42 SURFACE GEOLOGY. 



Glace, the Arve valley is blocked up by an enormous mass of coarse detritus, 

 which was probably the right hand lateral moraine of the glacier, when it for- 

 merly extended across the valley. It is nearly 200 feet high, as I judged, save 

 that on the north side of the valley the Arve has worn a passage to its bottom 

 through the moraine. Above this barrier was once a lake, and the result has been, 

 that at least three terraces have been formed on both sides of the river. The 

 highest of these terraces I found to be 670 feet above Chamouny, and 4100 above 

 the ocean. 



Still higher up the stream, just beyond the glacier, called Argentiere, is another 

 similar moraine, which produced some terraces, less distinct, however, than in the 

 lower basin. I did not pass into the bed of this ancient lake, but took an obser- 

 vation, towards the hamlet of le Tour, on a level with the terraces, as near as I 

 could judge with the eye, and found the height to be 926 feet above Chamouny, 

 and 4351 feet above the ocean; the highest point where I have ever seen terraces. 



After passing the summit between the valleys of the Arve and the Rhone, on 

 the Tete Noire Route, we came upon the Eau Noire, which descends into the 

 Rhone. The highest terraces I noticed on the Eau Noire, which are small and of 

 quite coarse materials, are 793 feet above Chamouny, and 4218 feet above the 

 ocean. But the valley of this stream, for several miles on the Sardinian side of 

 its course, affords a fine example of that sort of glacis terrace, which consists of 

 one broad slope towards the stream from the mountain side. The materials are 

 quite coarse, yet rounded, and evidently the result more or less of aqueous agency. 

 Yet along this stream the erosions of former glaciers are quite manifest, high up 

 on the precipices that bound the gorge. 



As we descend towards Martigny, on the Rhone, we have a view of the valley 

 of that river some dozen miles up the stream, traversed by the Simplon road. It 

 looks very much like an estuary recently abandoned, and I could see no terraces. 

 The detritus is spread, with nearly an even surface from one steep side of the 

 valley to the other, having a downward slope equal to that of the rapid stream. 

 Such, for the most part, is the character of the Rhone valley half way from 

 Martigny to Lake Leman. Frequently, however, the alluvial sides of the valley 

 slope towards the river in the glacis form, and sometimes I noticed more than one 

 of this kind of terrace, arranged in successive steps, like the level topped terraces. 

 At St. Maurice, where the river goes through a narrow gorge, and the road passes 

 from the Valais into the Vaud, we meet with terraces of the common form, which 

 I found to be 250 feet above Leman, and 1480 feet above the ocean; or on a level 

 with the surface at Martigny. The Rhone, however, at St. Maurice, is 70 feet 

 lower than at Martigny, according to Keller's map. 1 



1 In several of the valleys of the Alps, I was struck with a singular optical deception, which I have 

 not seen noticed by travellers. In ascending valleys with steep and lofty sides, the road sometimes 

 descends slightly for some distance, in consequence of the detritus, which spreads out over the whole 

 valley. In some cases of this sort, I felt a little anxiety to see the postilion urging on his horses at so 

 furious a rate, down what appeared to me a quite steep hill. But on looking back, I found that we 

 were scarcely descending at all. And, indeed, I found that a great part of the way we seemed to bo 



