A HISTORY OF NORTHAMPTONSHIRE 



surface itself slowly heaving, and for a time we may forget them and 

 attend to the larger movements. Supposing we draw a line through 

 Northamptonshire in the direction of its greatest length, then, speaking 

 generally, to the north-west of this line the characteristic rocks of the 

 county gradually disappear from the surface, and the Trias and older 

 rocks take their place, and also attain to great thicknesses. To the south- 

 east of our hypothetical line we also find the characteristic rocks of 

 Northamptonshire disappearing, but in this direction their place is 

 taken by Cretaceous and newer formations. These contrary conditions 

 along a line lying approximately north-west to south-east can only be 

 explained by differential movements equivalent to alternate rising and 

 sinking about some more stable intermediate area. Apparently the 

 north-west was the sinking area up to about the Middle Lias period, 

 and afterwards the rising one, whereas the south-west, only finally sub- 

 merged in Cretaceous times, was no doubt changed from a stationary 

 or rising to a sinking area at about the same time.* 



Northamptonshire happened to be so near to the fulcrum of the 

 differential movements we have been speaking of, and others acting 

 transversely for shorter periods in Lower Oolitic times, that when 

 within the sinking area it never received the full advantage of it, and 

 when within the rising it lost very little of what it had previously gained ; 

 thus qualitatively the Jurassic rocks are well represented, but quantita- 

 tively they are rather deficient. 



The Keuper 



The first effect of a gradual incursion of the sea into an area which 

 for a long time previously had been dry land would be to convert the 

 fragments of already disintegrated rock strewing its surface into pebbles. 

 The uneven character of the Old Land Surface in Northamptonshire 

 (see pp. 4, 5, 6,) necessarily implies that the pebble beds resting upon 

 it at different levels are not quite of the same age. Some of them 

 greatly resemble the Bunter, but there can be no doubt that they are all 

 of late Keuper to early Lower Lias age. 



By tidal action then the fragments of Carboniferous limestone, quartz- 

 porphyrite, and other rocks constituting the Old Land Surface, were 

 more or less rounded ; they became imbedded in a matrix of light green 

 sand and carbonate of lime, and so produced a kind of concrete which 

 tended to level up the new sea floor. 



Further levelling up of the inequalities of the floor was brought 

 about by deposits of variously-coloured sandstones, marls and clays. 

 (See sections of Orton, Gayton, Northampton and Kingsthorpe, pp. 

 4, 5, 6). These beds have usually been described as Trias simply, a 

 quite unnecessary precaution, since in two cases (Gayton and Orton) they 



' Granite has been found in the Kellaways Beds at Bletchley ; quartz, quartzite, and fossils 

 of Jurassic age in the Lower Greensand of Buckinghamshire and Bedfordshire ; granite, sand- 

 stone, shale, quartzite, and volcanic ash in the Chalk Marl of Cambridgeshire, indicating the 

 late period of total submergence in or near these localities. 



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