Yorkshire. G. W. Lamplugh, "Drift of Flamborough Head," Quart. Journ. Geol. Soc., vol. XLVII. p. 384.
Lincolnshire. A. J. Jukes-Browne, Quart. Journ. Geol. Soc., vol. XXXV. p. 397 and XLI. p. 114.
East Anglia. Clement Reid, Mem. Geol. Survey, "The Geology of the district around Cromer."
North Wales. T. McK. Hughes, "Drifts of the Yale of Clwyd" &c. Quart. Journ. Geol. Soc., vol. XLIII. p. 73, and A. Strahan, "Glaciation of South Lancashire, Cheshire, and the Welsh Border," ibid., vol. XLII. p. 486.
Switzerland. C. S. du Riche Preller, "On Fluvio-glacial and Interglacial Deposits in Switzerland," Quart. Journ. Geol. Soc., vol. LI. p. 369 and "On Glacial Deposits, Preglacial Valleys and Interglacial Lake formations in Sub-Alpine Switzerland," ibid., vol. LII. p. 556.
The reader will find references to other works on the Glacial Geology of other districts by consulting the general works referred to on the preceding page.
The foregoing remarks will convince the student that any attempt to show the distribution of land and sea during any part of the glacial period is not likely to meet with general acceptance, as so much depends upon the terrestrial or marine origin of the deposits of the lowlands, and the mode of formation of the shell-bearing drifts of high levels. The occurrence of elevation to a greater height than that which our country at present possesses during portions at any rate of the glacial period has been inferred on general grounds, but direct evidence in favour of it is furnished by the existence of a number of ancient valleys on the land around our coasts, whose floors are often considerably below sea-level, while the valleys are now completely filled up with glacial accumulations, except where they have been partially re-excavated by streams which for some distance run above the courses of the ancient streams.
The climatic conditions of glacial times can only be briefly touched upon in this place. If the periods of advance can be proved to be contemporaneous over wide areas, this points to alternations of colder and warmer periods, or at any rate of drier and wetter periods, though local advance may be due to a number of causes. It must be borne in mind that with the temperature remaining the same, advance of ice can be brought about by increased precipitation of aqueous vapour in the form of snow.
The question of the cause of the glacial period is one that only indirectly affects the stratigraphical geologist until he has accumulated sufficient evidence to indicate the cause. It must suffice to observe that the extremely plausible hypothesis of Croll (for which the student should consult Dr Croll's Climate and Time) does not explain the apparent gradual lowering of climate throughout Tertiary times till the cold culminated in the Pleistocene period, and the student will do well to remain in suspense concerning the cause of the Ice Age until further evidence has been brought to bear upon it.