574 DR. C. S. DU RICHE PRELLER ON GLACIAL [Aug. 1 896, 



IV. Preglactal Sub alpine Valleys. 



I have already shown in an earlier part of this paper that, so far 

 as the ice-sheet which covered the Swiss lowlands towards the end 

 of the Pliocene period can be subdivided, the Linth glacier was, in 

 the main, confined to the Zurich valley, while an arm of the Ehine 

 glacier spread even then over the Glatt district north of the Zurich 

 basin. The same applies, in my opinion, to the flow of the two 

 rivers before the first glaciation — in other words, the Glatt valley, 

 now practically dried up, was originally eroded by an arm of the 

 Rhine, 1 and the Zurich valley by the Linth jointly with the Sihl, 

 whose original, but subsequently barred outlet to the Zurich basin 

 lay, as already stated, in a line from Schindelleggi to the present 

 bay of Eichterswil. 



The erosion of the Zurich basin as a continuous valley from the 

 upper end of the present lake to Turgi involved the removal of 

 two saddles ; one, the Jurassic Laegern bar at Baden, and the other 

 the Molasse and Miocene Nagelfluh saddle which stretched across 

 from near Eichterswil to Eapperswil, as is evidenced by the two low 

 islands of Ufenau and Liitzelau, which now form a reef obliquely 

 across the upper end of the present lake, the Marine Molasse and 

 Miocene Nagelfluh rising just above lake-level. The Baden bar must 

 have been, in the first instance, sawn through by the backward 

 erosion of a torrent descending from the Laegern towards the basin 

 of the Aare, until it was sufficiently lowered to admit the Linth, 

 which subsequently completed the erosion of the lower valley. The 

 erosion of the Laegern bar must necessarily have preceded the first 

 glaciation, because, if it were otherwise, the Deckenschotter deposit 

 of the Gebensdorfer Horn, situated north of the Laegern, could not 

 be the glacio-fluviatile product of the Linth glacier. As regards the 

 Ufenau saddle, the process was probably precisely similar to that at 

 Baden : that is, the torrent originally descending from it towards 

 the Zurich basin, and sawing the saddle by backward erosion, was 

 subsequently superseded by the Linth. On the other hand, unlike 



1 The theory that the Glatt valley was originally eroded by the Linth, and 

 the Zurich valley by the Sihl, has been advocated by Wettstein and by Prof. 

 Heim (' Geologie v. Ziirich,' 1885, and Neujahrsschrift Naturf. Ges. Zurich, 

 1891), while Dr. Du Pasquier (Beitrage, vol. xxxi. 1891, p. 122) favours the pre- 

 sumption that even the Ehine Valley, from the Irschel hills to the confluence of 

 the Aare near Waldshut, was originally eroded by the Linth. In my view, these 

 two rivers are much too small to have eroded such broad valleys. The drainage- 

 areas of the Sihl, Linth, and Ehine at Schindelleggi, Walen Lake, and Sargans 

 respectively, are about 200 : 600 : 4200 sq. kilom., and, at a rainfall of 2 metres 

 per annum, less 25 per cent, for absorption, yield respectively 10, 30, and 210 

 cubic metres per second, the proportion being, therefore, 1:3:21. If we assume 

 that one-third of the volume of the Ehine was deflected at Sargans to the west 

 (while the main arm flowed N.N.W. through the Constance basin, and thence 

 towards the Danube until deflected to the W. towards Schaffhausen), we have an 

 adequate flow for the erosion of the present Walen Lake fjord, the Glatt valley, 

 and the Ehine Valley from Irschel to Waldshut by the western arm of the 

 ancient Ehine, and a sufficient volume for the erosion of the Zurich valley by 

 the combined Linth and Sihl. 



