E. Greenly — Denudation in North Wales. 69 



slopes from the foot of the crags. The northern one can be traced 

 by the eye a little way up into the crags themselves, but from below 

 we cannot tell whether the two channels, which disappear behind 

 a great rocky buttress, have or have not a common origin. 



The northern stream of debris crossed the road, broke down the 

 wall, and poured over on the other side a fan or cone of great stones 

 all the way down to the alluvial plain, while the finer material was 

 spread out upon the alluvium itself, and some even reached as far 

 as the river Ogwen. This fan is about 58 yards wide at the road, 

 and begins a considerable distance above it, the angle at its top 

 being a moderate one. 



Some of the debris is very coarse. I measured one block of 

 felsite 12x6 X 4 feet, standing on its narrow side, a little way 

 above the road. How far this had been carried by the torrent, I do 

 not know ; it may have been embedded in the drift before, but from 

 its position on the fan it must have travelled a good many yards. 



The southern stream crosses the road about 175 yards further on, 

 and is about 33 yards wide at that place. It is more conspicuous 

 in the distance than the other, and the amount of material at the 

 road is very great, but it does not go so far down into the valley, the 

 debris stopping on the steep slopes and not reaching the alluvium. 



Above these fans the work has been wholly erosive. At the head 

 of the northern one the channel cut in the drift and scree seemed to 

 mo to be more than 20 feet deep, and I think that it was cut down 

 to the solid slate here and there. On the steep rocky slopes at 

 the crag's foot the gully had been swept very clean and white. 

 Whether the erosive work afi"ected solid rock as well as drift, 

 I cannot tell. I saw no sign of a rock-fall in the crags, and to 

 ascertain whether there was any it would be necessary to go some 

 way up into the gully. The material of the fan, however, did not 

 seem to me quite angular enough to suggest any great fall of solid 

 rock, considering the short distance of transport. 



Whether this be so or not, it is clear that torrential denudation, 

 the work of only one afternoon, and probably of a very short time 

 in that afternoon, has cut channels through 20 feet or more of drift 

 and scree on the mountain side, moved blocks of felsite of as much 

 as 28S cubic feet, and spread out fans of stones and debris a quarter 

 of a mile in length. 



The fans and channel would be well worth being photographed. 

 In the Windsor Magazine of November last there is an account of 

 a disaster near Driffield in the Chalk wolds of Yorkshire, said to 

 have been caused by a ' waterspout.' The article is illustrated by 

 photographs, not only of damage to buildings, but of channels and 

 fans of debris very like these; and the point of origin is there quite 

 clear. These views, indeed, are of considerable geological interest. 



In view of the great importance of the subject of denudation, it 

 really seems a pity that instead of occasional papers, there should 

 inot be some kind of regular organization for collecting and recording 

 descriptions of what is actually going on at the present time. 



