32 The Geology of Cambridgeshire 



of the beds. Having no stratification along which the succes- 

 sive layers may slip when slightly folded, the rock had to 

 develope horizontal joints before the normal formation of the 

 dip and strike series could occur. These horizontal joints, 

 therefore, resemble tiny thrust planes, and as such are curved 

 rather than straight, so when stretching joints commenced 

 to form, they too were somewhat irregular, and now the whole 

 effect is strongly suggestive of false-bedding. Contraction 

 during the drying and later irregular solution by percolating 

 water may also have had their effect. 



The exposure is capped by a few feet of the so-called 

 Belemnite marls and Melbourn Rock, which here are not 

 particularly interesting and contain no fossils. They have, 

 however, a yellowish colour, and so serve to indicate the throw 

 of a couple of small faults recognizable below by their slicken- 

 sided surfaces and fault breccia. 



Another large pit in beds similar to those of Cherryhinton 

 is opened on the Caius Golf Links, by the side of the Gog 

 Magog Road. Fossils are not abundant, but Holaster and 

 occasional Pecten may be obtained from the workmen. 



Above the Holaster subglobosus zone is the zone of 

 Belemnitella (Actinocamax) plena, here represented by some 

 seven feet of alternating sandy marls and hard yellow chalk. 

 The marly beds contain lumps of apparently older white chalk, 

 and the surface of the chalk below is said to be eroded. 

 Fossils are rare. 



The line separating these marls from the overlying hard 

 Melbourn Rock is taken as marking the division between the 

 Lower and Middle Chalk. 



Middle or White Chalk. 



The lowest zone of the Middle Chalk is the zone of lilti/u- 

 chonella Cuvieri, and in Cambridge this zone is believed to 

 include the Melbourn Rock. 



