GEOLOGY 
the altered conditions of the Gault period and coming back with no 
further change than would occur in its natural process of development 
and modification elsewhere during the deposition of the Gault. 
The town of Leighton Buzzard derives its water-supply from the 
Lower Greensand, but the formation is not here a very satisfactory water- 
bearing one. The yield is sufficiently copious, but the presence of iron 
in a form exceedingly difficult to get rid of is a great drawback. The 
water is rather hard, but is much softened by ebullition.’ 
A well for the supply of Biggleswade has recently (1903) been sunk 
into the same formation, the Greensand having been reached at a depth 
of 110 feet. At 170 feet it was found to contain a seam of rock, which, 
with the sand immediately overlying it, is coloured green through the 
presence of grains of glauconite. 
UPPER CRETACEOUS—GAULT AND UPPER GREENSAND 
Upper Greensand . . Zone of Pecten asper 
Selbornian or Albian } Upper Gault . . . » Ammonites rostratus 
Lower Gault . . . » A. interruptus 
While in the Vectian epoch only a small portion of England was 
submerged, all in its eastern division; in the Selbornian the sea extended 
so far to the west as Devonshire, and probably covered the whole of the 
Midland as well as the Eastern Counties. The sea-bed sank as it gradu- 
ally extended, and may have reached a depth of several hundred fathoms 
before the close of the period. ‘The composition and fossil contents of 
the Lower Gavutt however indicate a comparatively shallow sea, not so 
deep as 100 fathoms and probably not averaging more than half that 
depth. The passage from the Lower Greensand to the Lower Gault is 
in some places continuous and in others shows a decided break, apparently 
owing to the former not having been entirely submerged when the 
deposition of the latter commenced. ‘Thus in a brickfield south of 
Leighton no distinct line of demarcation can be seen, the Gault clay 
gradually becoming more sandy downwards until it passes into a clayey 
sand with small pebbles, and that into a coarse yellow sand, obliquely 
bedded, the pebbly bed marking the base of the Gault and indicating 
current-erosion. In a sand-pit north of Leighton, on the other hand, a 
well-marked plane of division may be seen, ‘14 feet of dark-grey clay 
with small patches of bright-red clay at the base resting directly on 
yellow sand.” 
The Lower Gault stretches right across Bedfordshire, from south of 
Leighton to west of Potton, but in a portion of its course eastwards from 
Heath and Reach it is covered by drift deposits ; it is again exposed to 
the south of Flitton, Silsoe, and Shefford. On the slopes of the Lower 
Greensand hills to the north of the Ivel valley there are several small 
outliers of it, and there is a large one north of Shefford. It is the 
1 Analyses by Professor Attfield have been published in Trans. Brit. Assoc. of Waterworks Engineers, 
ili. 199, 224, 225 (1899). ne 
2 Jukes-Browne, Cretaceous Rocks of Britain, i. 284-5. 
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