THE ICE AGE AND ITS WORK. 251 



panying cut, wliere the cliaracteristic outline of such a valley is 

 well indicated, the water running up every tributary stream, as 

 described above. The lower section (4) shows the same feature 

 by means of a map of the river 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 with- 

 out exception form an even junction with the lake margin, just 

 as they would do if flowing into a river. Exactly the same fea- 

 ture 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 can not 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 overwhelmed it. 

 Of course, this rule does not apply to large tributary valleys car- 

 rying 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 sur- 

 face contour of all this class of lakes, so strongly contrasting 

 with that of valleys known to have been recently submerged, as 

 well as with the contour lines of valleys in non-glaciated districts 

 and in those which are known to be post-glacial seem to afford, 

 as nearly as the case admits, a demonstration that the lakes pre- 

 senting these features have been formed by erosion and not by 

 submergence. 



* Copied from a portion of the map at page 144 of Geikie's Great Ice Age, taken from 

 the Ordnance Survey Map. 



