Page:VCH Northamptonshire 1.djvu/38

 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. 8