338 THE TUNDRAS AND STEPPES OF PREHISTORIC EUROPE. 



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 loss. The loss, as we now have it, is a fluvio-glacial 

 silt or loam, very largely reassorted aud 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 suc- 

 cession 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 moutonnees 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 moun- 

 tains 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 consequent!" 

 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 



