THE VALAIS. 
II7 
material. The scar, still known as the Derochiaz, 
is even now visible, immediately above Aux Evouettes. 
Morlot was inclined to identify this catastrophe with 
the great rockfall of 456 a.d., which, according to 
Marius d’Avenches and Gregory of Tours, over- 
whelmed the town of Tauredunum. Others, how- 
ever, consider the Derochiaz fall to be more ancient, 
and point out that the fallen materials do not reach 
the foot of the mountain, but are separated by a 
stretch of absolutely flat ground. This they account 
for by supposing that the valley of Derochiaz was 
occupied by a small glacier, which acted as a sort of 
bridge, and over which the debris glided into the 
middle of the valley.* 
There was a great rockfall from the Dents du 
Midi in 1835, producing a torrent of black mud 
which flowed down the St. Barthelemy and covered 
the Bois Noir. Other rockfalls in this district have 
been already alluded to. 
The rocky hill of St. Triphon, opposite Ollon, 
must have been once an island. The sides plunge 
down almost vertically. There can be no doubt that 
the usual talus or scree exists at the base, but it is 
covered over by the alluvium, showing that the valley 
* Favre et Schardt, Beitr, 3. Geol. K. d. Schw., L. xxii. 
