20 The Geology of Cambridgeshire 



CRETACEOUS. 

 Lower Greensand. 



Towards the end of Jurassic time a notable uplift affected 

 the whole area of East Central England, and the old JUIM 

 sea area was broken up. 



South Cambridgeshire seems to have been near the summit 

 of the rising arch, but it is not quite clear at what period 

 the area was first affected. Certain it is, however, that 

 uplift was not persistent through the whole time interval 

 indicated by the unconformity. Kimeridge and Lower Port- 

 land Clays show little change in lithological character all along 

 their outcrop, and Mr Lamplugh and others in working out 

 the relations of the Speeton, Tealby and Snettisham Clays and 

 the Spilsby and Sandringharn Sandstones have shown that 

 each successive zone of these Neocomian strata overlaps its 

 predecessor on to the flank of the uplifted ridge. A similar 

 state of affairs is probable among the more estuarine beds of 

 the south, but much work remains to be done before such 

 overlap can actually be proved there. 



Uplift was followed by a gradual sinking of the now 

 denuded ridge, and towards the end of Lower Greensand 

 times the sea was able to break across it and again link the 

 water areas of the north and south. 



Under such conditions the Lower Greensand deposits of 

 Cambridgeshire were laid down, and it is not therefore sur- 

 prising that they are somewhat anomalous in character. 



The Lower Greensand of Cambridge consists of a variable 

 thickness of clean bright yellow or yellow-brown sands, much 

 false bedded on both large and small scales. When brought 

 up from deep borings it is found to contain numbers of green- 

 coated sand grains and occasional glauconite concretions which 

 seem to be casts of foraminiferal chambers. The sand is 

 almost entirely a quartz sand; its grains are generally suh- 

 angular, but the larger ones have their edges rounded and 

 their surfaces well polished. 



