184 
THE CRETACEOUS ROCKS OF BRITAIN. 
CHAPTER XV. 
THE LOWER CHALK IN BEDFORDSHIRE AND 
HERTFORDSHIRE. 
General Description. 
Near Dunstable, in Bedfordshire, the Lower Chalk is about 200 
feet thick, but it becomes rather thinner to the northward, and is 
not more than 170 feet between Hitchin and Ash well, in Herts. 
Near Dunstable there are passage beds between the Upper Gault 
and the Chalk Marl, but throughout the rest of the area included 
in these counties the base of the Chalk Marl is marked by a layer of 
phosphate nodules (Cambridge Greensand) resting on an eroded 
surface of Gault. 
The zone of Ammonites varians is about 80 feet thick near 
Totternhoe, and appears to consist throughout of soft marly chalk. 
The zone of Holaster subglobosus has at its base a conspicuous 
bed of Totternhoe Stone, which is sometimes 20 feet thick as at 
Totternhoe, sometimes divided into two beds by an intervening 
band of marl, as near Hitchin. 
The rest of this zone seems to be about 100 feet thick near Dun¬ 
stable, but is not more than 80 feet near Hitchin and Baldock. It 
consists of grey chalk in the lower part and of nearly white chalk 
in the upper half. There is occasionally a bed of hard grey rag 
- with large brown and yellow nodules, but it seems to be on a higher 
horizon than that near Tring. 
The subzone of Actinocamax plenus presents no unusual features 
except that the central hard white chalk is sometimes replaced by 
brecciated grey and white chalk. 
Stratigraphical Details. 
1. Zone of Ammonites varians. 
Chloritic Marl and Coprolite Bed. —At Eddlesborough 
and Eaton Bray, on the border of Bucks, the Chalk Marl passes 
down through soft glauconitic marl into the Selbornian sands 
(see Yol. I.) ; and greenish micaceous sand, with presumably the 
same upward passage into Chalk Marl, can be traced around the 
low-lying ground between Totternhoe and Tilsworth. East of 
this the Cretaceous strata are concealed by Glacial deposits, and 
the base of the Chalk has not been observed. 
