342 THE TUNDRAS AND STEPPES OF PREHISTORIC EUROPE. 



more reached the low grounds at the base of the Alps, but did not 

 flow so far as their predecessors of the preceding or second glacial 

 epoch. Tbe snow line of this third glacial epoch stood at an average 

 level of about 4,400 feet below the present. 



Each glacial epoch was necessarily marked by profound glacial 

 erosiou, and the consequent formation of massive sheets of ground 

 moraine in the lower reaches of the great valleys, and of huge terminal 

 moraines at or opposite their mouths. Enormous quantities of shingle 

 and gravel were at the same time swept outward by the rivers escap- 

 ing from the ice — each series of terminal moraines being thus closely 

 associated with its separate and distinct set of fluviatile deposits. No 

 difficulty is found in separating those successive accumulations of 

 gravel. They form terraces lying one within the other at three suc- 

 cessive levels. The highest rises upon an average 250 to 300 feet above 

 the present rivers; the surface of the middle terrace is about 100 feet 

 below the surface of the highest, and about the same distance above 

 the level of the lowest terrace. Each terrace rests upon solid rock, 

 and it is obvious, therefore, that the several epochs of gravel accumu- 

 lation have been separated by epochs of active river erosion. This 

 remarkable valley-within-valley formation is clearly the result of 

 climatic changes. The highest terrace indicates the action of flooded 

 rivers escaping from the glaciers of the first glacial epoch. These 

 glaciers then disappeared or shrank into comparative insignificance, 

 and an interglacial epoch of active valley erosion succeeded — the 

 rivers cutting their way down for a hundred feet or more into the solid 

 rocks. Next came the second glacial epoch, and the lowered valley 

 bottom was again deeply covered with gravel. The glaciers of this 

 stage then in their turn retired, and a second interglacial epoch super- 

 vened, when the rivers as before deepened their channels, working 

 down through the older gravels and excavating the underlying rocks. 

 Thereafter the third glacial epoch ensued, and a new series of gravels 

 was deposited at a lower level than the preceding accumulation. 

 Lastly, this third glacial epoch passed away and the rivers again 

 trenched the fluvio-glacial gravels, the upper surface of which is now 

 much above the reach of the greatest floods. 



What relation, then, does the loss bear to the glacial and inter- 

 glacial accumulations of the alpine lands'? Fortunately to this ques- 

 tion a definite reply can be given. It is dovetailed with the glacial 

 deposits in such a manner as to show that its formation has taken place 

 at successive epochs. 'I hus it occurs occupying an interglacial position 

 between the accumulations of the first and second, and between those 

 of the second and third glacial epochs. When we pass down the valley 

 of the Rhine a similar succession is encountered. In the wide plain 

 lying between the Vosges and the Black Forest, loss is met with on the 

 same geological horizons, overlying the gravel terraces of the first and 

 the second glacial epochs. Not only so, but even the youngest or lowest 



