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 A HISTORY OF BEDFORDSHIRE and the plant, Endogenites erosa, and remains of Iguanodon from Potton, have been derived from the Wealden as suggested by Mr. Walter Keeping. 1 In all probability however the deposition of the Wealden strata over the south-east of England, which must have occupied a great length of time, took place whilst the whole of Bedfordshire was above sea-level, and therefore whilst the Kimeridge Clay was subjected to sub-aerial denu- dation, by which its higher beds, with the overlying Portlandian, were removed. During the long period occupied in the accumulation of the Creta- ceous strata there was a nearly continuous and usually gradual change from comparatively shallow seas to those which were much deeper if not truly oceanic, occasionally interrupted by temporary elevations of the sea-bed. These alterations in the depths of the marine areas were accompanied by important changes in the character of the fauna. Many of the earlier Cretaceous forms became extinct and were superseded by others of newer types, noteworthy amongst which are the echinoderms Ho/aster, Galerites, and Micraster ; the lamellibranch Hippurites ; the cephalopods Hamites and Scapbites ; the fishes Lamna, Otodus, and Pty- chodus ; and the reptile Polyptycbodon ; species of all of these genera having been recorded for the county. LOWER CRETACEOUS Vectian or Aptian. Lower Greensand. Zone of T'ercbratula sella Owing to the absence of beds of Wealden age the Lower Creta- ceous rocks are in this district represented only by the Lower Green- sand, which is the equivalent of the higher division of the Vectian of the south of England and the Aptian of western Europe ; the lower division consisting in the Wealden area and the Isle of Wight of the Atherfield Clay, a very fossiliferous marine deposit having no representative in Bedfordshire. The Lower Greensand extends across the county from Leighton Buzzard to Potton, forming a very picturesque range of hills running generally from south-west to north-east. For some portion of its extent it is concealed by the boulder clay and alluvium of the valley of the Ivel, and on reappearing it attains a considerable elevation in the vicin- ity of Sandy, having there a rather steep and very prominent escarp- ment. Its maximum thickness is about 220 feet and its exposure is several miles in width, the area it covers being more extensive than in any other of the midland counties. In passing under the newer strata in a south-easterly direction it thins out rapidly, terminating in the adjoining county of Hertford somewhere between Hitchin and Ware. It was the absence of the knowledge of this fact which led the late Sir Joseph Prestwich to believe that it might form a valuable water-bearing stratum for the supply of London ; but although he was mistaken on this point, owing to no boring having at that time ( 1 85 1) been carried to a 1 In Geo!. Mag. dec. 2, vol. ii. p. 375.