180 
THE CRETACEOUS ROCKS OF BRITAIN. 
the Belemnite marls and the Totternhoe Stone, which crops out at 
the cross-roads, is only about 50 feet, instead of 75 or 80. It would 
appear, therefore, that some 25 or 30 feet of this white chalk has 
here been cut out by current erosion before or during the deposition 
of the overlying marl. 
At the northern end of the section the Belemnite beds consist 
as usual of two layers of marl, with an intervening bed of white 
chalk ; but the lower marl band is unusually thick, measuring 18 
inches, and is very shaly and gritty in the lower part. When 
followed southward, this lower part is seen to thin out entirely, 
the surface of the white chalk below rising as shown in the sketch 
(Fig. 42). A few yards further the bed d also thins out, allowing 
the two marls to come together and form one band about 2 feet 
thick. 
Fig. 42. — The Belemnite Beds in the chalk-pit at Butler’s Cross, 
near Wendover. 
f. Base of Melbourn Rock. 
e. Upper marl. 
d. Hard white chalk. 
c. Lower marl. 
b. Grey-gritty shale. 
a. Mottled rock 
The upper surface of the massive chalk (n) is slightly undulating 
and very sharply marked. The uppermost 10 or 12 inches of this 
chalk has a curious structure, being very hard and marbled with 
a grey chalk, of which in some places there is more than of the 
white chalk, but so intimately are the two mingled that the rock 
breaks through both. 
The nodule-bed near the base is also of special interest; the nodules 
are of all sizes up to that of a large potato. Internally they are of 
a pale butt' colour, but the outer portion is riddled with small tubules 
which are filled with the chalky matrix ; the green matter outside 
is merely a coating. The mass of the bed is rather coarse-grained, 
and under the microscope it is seen to consist chiefly of broken shell- 
fragments, with many glauconite grains and some Foraminifera. 
The material of the nodules is compact and fine-grained. 
The next exposure shows only the highest beds overlain by the 
Melbourn Rock. This is by the main road to Tring, less than half a 
mile east of Buckland Wharf, and here the following beds are 
seen : — 
