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 27
