and Euroioean Drift Deposits. 15 



wliich lie had been engaged since 1886. In this European study 

 about three months were spent in making the circuit of the Alps. 

 The several drift sheets were studied under a variety of topographic 

 conditions. Several months study in the !N^orth German lowland 

 extended from the Kussian boundary westward to the Netherlands. 

 The limits of the last giaciation and of the next older glaeiation, as 

 well as the characteristics of each of their drifts, compared with 

 a still older drift, exposed in recesses of the mountains along the 

 south side of the lowland, were leading subjects of study. In 

 Great Britain the study consisted of a few trips only along the eastern 

 coast and brief visits to the interior. This will account for the small 

 notice taken of British Pleistocene deposits. 



Leverett remarks : "It would be presumptuous for one to pretend 

 to clear up the matter of worldwide correlations of glacial deposits in 

 a single year's studj-; and no one perhaps realizes better than does 

 the writer what a small start can be made in this brief time. A full 

 correlation, however, may in time be reached by repeated efforts of 

 this sort. Even though tentative it seems worth while at this time 

 to state what impressions and results were obtained." 



In Table I are given in outline the main results arrived at by 

 Leverett. IS'os. 1, 2, 3, and 4 are the four well-marked cold 

 periods, whilst A, B, and C ai'e the interglacial warmer periods. 

 Eor fuller details the paper itself should be referred to. Attention 

 may be called to the fact that both in J^ortli America and on 

 the continent of Europe, the trend of glacial work is to support 

 the view that there were at least four cold periods separated by long 

 warm intervals. J. Geikie has shown that even after the Wisconsin— 

 Wiirm period there were climatic oscillations, probably of short 

 duration, which led to considerable increases and decreases in the 

 size of valley glaciers. This view is also held by Swiss and German 

 geologists of note. 



Penck and Bruckner ^ show that during each of the four cold periods 

 when the glaciers debouched upon the foreland enormous quantities 

 of gravel and sand were thrown out over the countrj^, filling iip and 

 obliterating all traces of many vallej^s. These outwash gravels were 

 formed wheii the mountain valleys were filled with great glaciers and 

 were grinding and plucking the rocks upon which they rested. The 

 fine rock flour found its way down the rivers to the sea. 



During each interglacial stage, when the glaciers had almost wholly 

 melted away, the rivers cut deep valleys through the outvrash gravels. 

 Thus valleys were eroded during warm periods and again filled by 

 outwash gravels during cold periods. There is very good evidence 

 that during the four main glacial periods the precipitation of moisture 

 was about the same as that during the warm periods, and that the 

 development of the glaciers was almost entirely due to fall of 

 temperature. 



We, therefore, see that during the glacial periods the rivers were 



so charged with debris that they could not transport it at normal 



gradients, and, as a result, they deposited pans of outwash material. 



AVith the disappearance of the great glaciers great erosion ceased, and 



■^ Die Allien Ciszeitalter. 



