LOWER CHALK—BEDFORD AND HERTS. 
187 
Fossils are not so abundant in this district as they are in the 
coprolite pits near Cambridge, and though Mr. Fordham collected 
carefully for many years, he only obtained about forty species of 
Invertebrates and seven or eight species of Vertebrate remains. 
A fuller account of the Cambridge nodule-bed will be given in 
the next chapter, but it may be observed that the nodules which 
were dug near Shillington, Arlesey, and Ash well were mostly black, 
and have clearly been derived from the Gault. The common and 
characteristic species are such as are found in the Gault nodule- 
beds of Buckinghamshire and in the Upper Gault of Folkestone ; 
while the fossils which characterise the Chloritic Marl and Bye Hill 
sand of Wiltshire and other southern counties, such as Am. [Schl.] 
varians, Am. [Acanth. 1 Mantelli, Pecten asper, Cucullcea mailleana, 
and Terebratella pectitd, have never been found among these 
Bedford and Cambridge “ coprolites.” 
Chalk Marl .—The thickness of the zone of Ammonites 
varians appears to be about 80 feet. The lower part is a dark grey 
marl, sandy near the base, drying light grey ; the higher part is 
a grey or bluish-grey calcareous marl, with little sand, but mixed 
with fine argillaceous matter in variable quantity. No beds of 
hard limestone have been seen in this district, but some hard cal¬ 
careous nodules occur in the railway cutting near Charlton, north¬ 
east of Dunstable. These concretions are heavy and compact, 
resembling septaria in having cracks filled with calc-spar ; some 
of them are a foot in diameter. They occur in two layers, which 
dip southward, and lie about 40 feet below the Totternlioe Stone, 
which is seen further on in the cutting. They are embedded in soft 
grey marl, which passes up into a bluish-grey marl below the 
Totternlioe Stone. 
Zone of Holaster subglobosus. 
Totternlioe Stone .—This stone attains its maximum thickness, 
20 to 22 feet, in the counties of Bedford and Hertford. It 
has been quarried in many places as a building-stone from very 
early times, and it is also a source of water-supply, many strong- 
springs 000111*14110' along the line of its outcrop, and giving rise 
to small permanent brooks, by the side of which most of the 
villages are situate. Its outcrop generally forms a slight rise or 
low escarpment, for, being firm and blocky, it resists denudation 
better than the soft underlying marl. An account of its minute 
structure will be given in Chapter XXIV. 
The bold spur which juts out from the Dunstable plateau and is 
crowned by the fine tumulus of “ Totternhoe ” (see Fig. 44) is 
pierced by numerous galleries driven in olden times to win the 
stone which crops out round the hill, but the only open section is 
a quarry on the northern side, and when stone is being quarried 
the following beds may 7 be seen : — 
