Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/148

110 110 S. Y. WOOD, JT7N., AND F. W. HARMEK ON THE mentioned to us by Mr. "Whitaker, it probably extended further into that county, though now probably mostly removed by the inter- glacial denudation which we have been discussing. Reviewing the whole case, however, it seems to us that the evidence shows that though Norfolk and Suffolk appear, with the exception of the ex- treme northern part, near Cromer (where the Contorted Drift, capped irregularly by the Middle Glacial, exclusively forms the country), to be occupied almost entirely by the Upper and Middle Glacial deposits, yet that this appearance is in a measure deceptive, and that the chief portion of all this area is underlain by a continuation of the Contorted-Drift deposit of North Norfolk more or less denuded throughout its range, so as to form troughs wider and deeper than the existing valleys, into which the succeeding Middle and Upper Glacial deposits have been bedded as well as spread like a mantle over the rest of the denuded surface. For a general view of the extent to which the Lower Glacial formation is exposed over the eastern half of Suffolk and Norfolk, we refer to the map which ac- companies our " Introduction " before referred to. The absence over the greater part of Essex of any traces of the Lower Glacial deposits precludes any satisfactory inference as to how far the valleys of that county may have been formed or modified by interglacial denudation. The middle portion of the valley of the Blackwater has its eastern side formed by an escarpment of London clay, which is part of a series of concentric curved escarpments to which that of the Chalk extending from Cambridgeshire to the Chil- tern Hills belongs. As it is evident that all these concentric escarp- ments had their inception in one disturbance, we may infer that, if one of them can be shown to be Preglacial, the rest of the concentric series are Preglacial also. The Upper Glacial clay lies up to that one of the series which is formed by the Chalk, and shows that it had originated prior to that clay. The Middle Glacial also in part of Bedfordshire occurs in such a way as to indicate that at the time of its deposition the chalk escarpment had acquired much of its present configuration ; but the Lower Glacial beds do not occur anywhere in such a position as to indicate whether they preceded the forma- tion of these escarpments. It is, however, most probable that they did not ; for these curvilinear concentric escarpments appear to have originated in disturbances which upheaved and terminated the older Tertiary formations ; and on this assumption the portion of the Blackwater valley referred to seems to be of Preglacial origin. In this part of the valley a bed of blue clay, full of chalk debris, and undistinguishable from the Upper Glacial of the Eastern Counties, was found, in sinking a well at Witham railway-station, to underlie the Middle Glacial gravel*. Some years ago we found what ap- peared to us to be the same bed, in the bottom of the valley, in some brick-earth pits at Appleford Bridge, near Witham, where it passed up, seemingly without any break, into sandy laminated brick- earth, which was overlain by gravel resembling that of the Middle Glacial so plentiful in the neighbourhood. As this exposure cor-
 * Geol. Mag. vol. v. p. 98.