Page:Early Man in Britain and His Place in the Tertiary Period.djvu/145

] Ingleborough, in Yorkshire. The tough clays with scratched stones, sometimes so hard to work that it is necessary to employ gunpowder, are considered by Dr. James Geikie to be the débris underneath the ice-sheet accumulated on land, and termed by the Swiss geologists "moraine profonde." These are met with chiefly in Scotland, but they have been observed by Mr. De Rance in South Lancashire, and at the Little Ormes Head in North Wales.

The climate must have been arctic in its severity during this period of glaciation, and this may have been partially due to the fact of the land standing at a higher level above the sea, and being lifted up into the colder regions of the atmosphere. It cannot, however, be wholly so explained, since it was the culmination of a series of changes by which the tropical climate of the Eocene passed into the warm Meiocene and temperate Pleiocene climates.

Then followed a period of depression beneath the sea. The glaciers, which had before carried their burdens of sand, clay, and stone far away from the present seaboard of Britain, now ended at the retreating shoreline, giving rise to icebergs, which deposited the lower boulder clay as they melted, and drifted as far to the south as the valley of the Thames. The mountains were