Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/531

1872.]

The actual existence of two out of the three climatal zones during the Pleistocene age is amply proved by the physical evidence of the Pleistocene strata, and altogether irrespectively of the mammalia. The researches of many eminent observers prove that after the close of the Forest-bed era the temperature of Northern Europe gradually became lowered, until it became arctic in character, and the complex phenomena were manifested which we call glacial—the deposit of clays with large angular and scratched blocks, the grooving, polishing, and rounding of the tops of hills and of the bottoms of valleys, and the accumulation of marine gravels.

At the commencement of the Glacial period in Britain an enormous ice-sheet, similar to that under which Greenland now lies buried, extended from the hills of Scandinavia over North Germany, the North Sea, Scotland, Ireland, Cumbria, and the lower portion of England, as far south as the line of the Thames valley, leaving the grooved hill-tops and the Boulder-clay to show its extension. The land then most probably, as Mr. Godwin-Austen, Prof. Ramsay, and Sir Charles Lyell believe, stood higher than it does now. To this succeeded a period of depression, during which the mountains of Wales and the hills of Derbyshire and Yorkshire were submerged to a depth at least 1300 feet below their present level; and the waves of the sea washed out of the Boulder-clay the shingle and sand which constitute the "middle drift" of the north of England, Scotland, and Ireland. Then the region north of the Thames valley was reelevated, and a period of glaciers set in, which, however, were of far less extent than those which preceded them, occupying isolated areas, and not uniting to form a continuous icy mantle to the country. There may have been, and probably were, many more changes in climate and geography than these three; but these are clearly and definitely marked in the whole of Britain north of the valley of the lower Thames, while the subdivisions are not so clearly traceable, and may be mere local phenomena.

From this it follows that there were at least three well-marked changes in the Pleistocene climate of Great Britain, if not of Northern France. The temperature of the Forest-bed era was lowered until it reached a minimum that found expression in the great continuous ice-sheet moving resistlessly down to the sea, over the lower hills and valleys, and traversing hills as high as 2500 feet. 2ndly. Then, coincident with the marine depression, the climate became warmer, until certain southern mollusks, such as Cardium rusticum, found