ON DOLGARROG DAM DISASTER. 



277 



Llewelyn, which lies to the south, but since the Clogwyn-r-Eryr ridge has been 

 pierced by a tunnel, the surplus waters of the Afon Dulyn have been added from the 

 north. 



The River Course. 



Though the area and level of Llyn Eigiau had, before the disaster, been materially 

 altered by the erection of a dam on the morainic ridge, and the stream-course had 

 been modified by subsidiary dams and leats, yet certain well-marked topographic 

 stages can still be distinguished. 



Stage I. Llyn Eigiau to Pwll-du. — After leaving the lake, the natural level of which 

 is 1,219 feet above O.D., the Afon Porth-llwyd meanders over a Drift-covered area 

 for two miles down to the farm of Pwll-du, 1,150 feet above O.D. At this point 

 rejuvenation of the stream begins, and a leat has been constructed to carry the water 

 round a spur of Moel Eilio to Dolgarrog. 



Stage II. Pwll-du to the Low-level Dam. — From Pwll-du the valley deepens and 

 cliffs in Boulder Clay appear. Half a mile farther downstream, at 850 feet above 

 O.D., the Low-level Dam, 40 feet in height, was constructed to hold up sufficient 

 water for one day's supply for the pipe-line to Dolgarrog which begins here. 



Stage III. The Low-level Dam to the lip of the Rhaiadr Porth-llwyd. — For the next 

 three-quarters of a mile of its course the river falls some 200 feet to the 650 feet 

 contour. But above that, at a height of 750 feet above O.D., the solid rock appears 

 in the river bed, and from this point the river flows through a rocky gorge with 

 undercut cliffs of Boulder Clay resting on the rock-shelves. 



Stage IV. The Rhaiadr Porth-llwyd. — At the 650 feet level the gradient steepens 

 so suddenly that this part of the stream-course may well be termed a lip. In less 

 than a quarter of a mile the stream falls over bare rock from 650 feet above O.D. to 

 a sloping ledge 350 feet above O.D. From where solid rock first appears (at 750 feet 

 above O.D.) to this point, the stream-bed lies in what is apparently an auto- 

 brecciated basic lava or intrusion. The sloping ledge is a pre-Boulder Clay 

 topographic feature forming the southern bank of a pre-glacial valley which apparently 

 deviates somewhat from the present stream-course and runs towards the north-east. 

 This valley is filled with glacial debris and contains very large boulders. Intensive 

 erosion here has resulted in the undercutting of a cliff of Boulder Drift 100 feet in 

 width and 50 feet in height. This cliff now forms the northern wall of the present 

 stream-course. 



Stage V. Below the Rhaiadr Porth-llwyd to the Conway. — From the ledge at 350 feet 

 O.D. to the floor of the main valley, the stream passes through a post-glacial ravine 

 cut in black pyritous shales into which an irregular mass of rhyolite-like rock has 

 been intruded. The ravine has steeply cut sides and is noteworthy for the number 

 of pot-holes both in its floor and at different levels on its flanks. The end of the 

 gorge near to the Conway Valley is cut through black slates overlain by Boulder 

 Clay. Thence the stream reaches the main river across an alluvial plain. 



The Effects of the Flood. 



Stage I. — A concrete wall, three-quarters of a mile in length, was constructed on 

 the moraine (here overlain by peat) along the eastern side of Llyn Eigiau and the 

 level of the lake was raised thereby from 1,219 feet to 1,239 feet above O.D. Special 

 precautions were taken to strengthen the wall across the southern outlet of the 

 lake, but a shallow saddle crossing the moraine about half a mile to the north of the 

 outlet was not specially safeguarded. In the midst of this potential overflow channel 

 slight seepage of water had apparently long been in progress, and when the heavy 

 rains which preceded the disaster raised the level of Lake Eigiau the seepage increased 

 inordinately. With augmented flow, the seepage developed to a well-defined spring 

 which, bursting close to the wall, soon enlarged itself to a well, or cauldron, 30 feet in 

 width and 20 feet in depth, over which the wall remained standing as a bridge. The 

 spring in bursting lifted the peat bed which covers the Boulder Clay and broke it 

 into rafts ; these the waters floated forward or cast outwards to strand on the margins 

 of the flood near by. 



Around the cauldron of emergence neither peat nor Boulder Clay are in any way 

 disturbed, and, notwithstanding that 120 million cubic feet of water flowed forth over 

 the moor, there is for the next 100 yards no perceptible water-channel, neither grass 

 nor heather being uprooted. A little way below the cart-track the hollow in the 

 moorland becomes more evident, and the flood, when it found this, confined itself to 



