Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/154

116 116 S. V. WOOD, JITN., AND F. W. BARHER ON THE has since taken place. Of snch a change, however, we can discover no indication, the great changes in relative level having in our view occurred in that part of England which lies south of the Thames, which was the theatre of disturbance at the close of the Glacial period, when the country rose from the sea *. The explanation therefore which Mr. Penning has offered of the absence of the Middle Glacial from the fens of Cambridgeshire, viz. that the currents from the north that formed it were entirely ex- cluded from the valley, seems to us altogether inadequate — because its absence is not confined to that valley, but prevails over most extensive districts, which must have become submerged by a depres- sion of less than even 200 feet ; and the explanation is, we think, rather to be sought in the position of the great branch of the land- ice to which we have adverted. We may either suppose that the original outspread of the Middle Glacial extended over this region, and that the advance of the land-ice ploughed it out and destroyed it along with much of the older formations on which it rested, or that the land-ice occupied the region during the accumulation of the Middle Glacial, and so prevented its deposit. It must be ad- mitted that neither of these hypotheses explains the absence of the formation beneath the Upper Glacial in South Essex, where this for some miles overlaps it. It is also a perplexing feature that some denudation has occurred in the bottoms of valleys, by which the Upper Glacial (or clay un distinguishable from it) rests directly on beds older than the Middle Glacial sand, as is shown in Section XV. in the case of the Ket valley, and of which instances are also to be found in the Waveney, Blyth, and Gipping valleys. This, if the clay so occurring be the Upper Glacial, seems to have taken place either during the accumulation of that deposit, or " that of the Middle Glacial, but to have been very partial or local. We are not prepared at present to offer any explanation of either of these difficulties ; but, with this exception, the latter of the two hypo- theses seems to harmonize with all the phenomena surrounding the question, as we will endeavour to show. In a paper by one of us on the correlation of the Scotch and English Glacial deposits f, the sequence of the Glacial formations posterior to the Contorted Drift (that is, posterior to the interglacial unconformity already discussed) was, it was attempted to be shown, both vertical and horizontal, and much more the latter than the former. In following the South- Yorkshire coast-section this is clearly seen ; for the clay which near the Humber-mouth and for some twenty miles north of it forms the lower part of the cliff, and, as proved by borings, descends to a considerable depth beneath the beach, is as full of rolled chalk as is the East>- Anglian Upper Glacial, with which, indeed, we identify it. Where the Glacial beds remain least denuded this clay is seen to be succeeded upwards (in sections 100 feet and more in height) by clay containing chalk debris in less and less quantities, till in the uppermost portion of the cliff-section, t Geol. Mag. April, 1872.
 * Quart. Journ. Geol. Soc. vol. xsxii. p. 198.