A HISTORY OF BEDFORDSHIRE 
erosive action of a tributary of the little river Ivel (the Flit) which has 
cut them off from the main mass ; they form however but a slight indi- 
cation of the former extent of the Gault to the north and west. 
The Lower Gault is usually a light or dark grey marly clay. 
Where it is worked for brickmaking near Silsoe it is called ‘ blue clay,’ 
and it is used mixed with the ‘black clay’ of the Lower Greensand 
before mentioned. At Arlesey its higher part is extensively worked, 
a section about 50 feet in depth showing a transition from dark clay 
at the base to that of a much lighter colour at the top. It there con- 
tains from 26 to 31 per cent of carbonate of lime.’ 
Near the base of the clay there is usually a bed ot phosphatic 
nodules which has been worked at various places for making into 
artificial manure. Many fossils have been found in and with these 
nodules, species or Ammonites and Belemnites prevailing, and by far the 
most abundant species being Be/emnites minimus. In a brickyard at Heath 
near Leighton the nodule-bed contains an admixture of Lower and 
Upper Gault fossils ; while at Campton near Shefford, apparently about 
the same horizon, 20 to 25 feet from the base of the Gault, there is 
a nodule-bed with fossils, all of which, with the exception of Terebra- 
tula biplicata, are of Lower Gault age.” 
An interesting relic in proof of the existence of the Gault beneath 
the south of the county is furnished by a brick which is built into the 
wall of the market room of the Cock Inn, Luton, bearing the legend 
‘F. Burr, 465 feet, Jan. 1828.’ This brick was made from material 
brought up from the bottom of a boring for an artesian well at the 
Old Brewery adjoining. From the estimated thickness of the beds from 
the Middle Chalk to the Upper Gault inclusive, it is safe to assume 
that the stratum in which the boring terminated was Lower Gault. 
Another bed of phosphatic nodules marks the base of the Upper 
Gau tT, which in this district is a much more calcareous formation than 
the Lower Gault, containing about 50 per cent of carbonate of lime. 
The sequence of formation was continuous, but the alteration in the 
mineral composition of the Upper Gault indicates that it was here 
deposited at a much greater distance trom the shore of the Cretaceous 
sea than was the Lower Gault. In fact it was laid down in a sinking 
sea with a receding shore-line. It is a marly clay much resembling the 
Chalk Marl in appearance and composition, Mr. William Hill having 
found it by microscopical examination ‘to consist of calcareous matter 
in a fine state of division enclosing many small particles which are pro- 
bably shell-fragments and some tests of Foraminifera.’* It is not of the 
same composition throughout, for this light-grey marl passes up into a 
darker grey sandy and micaceous marl.‘ 
When traced across the county in an easterly direction it is seen to 
diminish in thickness, its basal nodule-bed being brought nearer and 
nearer to the overlying Chalk, until, between Barton and Shillington, it 
1 Jukes- Browne, Cretaceous Rocks of Britain, i. 319. ® Ibid. p. 286. 
3 Loc. cit. * Loc. cit. 
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