Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/462

 plains near the ends of existing glaciers, which might be presumed to be filled-up lake-basins ; still these cases of glacial erosion, if such they be, are on a very small scale compared with the amount of denudation that this hypothesis requires; and there are many others, as, for example, in the Val del Campo and Val Viola*, where the great heaps of moraine stuff show the glaciers to have long remained just filling up the whole of a lateral valley, without any marked effect on the bed behind.

This difficulty is increased when we examine the bed of an existing glacier, a thing which the rapid diminution of some of the Oberland glaciers during the last few years has made possible. The surface of the Unter-Grindelwald glacier was (July 1870) about 100 feet lower, and the extremity had retreated full 500 feet higher up the steep hill-side, than in 1858. Formerly it descended to the level of the valley ; it now rests on a low cliff of rock ; and its stream no longer gushes out from an ice-cave, but runs deep in a rocky gorge. This shows that the glaciation of a valley, and the cutting a gorge in its bed may be simultaneous. (The Rosenlaui glacier affords another example of this.) It can now be seen that the final icefall descends over three or four steps of hard limestone ; the last, and perhaps the tallest of these, together with what we might call the first flag of the valley-floor, being at present exposed. If, then, the glacier has made these steps, if it is plastic enough to mould itself to them (which it is not in all cases), surely it ought, at the base of each wall, to have worn out considerable hollows, analogous to the pot-holes at the foot of a cascade. But there is nothing of the kind to be seen ; the rocks, beautifully polished and scratched in many places, exhibit the usual contours, and after forming an irregular lumpy terrace, showing often " Stoss- and Leeseite," round away to the next cliff.

The steps, not uncommon in valleys, such as those in the Val Formazza, and that which is visible from near Landro, in the glen which descends from under the Drei Zinnen to the Durren See, present similar but still greater difficulties in accepting the hypothesis of glacier erosion.

Again, how are valleys such as those of the Pusterthal or the Upper Etschthal, to be explained on a theory of glacial erosion ? The former is a well-marked and generally wide trench, about fifty miles long, extending east and west from Muhlbach, where the Rienz turns sharp to the south, to Sillian on the Drave (to say nothing of any further extension to the Gailthal). For the most part it is enclosed by mountains 8000 or 9000 feet high ; and the flat Toblach plateau, which parts these rivers and forms the watershed between the Adige and the Danube, is 3951 feet above the sea, " sloping gently, almost imperceptibly," in either direction. No one who has seen this great trough, whose sides are steep mountains, rising 4000 feet or more on either hand, whose base dips

Stelvio Passes.
 * Valleys connecting, on the Italian side, the lower parts of the Bernina and