22 THE CRETACEOUS ROCKS OF BRITAIN. 
250 feet thick ; thence it thins northward, till in West Norfolk it is 
less than 60 feet thick, though it expands again to 130 feet on the 
Yorkshire coast. From Wiltshire eastward it maintains a thick¬ 
ness of over 200 feet through Hants and Surrey, and is not less 
than 190 feet in the east of Kent. Southward, at Culver, in the 
Isle of Wight, it is still over 200 feet, but it thins rapidly westward, 
and in some parts of Dorset it is not more than 80 feet thick, though 
probably on the average about 100 feet in the north-west of that 
county. 
Where most clearly and fully developed, then, the Lower Chalk 
is made up of the several members shown in Fig. 1. 
/ 
Lower 
Chalk. ’'I 
Zone of 
Holaster 
subglobosus. 
Zone of 
Ammonites 
varians. 
Belemnite Marl. 
White and 
Grey Chalk. 
Totternhoe Stone. 
Grey Chalk and 
Chalk Marl. 
Chloritic Marl. 
Fig 1 . —Tabular view of the component parts of the Lower Chalk 
in districts north of the Thames. 
Comparative views of the development of the Lower Chalk in different 
parts of the country are given on page 23, and at the end of the volume 
(page 558). 
