V GLACIAL EROSION OF LAKE BASINS 119 



tour 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 presenting these features 

 have been formed by erosion and not by submergence. 



It may, of course, be urged that the reason the lakes in 

 question exhibit smooth contours is, because the lateral 

 inlets which once existed have become filled up with sedi- 

 ment. In some, perhaps in many, cases this may be the 

 explanation, but it can hardly be so in all, considering the 

 very limited amount of material brought down by many 

 of the smaller rocky streams, and the great depth to be 

 filled up in the case of the deeper lakes. In Lake Como, 

 for example, a fcAv of the lateral streams have formed 

 small projecting deltas, but they evidently run in steep 

 rock beds to the contour edge of the lake, not one of them 

 showing any sign of having had a submerged ravine now 

 filled up with alluvium. And the same is the case with 

 the lakes of Geneva, Brienz, and most others. When 

 lateral alluvial valleys at all commensurable in size with 

 the depth of the lake are proved to exist, this objection 

 may be made. At present it is a mere supposition. 



In connection with this subject may be noticed the 

 many cases in which Alpine valle3^s present indications of 

 having been greatly deepened by glacial erosion, although, 

 owing either to the slope of the ground or the uniformity 

 of the ice-action, no lake has been j^roduced. In some 

 valleys, as in that of Lauterbrunnen, the trough between 

 the vertical rock-walls was probably partly formed before 

 the ice age, but was greatly deepened by glacial erosion, 

 the result being that the tributar}^ streams have not since 

 had time to excavate ravines of equal depth with the main 

 valley, and therefore form a series of cascades over the 

 lateral precipices, of which the Staubbach is the finest 

 example. In many other cases, however, the side 

 streams have cut wonderfully narrow gorges by which 

 they enter the main valley. This work was probably 

 begun by a sub-glacial stream, and the action of the 



