35 2 Dr. Du Riche Preller — Lakes in Switzerland. 



in shallow Jurassic depressions slowly deepened by solution, the 

 water found an underground exit, the level of the lake was gradually 

 lowered, and its floor, drying up, was covered with vegetable matter, 

 leaving a pool in the deepest part, and reducing the basin to 

 peat moss. 1 



IV. False Lake Floors. 



By this unusual term I refer to certain extensive alluvia which 

 have all the appearance of having been deposited as lake floors in 

 standing water, viz. at a dead level, but which are extensive, flat 

 glacis cones formed in front of the moraine walls of glaciers, while 

 the latter, having reached the limit of their advance, remained 

 stationary. As already shown in the preceding paper, these glacis 

 cones, which have an average inclination of 1 in 200, were deposited 

 by the streams flowing round and escaping under the moraine wall, 

 until the glacier began to retreat, when the main stream overflowed 

 the moraine wall and, more frequently than not, found its way outside 

 the glacis cone and then eroded its own bed through the solid rock. 

 Sheet JSTo. 4, Figs. YIII-X, show the more important of these 

 cones formed in front of the terminal moraines of the five principal 

 glaciers at the extreme limits of the last glaciation. The two most 

 extensive cones 2 are those of the Rhone Glacier in the Aare Valley 

 below Soleure and Wangen, about 10 km. in length, and of the 

 Rhine Glacier below Schaffhausen of about the same length, while 

 the less extensive ones are those of the Aare, Ress, and Linth 

 Glaciers. The Aare circumvented the Rhone Glacier cone by over- 

 flowing at the margin of the glacier and cutting its bed through the 

 Molasse at a depth now 50 metres below the level of the cone (Sheet 

 No. 4, Fig. IX), while the Rhine, by a similar process, eroded its 

 new Molasse bed through the saddle between Mount Irschel and 

 Buchberg (Sheet No. 4, Fig. VIII), where the present level of the 

 liver is nearly 80 metres lower than the glacis cone. Similarly, 

 the Reuss broke through the Jura spur at Mellingen, leaving the 

 glacis cone on its left, and the Limmat eroded the Molasse at 

 Wettingen, leaving the glacis cone on its right. Thus the glacis 

 ■cones were left high and dry, with all the outward appearance of 

 lake valley floors. 



Conclusion-. 



1. "Whatever lakes or lacustrine expanses may have existed in the 

 areas described during interglacial periods previous to the last 

 glaciation, it is obvious that the present smaller lakes banked by 

 moraine can, in their original larger extent, have been formed only 

 during the retreat of the glaciers after the last general advance of 

 the latter, that is, like the five principal lake basins considered in the 

 preceding paper, towards the end of the Ice Age, while the smaller 



1 According to Dr. H. Walter (in a paper in 1896 on "Terrestrial Surface 

 Changes in Canton, Zurich "), within the last 250 years since 1668, out of 149 

 lakelets shown in Gyger's authentic lake-map of Northern Switzerland of that 

 time, no less than 73 dried up and were obliterated and 16 were largely reduced 

 in size. 



2 The term delta does not apply to these cones, as they were not formed in 

 standing water. 



