the remains of which happen to have been preserved, in spite of the erosion of the surface by the glaciers and the dash of the waves on the sea-shore, during the repeated depressions and elevations of the land described in the last chapter.
The presence of the late Pleistocene mammalia in the river deposits later than the boulder clays as far as the North Riding of Yorkshire, proves that they were in Britain after the land had been elevated above the sea, in which the icebergs had deposited their burdens of upper boulder clay in the midland and northern counties. They were, however, living in the south of England and in France, while the boulder clays and marine sands were being accumulated in the area north of London and Bristol. As this rose above the sea, they gradually passed farther north, and it is very probable that they were prevented from invading Ireland and Scotland by a barrier of sea, and that the higher parts of the country were rendered inaccessible by the glaciers, as yet unmelted.
Thus we may picture to ourselves southern and eastern Britain as inhabited by an abundant mammalian fauna during the last phase of the Pleistocene age; while ice and sea acted as barriers to the free migration which afterwards took place over the whole country in the Prehistoric age. We must further realise that all the climatal and geographical changes, known as glacial, happened while the late Pleistocene mammalia were living in the regions not covered by glaciers or overwhelmed by sea, and that they wandered to and fro as the barriers to their migration were altered. The glacial period did not define one fauna from another, and the only mark it made on the mammalian life was to push
