222 Dr. C. 8. Du Riche Preller — Zonal Lake Basins. 



a single span, appears physically and mechanically impossible, the more 

 so when the sequence of phenomena of a general glaciation is borne 

 in mind. This sequence may be briefly stated as follows : — 



Upon a general advance of the glaciers, the rivers gradually 

 decrease in volume, velocity, and erosive energy, and the level of the 

 lakes drops proportionately. When the glaciers reach the heads of 

 the lakes, the latter, if they have not already been filled up by 

 fluviatile deposition, gradually empty themselves through the natural 

 outlets, and the supply from their drainage areas gradually failing, 

 become reduced to meandering river channels and pools which, 

 taking the temperature of the ice, freeze as the glaciers advance. 1 

 When these, after traversing the filled-up lake basins, become 

 stationary, they deposit moraine walls round their terminal lobes, 

 and in front of these walls flat glacis-cones of fluvio-glacial material 

 are formed by the deposits of glacier streams flowing round and under 

 those walls. When, lastly, the glaciers begin to melt and retreat, 

 lakes form behind the moraine walls and gradually increase in length 

 and size in the wake of the receding glaciers. At their lower ends the 

 lakes at the same time find an outlet by a stream overflowing the 

 moraine bar at its lowest point, generally at the margin and not in 

 the middle of the bar, and thus the work of erosion begins both in the 

 bar and in the glacis-cone, though the main stream often, as it were 

 by caprice, erodes its bed round the edge, instead of through the 

 cone. As the glaciers recede beyond the lake basins, the affluents of 

 the latter, too, become active again, and thus new lakes are formed, 

 probably different in size, depth, and altitude from their predecessors, 

 only, in their turn, to be filled up again. 



The diagram (Fig. VIII), p. 218, shows the average fall of the 

 principal sub-Alpine rivers after the three glaciations, as evidenced by 

 the river deposits — indicating the valley floors — of each period. It 

 will be seen that the average fall, and therefore the erosive energy and 

 carrying capacity, decreased with each glaciation and is lowest in the 

 post-Glacial rivers of the present time. 2 It will further be seen that 

 the rivers must have been flowing at much higher altitudes than at 

 present, and that the same altitudes apply ipso facto to the lakes, the 

 valley floor and lake level, e.g. at Zurich, having been 400, 200, and 

 50 metres higher than at present, and the altitudes in respect of the 

 Lake of Geneva and the Rhone at Lausanne and Geneva being about 

 the same. Thus each glaciation must have been followed by a very 

 long period of erosion, the longest being that between the first and 

 the maximum glaciation, during which the valleys were approxi- 

 mately eroded to their present depth, while after the maximum and 

 the third glaciations the rivers had only to remove the products of 



1 For instance, a glacier, to traverse a filled-up lake basin 40 kilometres in 

 length, like that of Zurich, at the rate of 0-3 m. = 1 foot per day, would require 

 over 300 years, and in the case of the Lakes of Constance and Geneva double 

 that time. 



2 The diagram refers to the area of the junctions of the sub-Alpine rivers 

 between Zurich and Bale. In the Alpine valleys the former higher level of 

 400 metres would place the then valley floor, e.g. in the Linth Valley, on a level 

 with the present Klontal Lake, a hanging valley 400 metres above Glarus. 



