Page:VCH Bedfordshire 1.djvu/57

 GEOLOGY waterworn, while those in the clays, having been subjected to less at- trition, retain their angular form. Each separate flint is an index of a much larger amount of chalk which has been removed ; and so numerous are the flints over this area that it is a popular belief that ' stones grow,' the supposition being that the supply is maintained by their production within the soil by some occult influence. This is more particularly the case in the soil covering the superficial stratum known as clay-with-flints. Considered lithologically the Upper Chalk is a soft, white, almost pure limestone which in weathering separates into thin flakes. It con- tains flint both in the nodular and tabular forms. Its exposures in this district are neither numerous nor extensive ; they lie chiefly between Luton and the extreme south of the county. A chalk-pit at East Hyde exposes about 20 feet of this stratum. Near the top of the section is a thin layer of marly chalk similar to those which occur in the Middle Chalk but remarkably even. About 3 feet below this, and parallel with the lines of deposition, is a thin seam of flint which breaks into small fragments under weathering. There are also present numerous flint nodules, most of which are disposed in layers, a few being distributed unequally through the mass of chalk. The most typical fossils which have been recorded from this stratum in Bedfordshire are Echinocorys vulgaris, Ostrea normanniana, Pecten quin- quecostatus, and Belemnitella quadrata. Between the period of the Upper Chalk, which closes the Mesozoic epoch in western Europe, and the commencement of the Cainozoic epoch, important changes were effected in the life-history of this part of the globe, whole genera of organisms becoming extinct. As examples represented in the Mesozoic strata of the county may be named the marine reptiles Ichthyosaurus, P/esiosaurus, and the winged Pterodactylus, and also the cephalopods Ammonites, Scaphites, and Belemnites. The allied genus Nautilus, which occurs in several of the local beds, still survives, being now found in tropical seas. The Chalk once extended very much farther to the north-west than it now does. Proofs of such former extension of one formation over another are furnished by outliers, which are detached portions of the main mass. Instances of such outliers of the Chalk Marl on the Gault occur at Billington Hill near Leighton, and between Upper and Lower Gravenhurst. These however only give a slight idea of the former extent of the great Chalk formation. The Chalk of the south- midland and eastern counties was once continuous with that of Lincoln- shire and Yorkshire, having since been cut through by the rivers which flow into the Wash. EOCENE— READING BEDS A long interval must have elapsed between the close of the Meso- zoic era in western Europe and the commencement of the Cainozoic, and this was more prolonged in Bedfordshire than in the south of 23