Geology and Physiography 13 



tliose of the lower part of the Upper Gault in an ice-transported boulder 

 at Ely; and those of the auritus-zone in a pit one mile west of BurweU. 

 The section at Barnwell revealed over 50 ft. of grey and blue calcareous 

 clays, with an inconstant limestone band near the bottom. The thickness 

 of the Gault varies between 100 ft. and 200 ft. ; to the north-east it is 

 reduced to 60 ft. at Methwold, Norfolk. Here, the rock passes laterally 

 into a white calcareous clay, and finally into Red Chalk at Hunstanton 

 on the Norfolk coast. 



CAMBRIDGE GREENSAND 



The Cambridge Greensand is a thin bed of calcareous clay with glauconite 

 grains and phosphatic nodules, resting on a well-defined surface of Gault 

 Clay, but passing gradually into the Chalk Marl above. The glauconitic 

 grains may be foraminiferal casts; they are commonest at the base, and 

 disappear as they are traced upwards into the Chalk Marl. The phosphatic 

 nodules occur usually witliin a layer about one foot thick. They are 

 sometimes very rare, and are commonest in the deeper depressions in the 

 Gault surface. They, too, become fewer and smaller when traced upwards, 

 and usually disappear earlier than the glauconitic grains. The nodules are 

 often black or dark brown, but there are light brown examples, and all 

 intermediate stages are to be found. Many include fossils or are internal 

 casts of fossils; often they have adherent lamellibranchs (such as Dimyodon 

 nilssoni). More rare, are pebbles of igneous and sedimentary rocks; 

 similar pebbles have been recorded throughout the English Chalk, but are 

 sporadic. The sudden change of lithology between the Gault and Cam- 

 bridge Greensand makes the junction quite distinct. The upper surface of 

 the Gault is irregular, and is obviously an erosion surface ; small irregular 

 tubes filled with a matrix of the Cambridge Greensand penetrate down- 

 wards into it. The passage from Cambridge Greensand into Chalk Marl 

 is gradual, so that the thickness of the Cambridge Greensand cannot be 

 given accurately ; it is, however, approximately one foot thick, but may 

 be more in the deeper hollows of the Gault. Lithologically, the Cambridge 

 Greensand is the basal pebble-bed of the Chalk Marl. 



The fauna recorded from the Cambridge Greensand is very large, partly 

 because it was once extensively worked for its phosphatic nodules and 

 therefore offered abundant opportunities to collectors, partly because the 

 phosphatisation of its fossils increased their chances of survival. Some of 

 the species, such as Terebratulina triangularis, are always unphosphatised; 

 but the majority are internal phosphatised casts. Dr Spath considers that 

 all the ammonites may have come from the aequatorialis- to substiideri- 

 zones of the Upper Gault. 



