280 REPORTS ON THE STATE OF SCIENCE, ETC. 



ledge. Many pot-holes drilled by the post-glacial pre-flood stream have had the 

 down-stream lip battered and destroyed. Along this stage the flood met the bank of 

 Boulder Clay infilling the pre-glacial valley and dislodged many very large boulders, 

 which it carried through the steep and narrow ravine of Stage V and left upon the 

 Dolgafrog fan. Several big erratics like those which are found in the fan below still 

 project precariously from the Boulder Clay cliff, and two (of which one is estimated 

 to weigh 200 tons) have fallen on to the ledge itself since the disaster. 



Stage V. — In Stage V, although the whole of the surface of the upper parts of the 

 gorge have been hammered, it is remarkable how httle effect the battering has had 

 on the rhyolite. Only projecting corners have been removed and there is no sign of 

 any corrasion. In the narrow (lower) parts of the gorge the pre-flood surface with 

 all its pot-holes remains practically unaffected. The black slates, on the other hand, 

 gave way along their cleavage-planes, and slabs of moderate size were detached and 

 carried forward in considerable volume. At about 250 feet above O.D., a reinforced 

 concrete wall 10 feet in height and 6 feet in thickness had been erected to maintain 

 a water supply for the village. Of this only a very small part now hangs on the 

 southern bank. A view up the valley at this stage shows that the rocks near the 

 stream-course have been smoothed, but all the higher parts of the ravine are raw and 

 hackled like the face of a quarry from which every loose block has been removed. 



The fan below the termination of the ravine is a wilderness of great stones standing 

 at all angles. Many of these consist of the auto-brecciated basic rock which crops 

 out across the upper part of the gorge. Practically all have shapes and surfaces like 

 those of ice-borne boulders, and a few show pot-holes and smoothed water-channels, 

 sufficient to prove that streams of water have flowed over them and to suggest that 

 they have formed part of a river-bed. It is the opinion of the Committee that only 

 the freshly fractured slate slabs and a few of the angular blocks of the ' lip ' rocks 

 were quarried by the flood out of the solid beds. The great majority are ' second- 

 hand ' boulders and were scoured by the falling waters from the Drift section on the 

 ledge. One of these boulders measures 21 by 21 by 12 feet. It rests upon a smooth 

 boulder of hard dolerite 2-1 inches in length and 8 inches in thickness, which it has 

 broken into three pieces. At least four others are only slightly inferior to this in 

 size and quite a score of others have more than half that bulk. The fan is rudely 

 triangular in shape and extends across the former main road, where it is rather over 

 700 feet in width, the newly transported debris being there over 10 feet in thickness. 

 A rough sorting action can be observed. Near the apex lie the biggest boulders. 

 They decrease gradually in size to near the main road, where boulders of 6 to 8 feet 

 in diameter are common. The finer material was spread over the alluvial plain in the 

 direction of the River Conway. The present level of the fan at its apex is 100 feet 

 above O.D. and on the road 26 feet above O.D. Under this large fan lie the remains 

 of the wrecked church and of several houses. Over the position where the church 

 stood, nearly 50 feet of material, including at least one 100-ton boulder, have been 

 accumulated. 



The Committee's task was to obtain photographic records of the geological results 

 of the debacle. Forty-three photographs are herewith submitted, most of which 

 have been specially taken by Mr. W. H. Wilcockson, M.A., F.G.S., to whom the 

 Committee wish to offer sincere and hearty thanks. 



