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. 1 Near the base of the clay there is usually a bed or 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 Belemnites 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 bip/icata, are of Lower Gault age. 2 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 Gault, 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 irom 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.' 3 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 Rods of Britain, i. 319. 2 Ibid. p. 286. 3 Loc. cit. * Loc. cit.
