THE ICE AGE AND ITS WORJf. 249 



theory, however, there is no reason why the greatest depth should 

 occur in one part rather than in another, while separate basins in 

 the variously diverging arms of one lake seem most improbable. 

 The lakes of Thun and Brienz form two basins of what was evi- 

 dently once a single lake. The upper or Brienz basin is enor- 

 mously deep, over two thousand feet, and the reason is obvious. 

 The combined glaciers of the Lauterbrunnen and Grindelwald 

 Valleys entered the main valley in a direction almost opposite to 

 that of the Aare, piling up the ice against the great barrier of 

 the Rieder Grat, so that it at length flowed downward with great- 

 ly increased grinding power ; while lower down, toward Thun, 

 the valley opens widely and would thus allow the ice to spread 

 out with greatly diminished thickness. In our own country Loch 

 Lomond and Ullswater have been found to consist of several dis- 

 tinct basins, and in none of our lakes have any indications of 

 submerged river channels yet been found. 



The third point of difference between lakes of erosion and 

 those of submersion is the most important and the most distinct- 

 ive, and furnishes, I think, what may be termed a diagnostic 

 character of lakes of erosion. In most river valleys through a 

 hilly or mountainous country outside of the glaciated districts, 

 the tributary streams entering more or less at right angles to the 

 main valley are seen to occupy small valleys of their own, which 

 usually open out for a short distance at the same level before 

 joining the main valley. Of course, there are also torrents which 

 rush down steep mountain slopes directly to the main river, but 

 even these have usually cut ravines more or less deeply into the 

 rock. Now, if in such a valley we could mark out a contour line 

 two hundred, three hundred, or five hundred feet above the level 

 of the main stream, we should see that line continually turning 

 up each side valley or ravine till it reached the given level at 

 which to cross the tributary stream, and then turning back to 

 the main valley. The contour line would thus form a series of 

 notches or loops of greater or less depth at every tributary stream 

 with its entering valley or deeply cut ravine, and if the main 

 valley were filled with water this line would mark out the mar- 

 gin of the lake. As an illustration of this feature we may take 

 the southwest coast of England, which has never been glaciated, 

 but which has undergone a slight recent subsidence, as indicated 

 by the submerged forests which occur at several places. The re- 

 sult of this submergence is that the lower parts of its larger river 

 valleys have been converted into inland tidal lakes, such as Poole 

 Harbor, Dartmouth Harbor, Kingsbridge River, Plymouth and 

 Devonport Harbors, and Carrick Road above Falmouth. The 

 Dart River is an excellent example of such a submerged valley, 

 and its outline at high-water mark is shown at (3) on the accom- 



YOL. XLV. 20 



