118 STUDIES, SCIENTIFIC AND SOCIAL chap. 



Tweed, near Peebles, with the seven hundred feet contour 

 line marked on it by a dotted line.^ If the valley were 

 submerged to this depth the dotted line would mark the 

 outline of a lake, with arms running up every tributary 

 stream just as in the case of the river Dart. Although 

 situated in a glaciated district the valley here is post- 

 glacial, all the old river channels being deeply buried in 

 drift. 



If we now turn to the valley-lakes in glaciated districts 

 we shall find that they have a very different contour, as 

 shown by the two upper outline maps on the same page ; 

 (1) showing the upper part of Ullswater on a scale of one 

 mile to an inch, as in the Dart and Tweed maps, and (2) 

 showing the upper part of Lake Como, taken from the 

 Alpine Club map, on a scale of four miles to an inch. In 

 both of these it will be seen that the water never forms 

 inlets up the inflowing streams, but all of these without 

 exception form an even junction with the lake margin, 

 just as they would do if flowing into a river. Exactly the 

 same feature is present in the lower portions of these two 

 lakes, and it is equally a characteristic of every lake in the 

 Lake district, and of all the Swiss and Italian lakes. On 

 looking at the maps of any of these lakes one cannot but 

 see that the lake surface, not the lake hottom, represents 

 approximately the level of the pre-glacial valley, and that 

 the lateral streams and torrents enter the lake in the way 

 they do because they could only erode their channels 

 down to the level of the old valley before the ice over- 

 whelmed it. Of course this rule does not apply to large 

 tributary valleys carrying separate glaciers, since these 

 would be eroded by the ice almost as deeply as the main 

 valley. 



The three features of the valley-lakes of glaciated 

 regions now pointed out — the absence of submerged 

 ravines or river-channels either of the main river or of 

 tributary streams ; the basin-forms of the lake bottoms 

 and the frequent occurrence of two or more separate basins 

 even in small lakes ; and the simple form of surface con- 



^ Copied from a portion of the map at f. Hi of Geikie's Great Ice 

 Age, taken from the Ordnance Survey Map. 



