In that early period the continents and islands which we now see wholly separated were joined together at various points. The British islands formed a connected part of Europe. The Thames and the Rhine were one and the same river, flowing towards the Arctic ocean over a plain that is now the shallow sunken bed of the North Sea. It is probable that during the last great age, the Quaternary, as geologists call it, the upheaval of what is now the region of Siberia and Alaska, made a continuous chain of land from Asia to America. As the land was depressed again it left behind it the islands in the Bering Sea, like stepping-stones from shore to shore. In the same way, there was perhaps a solid causeway of land from Canada to Europe reaching out across the Northern Atlantic. Baffin Island and other islands of the Canadian North Sea, the great sub-continent of Greenland, Iceland, the Faroe Islands, and the British Isles, all formed part of this continuous chain. As the last of the great changes, there came the Ice Age, which profoundly affected the climate and soil of Canada, and, when the ice retreated, left its surface much as we see it now. During this period the whole of Canada from the Atlantic to the Rocky Mountains lay buried under a vast sheet of ice. Heaped up in immense masses over the frozen surface of the Hudson Bay country, the ice, from its own dead weight, slid sidewise to the south. As it went it ground down the surface of the land into deep furrows and channels; it cut into the solid rock like a moving plough, and carried with it enormous masses of loose stone and boulders which it threw broadcast over the face of the country. These stones and boulders were thus carried forty and fifty, and in some cases many hundred miles before they were finally loosed and dropped from the sheet of moving ice. In Ontario and Quebec and New England great stones of the glacial drift are found which weigh from one thousand to seven thousand tons. They are deposited in some cases on what is now the summit of hills and mountains, showing how deep the sheet of ice must have been that could thus cover the entire surface of the country, burying alike the valleys and the hills. The mass of ice that moved slowly, century by century, across the face of Southern Canada to New England is estimated to have been in places a mile thick. The limit to which it was carried went far south of the boundaries of Canada. The path of the glacial drift is traced by geologists as far down the Atlantic coast as the present site of New York, and in the central plain of the continent it extended to what is now the state of Missouri.