Vol. 58.] HANGING VALLEYS IN THE ALPS AND HIMALAYAS, 709 
their glaciers remained, the longer would this result be delayed: 
only the mouth of the valley being cut back into a gorge by the 
water flowing from the glacier, while the curve of erosion of the 
gorge would flatten gradually as the ice retreated. 
On this supposition, then, those Jateral valleys which have been 
longest freed from their protective covering of ice should have 
established the most accordant grade with the main valley ; and this 
indeed we find to be the case. There are no true hanging lateral 
valleys remaining on the left bank of the Ticino, which faces west 
and south (with the exception of the Val Piora, the preservation 
of which has been already explained), while those found on the 
right wall face north and east, and it is in these that the ice will 
have been t!) : last to melt: the head of the Val Piumogna being 
still occupied by a glacier. 
In this way we see how ice could be indirectly responsible for 
the hanging valleys, and how we can explain their marked occur- 
rence in glaciated regions, without attributing, as Prof. Davis has 
done, the overdeepening of the main valleys to direct erosion by 
ice. 
Before referring to the support that this explanation appears to 
receive from other districts, I would point out that there is one 
piece of evidence which seems strongly to favour the contention 
that the overdeepened floor of the Val Ticino was effected by 
water-erosion, and not by ice. If we attribute the overdeepening 
to ice-erosion, the action of the glacier should have been more 
marked at the lower end of the valley, near Biasca, than in the 
middle portion, near Airolo.? If, on the other hand, the over- 
deepening be due to an acceleration of the velocity of the river 
consequent on the elevation of the upper end of the district, we 
should expect the results to be greatest in the middle portion, and 
less marked at the lower end of the valley where the effects of 
elevation died away. 
If, now, we construct a section along the present floor of the al 
Ticino, from Airolo to Biasca, and restore the old floor before the 
overdeepening took place, using for this purpose the present posi- 
tion of the mouths of the hanging valleys between Airolo and 
Biasca, we find that there is a regular decrease in the respective 
heights of the hanging valleys, above the present thalweg of the 
Ticino, showing that the greatest amount of excavation has taken 
place in the fioor of the old valley in its middle portion, and that 
the amount of overdeepening gradually decreases in the lower 
portion.’ The heights are approximately shown on the accompanying 
diagram, drawn to the true scale of 1:50,000 (Pl. XXXVI, longi- 
tudinal section along the Valle Leventina), This appears to me 
conclusive evidence in favour of water having been the chief 
agent of erosion, and not ice. The Val Ticino, therefore, does 
* Reusch, Norges Geol. Undersog. No. 32 (Aarbog for 1900). 
2 Prof. Davis attributes the excavation of the Lago Maggiore below to ice- 
er osion. 
* It is here also that marked deposition commences. 
