116 STUDIES, SCIENTIFIC AND SOCIAL chap, v 



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 distinctive, 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 out- 

 side of the glaciated districts, the tributary streams enter- 

 ing 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 margin of the lake. As an 

 illustration of this feature we may take the south-west 

 coast of England, which has never been glaciated, but 

 which has undergone a slight recent subsidence as indi- 

 cated by the submerged forests which occur at several 

 places. The result of this submergence is that the lower 

 parts of its larger river valleys have been converted into 

 inland tidal lakes, such as Poole Harbour, Dartmouth 

 Harbour, Kingsbridge River, Plymouth and Devonport 

 Harbours, 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 

 accompanying cut, where the characteristic 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 



