CHAPTER YII. 



CORRELATION OF THE CAMBRIDGESHIRE DRIFTS WITH THOSE 

 OF THE EASTERN COUNTIES. 



In correlating the various glacial deposits which occur in 

 different parts of eastern England, the presence of the great 

 Chalky Boulder Clay is of great assistauce ; from its wide 

 extension and the constancy of its princijDal characters it forms 

 an excellent base or reference-stratum, so that, the presence of 

 this clay having once been definitely determined, we are at 

 once able to correlate the other members of the series which 

 may exist in the separate areas. 



This Boulder Clay, the Upper Glacial of Mr- S. V. Wood, 

 Junr., has been mapped both by him and by the Geological 

 Survey over the whole of Essex and over large parts of Cam- 

 bridge and Suffolk. Its extension through Cambridgeshire has 

 been indicated in the preceding pages, and it has been shown 

 that the main mass of this clay ends along the edge of the 

 Chalk escarpment above Royston, Saffron Walden, Linton, 

 Balsham, and Dullingham, at a height of from 500 to 350 feet 

 above the sea. It has however been pointed out by Mr W. H. 

 Penning^ that there are occasional outliers sloping down the 

 escarpment into the ancient valley of the Cam, and that the 

 subsequent re-excavation of this valley has resulted in the 

 separation of the Boulder Clay areas lying to the N.W. of the 

 main mass, with which they were once undoubtedly continuous. 



Thus the great spread of Boulder Clay which lies on the 



1 Qiiart. Journ. Geol. Soc. Vol. xxxii. p. 198. 



