572 



DR. C. S. DT7 RICHE PRELLER Olf GLACIAL [Aug. 1 8 96, 



the original outlet of that river was to the Zurich valley is evidenced 

 by the deep and broad depression eroded between the two mountain- 

 spurs of Etzel and Hohe Rhonen, near Kaltboden, as is shown in the 

 diagram (fig. 10). The ratio of slope of the two mountains places* 



the Preglacial Molasse-bed of the 

 'I river somewhat to the east of the 



Kaltboden exposure, that is, near 

 Geissboden, where a sharp reverse 

 curve of the Sihl marks the first point 

 of deflection, just below the present 

 confluence of the Sihl and Alp. The 

 Molasse-bed would here probably be 

 at contour 700 — that is, about 80 

 metres below the present moraine- 

 bed of the river, and no less than 

 120 metres below the crest of the 

 moraine-wall, which now bars the 

 old outlet. Prom here to the then 

 floor of the Zurich valley the river, 

 flowing in the direction of the present 

 bay of Eichterswil, must have formed 

 a rapid, having a fall of about 7 per 

 cent. (1 in 13), through a narrow 

 V-shaped valley or ravine, which is 

 now deeply buried under moraine, 

 and probably resembled closely the 

 Molasse ravine of the present river- 

 course between Schindelleggi and 

 Sihlbrugg. 



A second point of deflection o£ the 

 Sihl from the Zurich basin occurs at 

 Schindelleggi at contour 750, where 

 the river describes a sharp curve at 

 an angle of 50°. The entrance of the 

 j ^ deep ravine at this point, just below 



the railway-bridge, is shown in the- 

 transverse section (fig. 11), and presents the peculiar feature that the 

 left bank is formed by a wall of Lower (freshwater, so-called raised) 

 Molasse, and the right bank by a moraine-wall 50 metres in depth,,, 

 resting, however, on Molasse, as is evidenced by a sandstone quarry.. 

 This moraine-wall, which contains, besides angular blocks, enormous 

 boulders of Miocene Nagelfluh up to 3 metres in diameter, and shows 

 irregular, but yet distinct divisional lines, marking the superposition 

 of younger on older moraine of successive glaciations, is a con- 

 tinuation of the wall at the upper point of deflection near Geiss- 

 boden, and constitutes the bar thrown across the Preglacial outlet 

 by the Linth glacier, which thus deflected the Sihl and its successive 

 glaciers to the Reuss basin, until that river eroded its present bed. 

 parallel to, but outside the drainage-area of the Lake of Zurich „ 

 It is hardly necessary to mention that the Sihl glacier-torrent which- 



