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 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. 1 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 1 10 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 taper Upper Gault ... „ Ammonites rostratus Lower Gault ... „ A. interrupt™ 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 Gault 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.' 2 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, iii. 199, 224, 225 (1899). 8 Jukes-Browne, Cretaceous Rocks of Britain, i. 284-5. 13