Penck(68) estimates the intensity of the cold and of the humidity which prevailed during the glacial stages by the _descent_ of the snow-line in the Alps, which in the two periods of greatest glaciation reached from 1,200 m. (3,937 ft.) to 1,500 m. (4,921 ft.) below the present snow-level, with the consequent formation of vast ice-caps hung with glaciers which flowed great distances down the valleys of the Rhône and of the Rhine and left their moraines at very distant points. The moraines and drifts of the lesser glaciations, such as the first and fourth, stand considerably within the boundaries of these outer moraines and drift fields. On the contrary, the warmer climates of interglacial times are indicated by the sun-loving plants found at Hötting, along the valley of the Inn, in the Tyrol, which are proofs of a temperature higher than the present and of the _ascent_ of the snow-line 300 m. (984 ft.) above the existing snow-level of the Alps. [Illustration: SNOW LINES OF THE FOUR PRINCIPAL GLACIAL EPOCHS OF THE PLEISTOCENE PERIOD A-B Profile across Europe along the line A-B of map 5 Present snow line 4 Snow line of the Fourth (Würm) Glacial Epoch 3 " " " " Third (Riss) " " 2 " " " " Second (Mindel) " " 1 " " " " First (Günz) " " FIG. 13. An ideal earth section from the North Cape across the Scandinavian plateau, through the North Sea, Swiss Alps, Pyrenees, and Straits of Gibraltar, to the Atlas Mountains in northern Africa, along the line indicated on the map (Fig. 25, p. 65), illustrating the sea-level at the time of the greatest elevation of the continent during the Second Glacial Stage, as compared with the present sea-level; also the successive lines of descent of the region of perpetual snow during the four great glacial advances, as compared with the present snow-line. From studies by Dr. C. A. Reeds.] The alternation of the cold climates of the glacial stages with the warm temperate climates of the interglacial stages formed great oscillations of temperature (Figs. 13, 14). The fossil plant life indicates that during the periods of the First, Second, and Third Interglacial Stages the climate of western Europe was cooler than it had been during the preceding Pliocene Epoch and somewhat warmer than it is at the present time in the same localities. During the First, Second, and Third Glacial Stages there was certainly a marked lowering of temperature in the regions bordering the great glacial fields. This is indicated by the arrival in the northern glacial border regions of animals and plants adapted to arctic and subarctic climates. It has been generally believed that the whole of western Europe was extremely cold during these glacial stages, and that the heat-loving animals, the southern elephants, rhinoceroses, and hippopotami, were driven to the south, to return only with the renewed warmth of the next interglacial stage.