14 The Geology of Cambridgeshire 



The middle clay is often replaced by sandy beds and oc- 

 casionally disappears altogether. 



The commonest fossils are the ammonites of the Plicatilis 

 type, together with almost all the ammonites of the highest 

 Oxford Clay. Gryphaea dilatata is ever present, and the 

 same may almost be said of Exogyra nana. Pholadomya, 

 Pecten, Avicula and a few gasteropods, may generally be 

 obtained. Collyrites bicordata is the most characteristic 

 fossil, but there is no species which we can definitely record 

 as peculiar to the horizon. 



Away from the line of outcrop, rocks agreeing with the 

 Elsworth rock are known in various well sections. Those at 

 Bourne and at Chettering Farm, seven miles due east of 

 Holywell and three from Upware, are most interesting. In 

 each, the three-fold division is recognizable and the oolitic 

 character is well marked. At Upware the rock reappears at the 

 surface near the Inn and has been traced for about a mile to 

 the east. A well just north of the Inn exhibited it overlain by 

 some three feet of coral limestone rubble, and Mr Wedd, by 

 digging at the bottom of a small pit in the adjoining field, 

 found more of it overlain by some seven feet of undisturbed coral 

 limestone which, from its lithologic'al characters and included 

 fossils, he correlated with the well-known Coral Rag of the 

 south pit at Upware. More recently beds agreeing with the 

 upper beds have been recognized in a trial hole made through 

 the Coralline Oolite of the South pit, and it seems probable 

 that the Elsworth rock underlies all the Corallian deposits 

 of the Upware inlier. 



Above the Elsworth rock over most of the district lies the 

 Ampthill Clay, a clay much like the Oxford Clay but differing 

 from it in several points of detail. This contains a rich but 

 ill-preserved fauna which has enabled Mr Roberts and others to 

 correlate it with the Corallian of other districts. The Ampthill 

 Clay is darker in colour than the Oxford Clay and contains 

 a good deal more carbonaceous material, and though pyritous 

 in places, rarely has its fossils preserved in pyrites. It seems 



