Cambridge Greensand 27 



same direction. The whole section will be described later; in 

 the meantime it is interesting to note that there too the Gault 

 is fossiliferous and, though ammonites are rare, the common 

 Upper Gault lamellibranch Avicula gryphaeoides is abundant. 



Cambridge Greensand and Chalk Marl. 



About the end of the Gault epoch a further swelling of the 

 arch which had determined the Neocomian unconformity of 

 Central England brought the sea floor of the Cambridge area 

 again within the region of effective denudation, and a slight 

 unconformity between Gault and overlying Chalk Marl resulted. 



As is usual in such cases, a pebble bed rests upon the 

 eroded surface of the Gault, and as this is of economic im- 

 portance and is of a type somewhat unusual among similar 

 deposits, it has received a special name, the Cambridge Green- 

 sand. 



The great feature of the bed is the unusual abundance 

 of slightly worn black phosphate nodules which have been 

 washed out of the Upper Gault. These form about ninety-five 

 per cent, of the pebbles in the basal bed and the richness 

 of the fauna provided by them is remarkable. 



The remaining five per cent, of the pebbles are mostly 

 contemporaneous brown phosphate nodules, and these contain 

 a fauna proper to the Chalk Marl. Occasional lumps of 

 foreign rock, granite, gneiss, porphyry, basalt, purple grit, 

 quartzite, etc., rounded or angular, and up to a foot in 

 diameter, have also been recorded. The pebble bed varies 

 from a few inches to about a foot and a half in thickness ; 

 it rests on a pockety surface of Gault, the phosphate nodules 

 or Coprolites being most abundant in the hollows. 



The matrix is mostly a chalky marl but is much richer in 

 clayey material than the overlying beds. Grains of glauconite 

 supposed to be derived from denuded Gault are abundant 

 both among the pebbles and also for a foot or two above them, 

 and it is these that have earned for the bed its name Green- 

 sand. 



