Page:Popular Science Monthly Volume 45.djvu/265

Rh 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 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 without exception form an even junction with the lake margin, just as they would do if flowing into a river. Exactly the same feature 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 bottom, 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 carrying 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 surface 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 presenting these features have been formed by erosion and not by submergence.