Glaciers of the Isar and the Linth. 265 



the more conceivable, since the southern part of the covering of 

 Nagelfluh has a somewhat steeper dip than the more northern part; 

 this, however, may readily be regarded as its original structure. 



A positive proof of such changes cannot be found in the oldest 

 deposits of the district of the Bavarian highland lakes, which thus 

 contrast with those of the Lake of Zurich, because connected erosion- 

 terraces are wanting, and more especially because the position of the 

 strata of the Miocene Flinz cannot be accurately determined. But 

 though the possibility of such an origin of the lakes through local 

 changes of level cannot be proved to be inconceivable, it is so, 

 nevertheless, if it leaves the coincidence of the extension of the 

 glacier with the formation of the lakes, and the absence of lakes 

 beyond the margin of the glacier, to be explained as the result of 

 pure chance. 



As regards the third ground for the glacial origin of the Bavarian 

 highland lakes, the aye of the lake-basins, it may be remarked that 

 the gravels (I. b) are deposited on the slopes of Nagelfluh and Flinz. 

 As the gravels were formed before the advance of the glacier, the 

 valley which they cover, as well as the gaps in the covering of 

 Nagelfluh, must also have been in existence before the glacier, 

 as, moreover, Penck has already noticed (Vergletscherung, p. 357). 

 There remains, therefore, no further solid excavating work for the 

 glacier to perform, but merely the re-excavation of a part of the old 

 valley out of the gravels (I. b), leaving behind many fragments 

 of the same on the slopes, and further, the scooping out of the 

 hollow, 120 m. in depth, in the soft Flinz. It may be noticed that 

 the Flinz is a marly-clay which falls to pieces in water, and although 

 rock in appearance, it would offer less resistance to the course of the 

 glacier, than even loose pebbles. 



Staffel Lake and Rieg Lake lie between ridges of Molasse, parallel 

 to the Alps. The oblique partition separating them is formed of 

 huge masses of the gravels (I. b), which can hardly have been origin- 

 ally deposited in their present limits. Heim acknowledges, in view 

 of this, that a re-excavation of a part of the gravels in the same 

 northerly direction in which the Alpine valleys become open, is by 

 far the most probable mode of the formation of these lakes. Heim 

 also accepts a similar origin for the now extinct lake of Murnau and 

 the lake Kochel. As, however, regarding these lakes, it is only a 

 question of the re-excavation of basins of dislocation, in which the 

 harder ridges of Molasse on their margins and the islands of the 

 same have not been destroyed, we are both entirely of the same 

 opinion respecting them. 



On a consideration of the Bavarian highland lakes, Heim conceives 

 the re-excavation of valleys filled with loose gravels, as also the 

 scooping out of depressions in very yielding materials, and thereby 

 the formation of lake-basins through glaciers, possible, as heretofore 

 (comp. " Gletscherkunde," p. 382 at top, and p. 386), and in the 

 present instances, as very probable. On the contrary, proofs of the 

 formation of extended basins in hard rock and of the excavation of 

 the same through glaciers cannot be found in this district. 



