THE ICE AGE AND ITS WORK. 295 



submersion is the most important ;uid the most distinctive, and fur- 

 nishes, I think, what may be termed a diagnosis cliaracter of hikes of 

 erosion. In most river valleys through a hilly or mountainous country 

 outside of the glaciated districts, the tributary streams enteriug more 

 or less at right angles to the main valley are seen to occupy small val- 

 leys of their own, which usually open ont 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 tnit ravines more or less deeply into 

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

 200, 300, or 500 feet above the level of the main stream, we should see 

 that line continually tnruing up each side valley or ravine till it reached 

 the given level at which to cross the tributary stream, and then turn- 

 ing back to the main valley. The contour line would thus form a series 

 of notches or hiops of great<'r or less depth at every tributary stream 

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

 were tilled with water this line would mark out the margin 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 umlergone a 

 slight recent subsidence as indicated by the submerged forests which 

 occur at several places. The result of this submergence is tliat 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 sub- 

 merged valley, and its outline at high-water mark is shown at (3) on the 

 accompanying illustration (Plate xv), where the characteristic outline 

 of such a valley is well indicated, the water ruuning up every tributary 

 stream as described above. The lower section (4) shows the same fea- 

 ture by means of a map of the River Tweed, near Peebles, with the 700 

 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 val- 

 ley here is post-glacial, all the old river channels being deeply buried 

 ill diilt. 



It 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 Plate xv; (1) showing the upper part of Ullswater 

 on a scale of 1 mile to au 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 4 miles to an inch. In both of these it will bo seen 

 that the water never forms inlets up the inflowing streams, but all of 

 these without exception form au even junction with the lake margin. 



' Copied IVoiii a portion of tlie map at p. 14 I of Geikie's Great Ice Aye, taken from 

 the Ordnauce Survey Map. 



