278 REPORTS ON THE STATE OF SCIENCE, ETC. 



a deeper and narrower track. Here, under a very few feet of water, the peat-bed, 

 4 to 6 feet in thickness, seems to have floated up and, separating itself from the moraine 

 in cakes and strips, often many tens of square yards in extent, allowed the rushing 

 waters to uncover and erode the Boulder Clay. Across the 400 yards of moorland, 

 as far as the main Afon Porth-llwyd, a new stream-course 50 to 100 feet in width was 

 thus defined. The course was deepened generally by some 6 to 10 feet, with one or 

 more narrow channels locally incised within it to a depth of 20 feet. The finer debris 

 from this new cut was carried forward with the flood, but stones more than six or 

 eight inches in diameter were dropped as a delta at the edge of the main valley. Peat 

 rafts were also moved a little way, but most of them were detached in masses so large 

 and so well loaded with root-clay that the flood could not transport them to any 

 great distance. Many rafts of intermediate size were left stranded in the shallows 

 before the flood-waters reached the course of the main Afon Porth-llwyd. From the 

 confluence of these streams down to Pwll-du the valley is wide, and its gradient is so 

 low that the velocity of the waters was never considerable. In this reach the banks 

 show no signs of erosion, and even in two pronounced meanders above the farm of 

 Pwll-du the stream-bed was not altered. 



Stage II. — Below Pwll-du the banks of Boulder Clay were eroded and undercut 

 at each bend of the stream, the flood again becoming heavily charged with small and 

 large stones and boidders. When it reached the still waters of the storage-basin 

 formed by the Low-level Dam, it spread out and dropped its load of boulders and 

 finer material as a fan at the head and over about one-third of the total extent of the 

 floor of this sheet of water. 



Stage III. — The Low-level Dam was an earth-dam with a concrete core, concave 

 towards the upland area. It was made of the local Boulder Clay with its included 

 boulders, and was about 40 feet in height, with a spillway from the leat at the side 

 of its southern end, some 4 feet lower. This leat collects waters from the flanks of 

 Moel Eilio, below the Pwll-du leat, and brings them into the Low-level storage basin. 

 The flood-waters from Llyn Eigiau banked themselves against the dam and increased 

 the flooded area very considerably. Eventually they reached such a height that 

 water overflowed the dam to a reputed height of two feet, and thus began to erode 

 the piled-up Boulder Clay on the downstream side. This overflow in itself was not 

 serious, but two feet of water over the dam gave a height of six feet above the by- wash 

 or the spillway. The by-wash, though it could not accommodate all the flood, did 

 send down a very large quantity of water which, rushing with unaccustomed velocity, 

 cut into the toe of the earth-dam. Thus near the southern end of the dam the 

 concrete core was probably exposed, and after an interval it broke. A gap 120 feet 

 in width was made and a tremendous flood (11 million cubic feet of water) was 

 suddenly released. It met the pipe-line mounted on its concrete pillars, buried that 

 pipe-line in gravel, and was temporarily checked. The obstruction gave way 

 in turn and the flood swept on to the Conway Valley. Sections of the pipe-line went 

 with it, and some of the concrete piers were uprooted. Some of the material soon 

 became stranded, but some made the whole journey to Dolgarrog, where it now forms 

 part of the fan. The flood-water undercut the Boulder Clay on the rocky shelves, and, 

 scouring them clean of all movable material, swept out loose supports and allowed 

 joint-blocks of solid rock to move forward down the gorge. There is definite evidence 

 that some very large blocks of rock were moved even at the top of the gorge. A 

 boulder 15 by 15 by 15 feet, lying near the lip, still has dead vegetation attached to 

 its under side, hanging head downwards. The boulder is trapped against joint- 

 blocks of unmoved rock, and has caught and smashed a slab of slaty ash, which is 

 now held beneath it. While it has not actually been proved that any large mass of 

 the solid formation was carried to the foot of the gorge at Dolgarrog, this is considered 

 possible. As a result of the under-cutting along the stream-course, the gradient has 

 been disturbed and the slope locally made steeper than the angle of rest for joint- 

 blocks ; thus certain steeply inclined master-joints behind projecting masses have 

 been given opportunity to open under gravity. During the flood, boulders fell into 

 some of these gaping joints, and these may have acted as wedges. All this is well 

 shown at the ' lip,' where evidence of the mighty power of the flood is particularly 

 impressive. 



Stage IV. — From the lip downwards, the most noticeable phenomena are the 

 effects of the blows struck by the boulders which hit the solid rock in their tumble 

 over the waterfall. New fractures, often triangular in form and many square feet 

 in area, can be seen on projecting corners and edges all the way down to the slanting 



