gravel terrace (that of the third glacial epoch) is in like manner sheeted in löss. The loss on these three separate horizons is for the most part wind blown, and exactly resembles that of middle Europe generally, showing the same structure and arrangement, and containing a similar assemblage of organic remains.
To what extent each of these "horizons" of löss may be represented in the low grounds of middle Europe we can not definitely say. But as the materials of the loss are for the most part of fluvio-glacial origin, it is obvious that such accumulations must have been formed during each successive advance of the alpine glaciers. As each glacial epoch passed away those accumulations were greatly modified by the wind, and drifted into the valleys that drain the Alps, where they were subsequently covered and to some extent preserved under the morainic and fluvio-glacial deposits of the succeeding epoch of glacial advance. It seems probable, therefore, that the wind-blown loss of the low grounds of middle Europe does not belong exclusively to any one particular stage of the glacial period. It is impossible, however, at present to divide it up into separate stages. But we may feel sure that if tundra and steppe faunas succeeded each other again and again in the valley of the Rhine, they could hardly fail to have done the same in the wide plains of middle Europe.
It will be remembered that at the Schweizersbild the deposits containing remains of tundra and steppe faunas rest immediately upon fluvio-glacial gravels. These gravels were laid down during the third glacial epoch. It is quite certain, therefore, that the faunas referred to must have entered Switzerland after the retreat of the glaciers from the low grounds. But how long an interval may have elapsed between the disappearance of the glaciers and the advent of the lemmings and their congeners we can not tell. All we know is that after the appearance of the tundra fauna in Switzerland the climate, at first cold and arctic, gradually became less extreme, so that in time a steppe fauna, and afterwards a forest fauna, succeeded. In other words, no perceptible hiatus separates the present from the conditions that obtained when the reindeer hunter vanished from the alpine lands. He was succeeded by Neolithic man, just as the latter was followed by the men who used bronze and iron implements and tools. So far as the evidence of the Schweizersbild rock shelter is concerned, we should infer that no great alternations of cold and genial epochs followed after the final retreat of the great glaciers of the third glacial epoch. But, as we shall see presently, the tale told by that interesting rock shelter is incomplete. Certain considerable climatic changes did take place after the third glacial epoch had passed away. The evidence of such change, however, though not wanting in the alpine lands, is much more clearly displayed in northwestern Europe. To the testimony yielded by the glacial and interglacial deposits of that region, therefore, we shall now direct attention.
