Page:VCH Bedfordshire 1.djvu/39

 GEOLOGY yielding, besides aquatic forms of life, remains of land plants and animals. It varies much in thickness within short distances, and in fact is not a persistent bed, sometimes occurring only in ' pockets ' or ' pipes ' let down into the underlying strata. The Series is a richly fossiliferous one in some places, but not here, and it represents in time the Stonesfield Slate of the south of England, from which have been obtained many fine fossils that adorn the walls of our museums. While estuarine and freshwater conditions continued in the north, marine conditions set in from Lincolnshire southwards, commencing with the deposition of oolitic limestone. This does not indicate a deep sea, for the limestone was in all probability the detritus of coral reefs, which are built up in shallow water on a slowly-sinking sea-bed. The influx of the sea must have been from the south, and the warm currents thus brought in would be favourable to the growth of corals. The Great Oolite here consists of two divisions which are not very dis- tinct from each other — the Great Oolite Limestone and the Great Oolite Clay. The former is by far the most persistent, extending through the midland and southern counties, while the latter is repre- sented in the south of England by the Forest Marble, a shallow-water and perhaps partly estuarine deposit which gradually takes its place. The Great Oolite Limestone is quarried for lime-burning and for use as a building-stone at several places near Bedford, where its thick- ness varies from 25 to 32 feet. It extends from Kempston south of Bedford westwards to Cold Brayfield, Carlton, and Harrold, and north- wards to Puddington and Farndish. It usually consists of pale grey, dark blue, and bluish-grey limestone, either earthy, oolitic, or flaggy, in beds of varying thickness (from 1 foot to 10 feet) separated by thinner beds of pale grey, dark blue, or mottled clay or clayey marl, both lime- stone and clay frequently being crowded with specimens of Ostrea sowerbyi. The limestone is occasionally false-bedded or current-bedded, which indicates shallow-water deposition, this inference being confirmed by the great variations in the thickness and character of the beds which take place within short distances. After O. sowerbyi the next most fre- quent fossil is O. subrugulosa. Myadas are abundant, and remains of saurians and fishes also frequently occur. The water-supply of Bedford is derived from this formation. The water has been analysed, with that from the River Ouse adjacent to the pumping-station, by Professor Attfield, F.R.S. 1 Both waters are hard, but the well-water is about twice as hard as the river-water. The Great Oolite Clay is very variable in colour, calcareous in places, contains selenite, and has at or near its base a nodular ironstone band about which the clay is sometimes dark and carbonaceous. It occurs near Bedford but is not persistent ; the Cornbrash, at West End, Stevington, resting directly on bluish oolitic limestone. It is only a few feet in thickness. 1 See Trans. Herts Nat. Hist. Soc. iv. xxviii.-xxix (1888). 5