30 Tlie Geology of Cambridgeshire 



Lower or Grey Chalk. 



Resting conformably on the Chalk Marl is the Grey Chalk 

 or Zone of Holaster subglobosus. The basal beds of this series 

 are of somewhat unusual character and are variously known 

 as Totternhoe Stone, Burwell Rock or Clunch. These beds 

 differ markedly from the rest of the Chalk of the district in 

 that they contain an appreciable proportion of detrital quartz. 

 They are very compact and harsh to the touch, but this is due 

 to the presence of a cement among the shell fragments rather 

 than to the actual quartz grains. The rock is well and 

 regularly jointed, and lying as it does between porous Chalk 

 above and impervious Chalk Marl below generally determines 

 the line of springs which follows the base of the Chalk escarp- 

 ment. The rock is rich in phosphate, and green lumps of 

 phosphatized chalk are common, especially at the base. 



The only exposure of the lowest beds is to be found in the 

 large pit nearest the station at Burwell, where the rock is still 

 worked as a building stone. The higher beds are also worked 

 in this pit and in another some four hundred yards to the 

 north of it and are burnt for lime. Fossils are most abundant 

 in the lowest beds, but the limeburners collect from the higher 

 ones only. The most characteristic fossils are the grinders 

 and other teeth of sharks, the various Pectens Heaveri, 

 orbicularis, and fissicosla, several species of Inoceramus, Tere- 

 bratula semiglobosa, Rhynchonella Mantelliana, and Holaster 

 subglobosus. The rock is twenty feet thick at Burwell but 

 varies much. 



Another tolerable exposure is still open just south of 

 Haslingfield and there the peculiar Ostrea frons and various 

 ammonites may be collected. 



At one time the Totternhoe stone was worked all along 

 its outcrop, but there is now no further demand for soft 

 building-stone suitable only for interior decoration, so the 

 working has ceased and the pits are much overgrown. 



