accumulated, and may well have endured for some time alter glacial conditions bad passed away. In these temperate latitudes, however, they were bound ere long to melt and allow the overlying alluvial deposits to settle down in the manner already described.
There are thus various lines of evidence which lead to the conclusion that during a glacial epoch the lower reaches of all the great valleys opening out from glaciated regions, as well as large tracts of the wide plains extending in front of the northern mer de glace, would be more or less drowned in temporary lakes of turbid water, over the beds of which a line sediment of somewhat uniform character must have been deposited. And such is generally believed to be the origin of the materials of the löss. The loss, as we now have it, is a fluvio-glacial silt or loam, very largely reassorted and rearranged by the wind. Its history, therefore, is involved with that of the Ice age, and we must consequently turn our attention to the unquestioned deposits of that period, with a view to discover, if we can, at what particular stage of it the glacial silts were worked over by the wind, and tundra and steppe faunas successively occupied the low grounds of middle Europe.
Let us first, then, trace as briefly as may be the history of the glacial and interglacial deposits. Avoiding detail, we shall confine attention to the more salient features of the evidence and try to picture the succession of events from the beginning to the close of glacial times.
The facts upon which geologists base their conclusion that a vast ice sheet formerly covered much of northern and northwestern Europe, while great snow fields and glaciers existed not only in the Alps, but in many of the minor mountain ranges of central and even of southern Europe, may be very briefly summed up.
First, we have the evidence supplied by morainic accumulations of all kinds—bottom moraines or bowlder clays and terminal moraines. Second, we have the proofs of former glaciation afforded by striated rocks and roches moutonnées and by the crushed, broken, tumbled, and confused rock surfaces that occur so frequently underneath the bottom or ground moraines. Third, we have the presence of certain remarkable ridges of gravel and sand which appear to have been formed in tunnels under the ice, and of enormous sheets of similar materials which have been spread out by the waters escaping from the terminal front of the inland ice of northern Europe, while in all the great valleys leading down from the Alps and other glaciated mountains we see broad terraces of alluvial detritus which have been deposited by torrential streams and rivers. All those fluvio-glacial deposits, when followed from the low grounds into the regions occupied by moraines, are found to dovetail with the latter and are consequentl—of contemporaneous origin.
By mapping rock stripe and noting the general trend of the erratics which constitute so large a portion of the ground moraines we acquire a knowledge of the directions followed by the inland ice and the great
