10 
THE CRETACEOUS ROCKS OF BRITAIN. 
Isle of Wight. Where fully developed, as in the south-eastern 
counties, the Middle Chalk is about 240 feet in thickness, hut in 
some districts it is less than 100 feet. 
The Upper Chalk is less variable in lithological character 
than the Middle or Lower divisions; it consists in the lowest part 
of hard nodular chalk, in which beds of compact limestone fre¬ 
quently occur, one of which is known as the Chalk Rock ; the 
greater part of this division, however, consists of soft white chalk, 
in which flints are generally abundant, but there is a certain zone 
or portion of it where flints are less common and which is some¬ 
times almost flintless. 
Where fully developed the Upper Chalk is more than 1,000 feet 
thick, but its actual thickness varies greatly in different parts of 
the country, owing to the great erosion which it suffered in the 
interval between the Cretaceous and the Eocene periods. Thus in 
the central parts of the London Basin, its thickness has been reduced 
to between 200 and 300 feet, and though it may never have been 
1,000 feet thick in that area, it is probable that several hundred 
feet of chalk were removed before the formation of the lowest Eocene 
beds. 
