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 



* Henry John Eunson, ' The Range of the Paljeozoic Rocks beneath Northampton,' 

 Quart. Journ. Geol. Soc. (Aug. 1884), vol. xl. p. 492. 

 ^ Ibid. 



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