The entire climatic, life, and industrial cycle of which the Magdalenian marks the conclusion has been presented in Chapter IV (p. 281). After a very long period of cold and somewhat arid climate following the fourth glaciation, it would appear that western Europe in early Magdalenian times again experienced a stage of increasing cold and moisture accompanied by the renewed advance of the glaciers in the Alpine region, in Scandinavia, and in Great Britain. This is known as the _Bühl_ stage in the Alps, in which the snow-line descended 2,700 feet below its present level and the great glaciers thrust down along the southerly borders of Lake Lucerne a series of new moraines distinctly overlying those of the fourth glaciation. Another indication of the lowered temperature and increased moisture in the same geographic region is found in the return of the arctic lemmings from the northern tundras; these migrants have left their remains in several of the large grottos north of the Alps, especially in Schweizersbild and Kesslerloch, composing what is known as the _Upper Rodent Layer_, with which are associated the implements and art objects of the early Magdalenian culture stage. We have adopted the minimum estimate of 25,000 years since the fourth glaciation, but Heim(4) has estimated that the much more recent prehistoric event of the advance of this minor _Bühl_ glaciation began at least 24,000 years ago, that it extended over a very long period of time, and that the _Bühl_ moraines in Lake Lucerne are at least 16,000 years of age. The three climatic changes of Magdalenian times are therefore as follows: First, the _Bühl_ Postglacial Stage in the Alps, which corresponds with what Geikie has named the Fifth Glacial Epoch, or _Lower Turbarian_, in Scotland; for he believes that a relapse to cold conditions in northern Britain was accompanied by a partial subsidence of the coast lands, that snow-fields again appeared, that considerable glaciers descended the mountain valleys, and even reached the sea. At this time the arctic alpine flora of Scotland also descended to within 150 feet of the sea-level. The result of this renewed or fifth glaciation in western Europe was the advent of the great wave of tundra life and the descent to the plains of all the forms of Alpine life. [Illustration: FIG. 180. Correlation of the Postglacial climatic changes with the four stages of Upper Palæolithic culture: the Aurignacian coincident with the final retreat of the fourth glaciation; the Solutrean coincident with the interval preceding the _Bühl_ advance; the Magdalenian coincident with the _Bühl_ and _Gschnitz_ Postglacial advances; and the Azilian coincident with the _Daun_ or third Postglacial advance. (Compare Fig. 14.)]