.'578 



DR. C. S. DT7 EICHE PEELLER ON GLACIAL [Aug. 1 896, 



.figuration and depth of the Preglacial valley by transverse sections. 

 Of such transverse sections I have plotted no less than twenty, from 

 which I have deduced a mean section for each of the three divisions 

 of the Ziirich valley, covering in the aggregate a distance of over 50 

 kilometres, as follows : — 



kilometres* 



(1) Turgi (confluence of the Limmat and Aare) and Baden 6*5 



(2) Baden (upper end of basin) and Zurich end of present lake. . . 22*5 

 ,(3) Zurich and Wollerau, or IJfenau bar (Preglacial confluence 



of Linth and Sihl) 22*5 



51-5 



Again, from these divisional mean sections I have deduced a 

 general mean section, both the former and latter being shown in 

 the diagrams (figs. 13, 14, 15, & 16), and drawn to an uniform 

 horizontal and vertical scale. As is seen, the old valley widens and 

 deepens gradually from its lower to its upper end, and the general 

 mean section gives a width between the crest-lines of about 7 kilo- 

 metres and a depth of 300 metres, 1 the general ratio of slope being 

 5°, equal to 8*7 per cent., or 1 in 11*5, while the vertical dip, or 

 proportion of depth and upper width, is 1 : 23, and the cross-section 

 of the valley therefore represents an extremely flat curve. 



If we compare this valley with two contiguous old valleys — namely, 

 that of the Grlatt, eroded by an ancient arm of the Rhine, and 

 that of the Reuss — we find that they all present the flat dome-shaped 

 ridges and the correspondingly shallow depressions which are 

 eminently characteristic of all Molasse valleys eroded by large 

 rivers. The Albis or Uetliberg range, dividing the Zurich and Reuss 

 basins, is the only one whose Preglacial dome-shaped configuration 

 has been profoundly altered by the Postglacial erosion of its right 

 flank by the Sihl, and of its left flank by the Reppisch, in conse- 

 quence of which the original dome was converted into a y\ (inverted 

 wedge-shaped) central ridge, with a low ridge on each side. 



1 The totals of the calculation are as follows, the sections being considered 

 flat parabolas, according to the formula § sk. 



Upper width. 



s 



metres. 



I. #(2210 X 



II. |(4820 X 



III. |(9700 X 



Depth. 



metres. 

 157) 

 191) 

 400) 



Length. 



metres. 

 X 6500 

 X 22500 

 X 22500 



51500 



Cubic contents. 



cubic metres. 

 1503 million. 

 13809 „ 

 58199 „ 



73511 



Hence the mean section is 1,427,000 sq. m., the mean width 6"6 kilom., and the 

 mean depth 323 m. In round figures, the total quantity of material removed 

 by erosion for about 50 kilom. may therefore be taken at 70,000 million cubic 

 metres, the mean section at 1,400,000 sq. m., and the mean width and depth 

 at 7 kilom. and 300 m. respectively. 



