14 



Geology and Physiography 



THE CHALK 



The Chalk Marl, about 80 ft. thick, is a compact bluish argillaceous 

 limestone, weathering brown. Gasteropod and ammonoid casts are 

 characteristic. It is exposed at the Norman Cement Works, just south-east 

 of Cambridge, and near Barrington. 



The Bimi'ell Rock (15 ft.-20 ft.), seen at Burwell, is well jointed, 

 brownish in colour, and contains small brown phosphatic nodules, and 

 an abnormal proportion of small shell-fragments. It passes upwards into 

 the Grey Chalk (70 ft.), which is distinguished by curvilinear jointing, 

 often almost horizontal and simulating bedding. Holaster subglobosus is 

 common in the lower part, but is replaced by H. gregoryi in the upper 

 (seen in the pit on the Golf Course on the Gogs). At the top is a variable 

 series with yellowish laminated marl seams in which Actinocamax plenus 

 reaches its greatest size: this is the attenuated representative of the Bclem- 

 nite Marls. 



The subdivisions of the Chalk are summarised in the following table : 



The Melbourn Rock (about 10 ft. thick) is a hard limestone; on weathered 

 surfaces it is seen to be nodular. Greenish marl seams occur, and sometimes 

 the marl wraps round isolated nodules. Inoceramus labiatus, Rhynchonella 

 cuvieri and Discoidea dixoni are characteristic. Above the Melbourn Rock 



