Jura Lakes in Switzerland. 345 



left of Lake Zurich, at altitudes 840 and 800 metres, or about 

 400 metres above that lake, their length being 10 and 5 km. and 

 their mean width 2 and 1 km. respectively. Both valleys were 

 choked by the Linth Glacier advancing more or less at right angles 

 through the Zurich Yalley, and the lakes thus formed existed until 

 during the recession of the glacier they overflowed the moraine walls, 1 

 and the Sihl eroded for itself an entirely new course through moraine 

 and Molasse parallel to Lake Zurich, while the Waeggi-Aa found its 

 old way again direct to that lake. The floors of both basins sloping 

 considerably towards their exits, where erosion was, moreover, very 

 active, the lakes gradually emptied themselves through their exit 

 channels, which have an average fall of 1 in 40 and 1 in 30 respectively, 

 as shown in the diagrams (Sheet No. 1, Fig. Ill 2 ). The altitudes of 

 the exits of both basins, which are practically hanging valleys about 

 400 metres above the present Lake Zurich, point to the same 

 conclusion as that arrived at in relation to the altitude of the 

 Klontal Yalley. 



4. Lac de Joux (Sheet No. 1, Fig. II), well known for its winter 

 sports, occupies, at '1,008 metres altitude, a Cretaceous basin between 

 the two ridges of Mont Risoux and Mont Tendre, the former of which 

 marks the crest-line of the Jura. As shown in the sketch-plan, the 

 lake formerly extended fully 15 km. or double its present length 

 of 7 - 5 km., higher up the valley, practically to the Lac des Iiousser, 

 from which issues its principal affluent the Orbe. The average width 

 of the present lake is 1 km., and its greatest depth 34 metres. At 

 its lower end it connects with a small outer lake, Lac lirenets, 

 practically an outflow regulation and storage tank, at the end of 

 which the Orbe plunges into a funnel or 'entonnoir', characteristic 

 of the Jura, whence it issues 2 km. below at altitude 789 metres, 

 viz. at a fall of 1 in 10, and then winds in a steep ravine past Vallorbe 

 through the Jura until it discharges into the Yverdun Canal at the 

 head of Lake Neuchatel. Lac de Joux owes its existence to one of 

 the numerous shallow depressions in the Jura terraces parallel to the 

 axis of the chain, and its gradual deepening to the action of solution 

 in its Cretaceous bed, while its abnormal shrinkage is due to 

 evaporation and percolation. Fortunately, the entonnoir, being at its 

 exit instead of a fissure in its floor, saved the overflow valley from 

 drying up. The lake and the River Orbe constitute important power 

 factors in the industrial development of that part of the Jura, as do 

 Lake Klontal and Lake Aegeri in their respective areas. 



II. Lakes with Moraine or Fltjviatile Ears. 

 In this group I have comprised the smaller lakes which, with the 

 one exception of Lake Sarnen, lie in the -Molasse formation and at 

 altitudes between 435 and 540 metres. For the sake of brevity their 

 dimensions are given in the following table (p. 350) : — 



1 An illustration of the moraine wall cut by the Sihl at Schindelleggi will 

 be found in my paper, Q.J.G.S., vol. lii, p. 570, 1896. 



2 Both these ancient lakes are, in the near future, to be reconstituted for 

 hydro-electric power purposes by valley bars, the Sihl basin alone being 

 estimated to yield 60,000 horse-power. 



