Ampthill Clay 15 



to have been deposited more slowly than the Oxford Clay, for all 

 its shelly fossils are encrusted with growths of Serpula and it 

 contains occasional phosphate nodules. 



In the south and west of the district a good deal of the 

 Ampthill Clay was removed by denudation before the deposi- 

 tion of the Lower Greensand and hence the sections there are 

 incomplete. The greatest thickness is attained in the district 

 about Over where it may be about one hundred and fifty feet 

 thick. Towards Upware it thins rapidly and at Chettering is 

 but twenty-six feet in thickness. 



The best exposures are at Gamlingay, where Ampthill Clay 

 is worked in three pits. Another pit is worked at Everton, 

 west of Great Gransden, and the most westerly pit at Hadden- 

 ham is probably in its upper layers. 



The Belle Vue Brickyard at Gamlingay shows an interest- 

 ing section. Flooring the pit is a hard calcareous bed whose 

 lower layers contain enormous quantities of Serpula. This is 

 supposed by many to be the local representative of the 

 Elsworth rock. Some nine or ten feet above it is another 

 hard bed, and above this some seven or eight feet brings us to 

 the base of the Lower Greensand which unconformably overlies 

 the Ampthill Clay. Following along the length of the section 

 we are able to make out a definite transgression from east to 

 west. The junction of clay and sand is absolutely sharp, but 

 the last two or three feet of the clay are quite different in 

 character from the rest, and are rejected by the brick-makers 

 who call the material Bung. This ' bung ' is evidently much 

 crushed and slickensided and is occasionally rucked up into 

 little anticlines which encroach upon the domain of the sand 

 above. 



The commonest and most characteristic of the fossils are 

 the flattened oysters Ostrea discoidea. Belemnites abbreviatus 

 and biplicate ammonites may generally be collected, and there 

 are very many crushed Pectens and other lamellibranchs. 



Towards Upware, as we have said, the Ampthill Clay thins, 

 and gives place to the coral island deposits shown in the 



