These conditions were rendered possible by the former greater extension of our continent into the Atlantic, when the major portion of the North Sea and the English Channel were dry land, and the British Islands formed part of the continental area.
Considerable climatic changes continued to take place after the passing of the third glacial epoch. These have left their traces in the alpine lands, but they are nowhere so clearly seen as in northern and northwestern Europe. Temperate conditions supervened in north Germany, the flora and fauna closely resembling those of the present. But eventually a relapse to glacial conditions followed, and from the Scandinavian snow fields another invasion of north Germany took place. Norway, Sweden, and Finland were now once more shrouded in ice, and a great Baltic glacier came into existence, the gigantic terminal moraines of which are met with in Denmark, Schleswig-Holstein, and Prussia. The Scottish Highlands and other mountainous parts of the British Islands at the same time nourished local ice sheets and large valley glaciers, which in many cases descended to the sea. The alpine lands in like manner witnessed a recrudescence of glaciation, large glaciers flowing into the great longitudinal valleys, but nowhere deploying as before upon the low grounds. It is to this stage, probably, that we should assign the tundra fauna of the Schweizersbild. (See Map D.)
The succession in that interesting rock shelter has shown that as the severity of the climate relaxed, steppe and forest faunas successively followed the disappearance of the tundra forms. The climate of Europe generally became temperate, and immense forests overspread wide regions. It was during the approach of these conditions, as we have seen, that Paleolithic man seems finally to have vanished and the Neolithic races to have made their earliest appearance in Europe. The British Islands at this time formed part of the continent and the Baltic existed as a great fresh-water lake. The lower buried forests of our peat bogs are among the conspicuous remains of this stage. Eventually, however, submergence ensued, the British Islands were severed from the continent, and the sea again invaded the Baltic basin. It is notable that the character of the marine fauna which at this stage lived off the coasts of Scandinavia and Britain is indicative of more genial conditions than now obtain. The climate, however, gradually became colder, the vertical and horizontal range of the forests was restricted, and snow fields again appeared among the higher mountains of our islands. In Scotland glaciers here and there came down to the sea, and dropped their moraines upon the beaches then forming; the large majority, however, terminated inland. At that time the snow line in north Britain ranged between 2,000 and 2,600 feet. Similarly, in Norway and in the Alps an advance of glaciers took place—the snow line in southern Norway being about 2,400 feet, while in the alpine lands it seems to have averaged 7,500 feet, or some 1,600 feet lower than the present.
