34 Dr, Du Riche Preller — Three Glaciations of Switzerland. 



produced the rapids of Kaiserstubl, Laufenburg, Eheinfelden, 

 and others, while similarly the Aare has in various places, e.g. at 

 Brugg and Laufthor, eroded the gravel and moraine of the three 

 glaciations down to the molasse, and the Limmat at Wettingen 

 below Zurich, and again between Baden and Turgi near its confluence 

 with the Reuss and Aare, is now eroding the molasse itself. 



The following Table shows at a glance the succession and extent 

 of the three glaciations : — 



As regards, in conclusion, the question whether we live in another 

 genial interglacial period, and have to face the contingency of a 

 further recurrence of glaciation and of a refilling of the valleys fol- 

 lowing upon the present period of erosion, that contingency is not 

 nearly so remote as might appear. During the present century the 

 Alpine glaciers have shown considerable fluctuations. In the first 

 quarter of the century, they steadily advanced, and this advance 

 reached it maximum between 1820 and 1830. They then receded, 

 but after various fluctuations advanced again between 1850 and 

 1860. Then followed another period of shrinkage, the glaciers 

 reaching a minimum in 1880, since which time there has been a fresh 

 advance at an ascertained average rate of 100 metres or about 330 

 feet per annum, say one foot per day. These fluctuations are easily 

 accountable when we consider that the mean annual temperature 

 within a given district or watershed not infrequently varies as much 

 as 3° C or about 6° F. in consecutive years, and that a decrease of 



1 Peat-moors, such as those in the Glatt valley N.E. of Zurich and on the hills 

 (Zurich berg) which separate that valley and the lake of Zurich, are always an 

 infallible criterion of ground moraine underlying them. This ground moraine or 

 boulder-clay generally rests on the bottom of pre-glacial molasse valleys ; it offers 

 great resistance to the percolation of water and even to fluvial erosion, and remains 

 compact. Hence the soil overlying the boulder-clay does not favour cultivation. 

 Surface moraine or moraine sand, on the other hand, is highly conducive to vegeta- 

 tion, owing to its permeability and the consequent decomposition of alkali producing 

 vegetable matter. 



