28 The Geology of Cambridgeshire 



Passing up from the Greensand we find a continuous 

 diminution of the proportion of clay and a corresponding 

 increase in the amount of chalk, and this continues n-ht 

 up to the top of the Chalk Marl. 



The natural mixture of chalk and clay is very favourable 

 to the making of cement, and the gradation of composition 

 enables manufacturers who judiciously mix their different beds 

 to obtain cement of constant composition ; they are also able to 

 vary that composition, and so make the different grades of 

 cement. 



Unfortunately these manufacturers, when they obtain 

 cement chemically similar to more famous cements which 

 are known by names which declare the stratigraphical horizon 

 of their origin, do not hesitate to apply these stratigraphical 

 names to their Chalk Marl cement, and unless this be stopped 

 much confusion is likely to ensue. 



The thickness of the Chalk Marl in South Cambridge is 

 about sixty feet and the whole is worked for cement. North- 

 ward it thins gradually, and at Cambridge only the lower thirty 

 feet is marly enough for cement making. At Hunstanton, the 

 whole is only sixteen feet thick and has become a very pure 

 white limestone the Sponge Bed. 



Exposures of Chalk Marl are very numerous in Cambridge, 

 and were formerly much more so. Generally speaking, the 

 whole outcrop of the Cambridge Greensand has been worked 

 over and its coprolites extracted to a depth of about twenty 

 feet, but all these workings are now closed and it is only where 

 cement pits happen to be worked to the base of the marl that 

 we are able to examine its peculiarities. 



At the pit of the Royston Cement Company, about a mile 

 south of Barrington, the Chalk Marl is worked for cement and 

 the underlying Gault for bricks, and as these are worked 

 separately a considerable shelf of the greensand and phosphate 

 bed is left exposed. 



The exposure in the more easterly pit is most interesting 

 but the western one shows the higher as well as the lower 



