206 
THE CRETACEOUS ROCKS OF BRITAIN. 
CHAPTER XVII. 
THE LOWER CHALK IN NORFOLK. 
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General Description. 
In the southern part of Norfolk, between the rivers Little Ouse 
and Wissey, the base of the Chalk lies below the Fenland, hut it 
comes to the surface near Stoke Ferry, and continues above sea- 
level, thdugh often obscured by Drift till it passes beneath the sea 
near Hunstanton. 
The Lower Chalk undergoes great changes as it passes from the 
south to the north of this county, and its thickness is at the same 
time greatly diminished. Near Stoke Ferry the total thickness of 
the Lower Chalk is probably about 125 feet, but this decreases 
rapidly northward; at Marham it is about 90 feet, and at 
Hunstanton it is reduced to about 56 feet. 
The Chalk Marl or Zone of Ammonites varians.— This has 
a bed of glauconitic chalk at its base between Stoke Ferry and 
Shouldham, but the glauconite grains are very small. Farther 
north this bed disappears, and a hard-white chalky limestone rests 
directly on the Gault marls. In the southern part of the county, 
as far north as Stoke Ferry, the Chalk Marl consists of alternating 
soft and hard beds, and it maintains a thickness of 70 to 75 feet; 
but north of Stoke the soft beds seem to thin out, and the harder 
beds become more purely calcareous, owing to the elimination of 
the siliceous and argillaceous ingredients, so that in the northern 
part of the county the zone of Am. varians consists entirely of hard 
greyish-white chalk, and at Hunstanton is only 18J feet thick. 
This must be regarded as a condensed equivalent of the Chalk Marl. 
The change may be compared with that which takes place in 
Dorset, where the marly beds similarly thin out (see p. 108). 
Zone of Holastcr subglobosus.— In Norfolk, Holaster subglobosus 
is again found in the beds below the Totternhoe Stone and con¬ 
sequently ceases to be a good index for the higher zone, while 
Off aster sphwricus has not yet been found in this county. 
This zone has the usual bed of Totternhoe Stone at the base, 
and at Stoke Ferry the thickness of this stone is about 4 feet, but at 
Hunstanton it is only 2 feet. It is a tough grey gritty chalk, with 
a layer of irregular green or yellow-coated nodules at its base, like 
those in the “ brassil ” of Burwell. As usual, it is mainlv com- 
posed of comminuted fragments of shell (60 to 70 per cent.), and 
glauconite is present in some quantity, but quartz grains are few 
and very small. 
