FLOODS OF THE ARDECHE. 261 
It is easy to see that the damage occasioned by such floods as 
I have described must be almost incalculable, and it is by no 
means confined to the effects produced by overflow and the 
mechanical force of the superficial currents. In treating of 
the devastations of torrents, I have hitherto confined myself 
principally to the erosion of surface and the transportation of 
mineral matter to lower grounds by them. The general action 
of torrents, as thus far shown, tends to the ultimate elevation of 
their beds by the deposit of the earth, gravel, and stone conveyed 
by them; but until they have thus raised their outlets so as sen- 
sibly to diminish the inclination of their channels—and some- 
times when extraordinary floods give the torrents momentum 
enough to sweep away the accumulations which they have 
themselves heaped up—the swift flow of their currents, aided 
by the abrasion of the rolling rocks and gravel, scoops their 
beds constantly deeper, and they consequently not only under- 
mine their banks, but frequently sap the most solid foundations 
which the art of man can build for the support of bridges and 
hydraule structures.* 
In the inundation of 1857, the Ardéche destroyed a stone 
* In some cases where the bed of rapid Alpine streams is composed of very 
hard rock—as is the case in many of the valleys once filled by ancient glaciers 
—and especially where they are fed by glaciers not overhung by crumbling 
cliffs, the channel may remain almost unchanged for centuries. This is observ- 
able in many of the tributaries of the Dora Baltea, which drains the valley of 
Aosta. Several of these small rivers are spanned by more or less perfect 
Roman bridges—one of which, that over the Lys at Pont St. Martin, is still in 
good repair and in constant use. An examination of the rocks on which the 
abutments of this and some other similar structures are founded, and of the 
channels of the rivers they cross, shows that the beds of the streams cannot 
have been much elevated or depressed since the bridges were built. In other 
cases, as at the outlet of the Val Tournanche at Chatillon, where a single rib 
of a Roman bridge still remains, there is nothing to forbid the supposition 
that the deep excavation of the channel may have been parily effected at a 
much later period. 
The Roman aqueduct known as the Pont du Gard, near Nismes, was built, 
in all probability, nineteen centuries ago. The bed of the river Gardon, a 
rather swift stream, which flows beneath it, can have suffered but a slight 
depression since the piers of the aqueduct were founded, 
