Glaciers of the Isar and the Linth. 263 



Arnmer Lake and Wuerrn Lake are limited above and below 

 by recent deposits. Their valley basins are bounded below by 

 moraine walls, which the outflow from the lake cuts through, 

 forming extremely narrow valleys. In the sections laid bare, the 

 gravel (b) is shown under the moraine, and in the Wuerm Valley 

 the Flinz is also exposed. Lower down the valley, beyond the 

 moraine walls bounding the lake-basins, the gravel (I. b) forms 

 extended terraces, partly interrupting the covering of the Nagelfluh 

 and the outer moraine. 



IV. — The Relation of the Lake of Zurich to the Quaternary Deposits. 



Wuerm Lake and Ammer Lake, both in their relation to the 

 Quaternary deposits, and also to the Molasse valley to which they 

 belong, show very different phenomena to those of the Lake of 

 Zurich and of the other large lakes of the Alpine borderland of 

 Switzerland. The valleys in the Molasse in these latter are much 

 deeper. A deposit like the gravel (Schotter) (I. b) in Bavaria, which 

 both above and below the lakes exhibits the same uniform slope, is 

 altogether unknown in the Lake of Zurich, in which the only gravels 

 found are those mentioned under (II. b). On the declivities of the 

 valley of the Lake of Zurich, the Molasse rock is itself carved into 

 distinct erosion terraces, independent of the stratification, and these 

 are often only sparsely covered with glacial debris, and not seldom 

 altogether bare, whilst a similar condition of things is not observable 

 on the borders of Wuerm Lake and Ammer Lake on account of the 

 slight elevation of the outcrop of the Flinz and of the valley slopes, 

 and of the soft character of the material. In Switzerland, for the 

 most part, the valleys below the lake basins remain widely open, 

 and show moi-e especially the characters of main lines of ancient 

 valleys. Hardly anywhere is the outlet of a lake through a narrow 

 gully. The Molasse in the district of the Lake of Zurich is plainly 

 not horizontal, but it forms a shallow trough between the Alps and 

 Jura : the depression in this has affected the ancient erosion-terraces 

 in unequal proportions. 



The Quaternary deposits rest on the dislocated Molasse terraces of 

 the declivities of the valley, so that the lateral moraines, with their 

 somewhat steeper inclinations in the direction of the valleys, cut the 

 margins of the Molasse terraces at an oblique angle. 



On the other hand, Rieg Lake and Staff el Lake in some measure 

 call to mind certain of the smaller lakes of the Alpine borderland, 

 as, for example, Greifen Lake and Pfaeffikon Lake in the Glatt 

 district. 



V. — The Question of the Origin of the Lalce-Basins. 



In our excursion in Upper Bavaria, Heim had the opportunity 

 of ascertaining the facts respecting this district published by 

 Penck in his " Vergletscherung der deutschen Alpen." ' On the other 



1 A review of this work is in the Geological Magazine for 1883, Decade II. 

 Vol. X. p. 177. 



