 

 at Warboys, where the same ammonite is plentiful. The Oxford Clay is 

 dark blue or grey, with a few thin argillaceous limestones, bands of 

 septarian nodules, selenite and pyrites (the fossils are often pyritised). 

 Dr Arkell has suggested that the Warboys exposure is of the mariae-zone; 



' By A. G. Brighton, M.A. 



^ To the north, in Norfolk, the strike changes to approximately north and south. 



3 I am indebted to the Director of the Geological Survey for permission to include 

 some unpublished information about the Jurassic Clays in the Fenland. Part of the 

 area is now undergoing revision by the Geological Survey. 



