Page:VCH Northamptonshire 1.djvu/39

 GEOLOGY are immediately succeeded by Rhastic beds without any signs of uncon- formity, and at three (Gayton, Northampton and Kingsthorpe) have yielded salt water, rising to a great height, thus proving their connection with the Warwickshire Keuper. It may be added that salt water was also met with in a boring at the L. & N. W. Railway Bridge Street Station, Northampton, in 1846. The water came from about 650 feet below the surface, i.e. about 559 feet below sea-level, and rose to within 8 feet of the top of the boring. The Rh^tic Beds For a considerable time preceding the Rhastic period, a vast regional depression had in all probability been taking place, which masked any differential movement that may have accompanied it ; each succeed- ing deposit covered a larger area than its predecessor, up to the time when no dry land was left near ; and not only so, but greater uniformity in the character of the sediment resulted as the land supplying it receded, and so permitted of the sorting action of deeper water. We therefore find the Grey Marls of the Rhastic period probably represented by some 6 feet of grey and cream-coloured marls at Gayton only, the most westerly section to which we can appeal. The Black Shales are well developed at Gayton (22 feet), the Avicula-contorta zone being identified by such fossils as Avicula contorta, Cardium rhceticum and Pecten valoniensis, and the horizon of the celebrated Bone bed by the remains of fish, such as Acrodus, Gyrolepis and Saurichthys. At Orton these beds can only be identified by the exact matching of some 10 feet of green shale and sandy marl with material at Gayton. The White Lias is almost equally well developed at Gayton and Orton, it is characterized by iron pyrites in both places, but the only fossils recorded ' — Pectens, Ostrea and reptilian remains — are from Gayton. The absence of Rhstic beds at Northampton has presented difficul- ties* but admits of a simple explanation. If we take the top of the Middle Lias as a datum, and consider that it was deposited under very uniform conditions as to depth over a large area (see p. 12), then, since that time, relatively to Northampton, Orton has been raised some 219 feet, and Gayton 167 feet. Before this movement, therefore, it would appear that the Old Land Surface at Northampton must have been about 57 feet higher than at Gayton, and 33 feet higher than at Orton, and not lower as at the present time, so we can understand why it never received true Rhstic deposits. The peculiar littoral deposits resting on the Old Land Surface at Northampton, and other specific characters of this section can now be better understood, for the deposits are in part contemporaneous with the Rhstic and lowest beds of the Lower Lias of other localities. The combined thickness of Lower and Middle Lias is less at Northampton than Quart. Journ. Geol. Soc. (Aug. 1884), vol. xl. p. 492. ^ Ibid. 9
 * Henry John Eunson, ' The Range of the Paljeozoic Rocks beneath Northampton,'