GEOLOGY 
disappears altogether, the Chalk Marl resting directly upon the Lower 
Gault. This thinning-out is most probably due to erosion, for the nodule- 
bed which forms the base of the Chalk Marl contains phosphatic fossils 
derived from the zone of Ammonites rostratus, that is to say of Upper 
Gault age. 
From recent borings the Gault appears to be much thicker in the 
south of the county than has hitherto been supposed. 
The Upper GREENSAND is not a continuous bed; after running 
through the greater part of Buckinghamshire it dies out before it 
reaches Bedfordshire, re-appearing again near the boundary of the two 
counties. It can be traced, but not very clearly, in the parishes of 
Eddlesborough, Eaton Bray, and Tilsworth ; its maximum thickness in the 
county being 20 feet. ‘It consists of fine yellowish-grey micaceous sand 
passing up into dark-green glauconitic sand, which in turn passes up 
into the glauconitic sand that forms the base of the Chalk Marl.” Only 
one fossil, Aucellina (Avicula) grypheoides, has been recorded from it in 
the county and that with some doubt, its horizon being uncertain. 
In the opinion of Mr. Jukes-Browne the restricted area of the 
Upper Greensand in this district is not due to the erosion of a much 
larger deposit, but to the present outcrop marking the easterly extension 
of the original shore-line. He also thinks that beyond its eastern limit 
near Kateshill there was a land-surface which was unaffected either by 
erosion or deposition until the period of the Chalk Marl, when the 
area was submerged beneath the Cretaceous sea. ‘The absence of the 
Chloritic Marl in this locality tends to confirm this view. 
UPPER CRETACEOUS—THE CHALK 
Senonian Upper Chalk Chalk-with-flints. . Zone of Micraster cor-bovis 
Chalk Rock . . . »  Ldeteroceras reussianum 
Turonian Middle Chalk { Soft white Chalk. . »  Lerebratulina lata 
Melbourn Rock . . 3  Rhynchonella cuvieri 
Soft grey Chalk . . »  Actinocamax plenus 
Tough blocky Chalk. »  Holaster subglobosus 
Cenomanian Lower Chalk { Totternhoe Stone . »,  Pecten fissicosta 
Chalk Marl . . . » Ammonites varians 
Chloritic Marl . . »»  Scaphites equalis 
The Chalk is essentially a pelagic formation, and during its deposi- 
tion the whole of England, with the exception of the mountainous 
districts of Cumberland and Westmorland, North and South Wales, 
and Devonshire, was submerged. The depth of the sea gradually 
increased on the whole, but varied greatly from time to time, and at 
one period especially, when the Chalk Rock was being tormed, it must 
have been comparatively shallow; but there is no indication of the prox- 
imity of a shore-line in our area during the deposition of the Chalk. 
The lowest bed of the Chalk is very different from the white lime- 
stone which gives the name to the formation ; the Cutoriric Mart, which 
immediately succeeds the Upper Gault, being usually a grey or bluish 
1 Jukes-Browne, Cretaceous Rocks of Britain, i. 287. 
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