24 The Geology of Cambridgeshire 



of Cambridgeshire to a late stage in the Lower Greensand 

 period. Mr W. Keeping in 1883 fully confirmed this result, 

 and still more recently (1903), Messrs Lamplugh and Walker, 

 examining a limestone band near the top of the Lower Green- 

 sand of Leigh ton Buzzard, Bedfordshire, have found a fauna 

 which according to them has strong affinities with that of the 

 Upper Greensand of some parts of England. 



Gault. 



Overlying the Lower Greensand is the Gault. 



The mutual relations of the two are not always very clear, 

 but in Cambridgeshire there seems to be complete conformity 

 with overlap by the higher beds in all directions. 



A rapid subsidence in late Neocomian times had brought 

 Lincolnshire and Norfolk within the area of pelagic conditions, 

 and, had not the supply of sediment from the south been 

 comparatively large, Cambridgeshire, too, would have shared 

 in those conditions; and as it is, a very notable change in the 

 thickness of the Gault is observable within the limits of the 

 county. Thus well-borings at Ashwell have proved nearly 

 200 feet of Gault clay, while at Cambridge there is but 150 feet, 

 at Soham 90, and at West Dereham, just over the Norfolk 

 border, only 60 feet. Still further north, at Roydon, there is 

 only about 19 feet of clay with a two-foot band of yellow and 

 red marls in the middle of it, and at Dersingham the whole 

 is represented by less than ten feet of the red marls, and 

 between that place and Heacham these pass into the well- 

 known Hunstanton Red Rock, which though only three feet 

 thick represents more than the two hundred feet of clay found 

 in a district less than 50 miles distant. 



This change of thickness, great as it is, doubtless does not 

 represent the whole change really involved. Between Soham 

 in Cambridgeshire and Barton in Bedfordshire, the Gault is 

 overlain by Cambridge Greensand, a bed which contains many 

 phosphate nodules believed to have been locally derived from 

 the Gault by subaqueous denudation, and it is generally stated 



