GEOLOGY 



also the slow advance of the latter ; whilst a residue of much clay and 

 little or no real gravel, except in particular situations, must be accounted 

 for by a very slow melting of the ice, either from below upwards, by the 

 rise of earth heat when actual refrigeration ceased, or if under subaerial 

 melting (as it must have been very largely towards the end), by the 

 slight fall of the water, due to depression of the land. Chalk, flint, 

 and Bunter pebbles are the most abundant erratics ; granite, greenstone, 

 jasper, lydian stone, white quartz, mica-schist, carboniferous limestone, 

 gritstone, coal-measure sandstone and shale, etc., also occur, all indicat- 

 ing a distant origin for much of the ice. Some of the stones are striated, 

 but more particularly the large and moderately hard local rock fragments 

 generally found at or near the base. 



The great thickness of the glacier and its universal extension over 

 the county are not in doubt, for every hill that has been carefully 

 examined shows traces of Drift. 



Post-glacial Gravels 



No marine or fresh water shells of contemporaneous age have been 

 found in the Drift of Northamptonshire (excepting such as admit of 

 another explanation, cf. Brigstock, p. 26), and speaking generally 

 there is no distinct bedding in the Boulder Clay ; in other words it was 

 not deposited in water. Much evidence is available, however, from out- 

 side the county, of a depression of some 140 to 170 feet during the 

 period of extreme glaciation, and restitution to its present level after- 

 wards. 



We accept such a depression for Northamptonshire because it is 

 quite consistent with observed phenomena, and permits of a better ex- 

 planation of some succeeding events than could otherwise have been 

 given, such as post-glacial gravels only on fairly high ground, or in the 

 river valleys, where, as so-called river gravels, they occur at different 

 heights. 



Development of Modern Scenery 



The larger features of Northamptonshire Physiography were un- 

 doubtedly developed before the Pleistocene period, nevertheless consider- 

 able modifications were brought about by glaciation of the county ; for 

 instance, the hills are now specifically or relatively less high than they 

 were by the amount of material removed from them by ice, and much of 

 the lower ground probably higher than before from the Drift deposits 

 left on it being thicker than the rock removed. 



As the last ice sheet was melting old lines of drainage tended to 

 resume control of the water discharge, and in this they were largely but 

 not completely successful. It must be remembered that when cutting 

 back of the ice-filled valleys recommenced, it was not from present sea- 

 level, because the eastern part of the county, and therefore the lower parts 

 of the old valleys, were submerged ; so by the time that the land had 

 regained about its present elevation, many new lines of drainage had been 



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