Vol. 52.] DEPOSITS, ETC., IN SUBAXPINE SWITZERLAND. 



56: 



Fig. 6. — Section at Au. 



Au. — About 9 kilometres from Kiissnacht, but on the left side 

 of the lake and close to the shore, is situated the peninsula of Au, 

 entirely composed of glacio-fluviatile gravel, the origin and the age 

 of which have long been a fruitful source of discussion. Escher 

 von der Linth and others regarded it as of Tertiary age and con- 

 temporaneous with the Uetliberg JSagelfluh — that is, pre-Glacial'in 

 the old acceptation of the term ; while latterly, among others by 

 Dr. Aeppli, 1 it has been classed with the Low Terrace, namely, the 

 Pleistocene gravels of the third glaciation. In the paper read last 

 session, I mentioned it incidentally as probably being a detritus- 

 cone deposited by the river Sihl during the older Pleistocene or 

 second Interglacial period, but a close examination of the locality 

 has led me to modify this view. The peninsula (fig. 6) forms an 

 oblong dome-shaped cone 

 rather less than 1 kilo- 

 metre in length and 

 about 50 metres in depth, 

 the summit being at con- 

 tour 456, while the mean 

 lake-level is at contour 

 409. All along the edge 

 of the lake-level, but 

 more especially at the 

 western end, and facing 

 the lake, there appears a 

 cliff with both natural 

 and artificial caves, of 



hard, compact, and irregularly stratified conglomerate from 5 to 

 15 metres in depth, many of whose pebbles, varying in size up to 

 that of a man's fist, are leached and disintegrated and show traces of 

 former crushing and other impressions. In one exposure, where the 

 stratification is clearly marked, a stratum of sand 5 metres thick is 

 intercalated between the Cavernous Nagelfluh, and the whole 

 deposit has a dip up the valley of about 20°, which is, however, 

 due simply to its deposition as a detritus-cone, and not to earth- 

 movements. At another point the Cavernous Nagelfluh is covered 

 by a looser and coarser gravel, largely composed of rough and very 

 imperfectly rolled pebbles of Sernifite, evidently deposited subse- 

 quently and by a stream flowing transversely over the conglomerate 

 into the lake. In a farm-shed near the summit of the peninsula, 

 at contour 445, one of the walls, 12 x 4 metres, is formed by 

 gravel also dipping up the valley and composed, in upward 

 succession, of 1 metre of sand containing pebbles, 2 metres of 

 very coarse and loosely cemented gravel in which are embedded 

 boulders up to 1 metre in diameter, and 1 metre of loose, some- 

 what less coarse gravel. This is obviously, like the Sernifite-bearing 

 gravel lower down, a much younger deposit than the conglomerate 

 of the cliff bordering the edge of the lake, the latter being, in my 



1 Beitrage z. geol. Karte d. Schweiz, vol. xxxiv. (1894) p. 94. 

 Q. J. G. S. No. 207. 2 q 



rRildm. 



Cavernous Nagelfluh 

 LAKE ZURICH 

 406 m.aboue sea-level 



