512 ME. 0. T. JONES ON TEE HARTFELL- VALENTIAN [Nov. I9O9, 



of country, both north and south of the line of section. As a rule, 

 exposures are not good, and graptolites are difficult to find, except 

 near the base and summit of the group. On the north they occur 

 around the tarns called Llynoedd Ieuan, and in the precipices over- 

 looking Cwm Ergyr ; while on the south they form the hills of 

 Erw Berfa and Ty'nlhvyn, which divide the valley of the Eheidol 

 from those of the Myherin and the Mynach. 



If a traverse be made from the Rheidol Valley across the former 

 hill, about a mile south of the previous one, the Myherin is reached 

 where it is joined by the Khuddnant to form Afon Mynach, on 

 which occur the well-known falls at the Devil's Bridge. The 

 mudstone-and-grit group is thrown into numerous undulations in 

 which anticlines and synclines succeed one another with a certain 

 degree of regularity, as shown by the long north-and-south ridges. 

 In the continuation of the section along the Rhuddnant the 

 mudstones contain thin grit bands for a distance of nearly a mile 

 up that valley ; then they disappear quickly. The upper part of 

 the valley offers some of the wildest scenery in the district: for 

 more than a mile the north side is formed of bare rock, which 

 descends almost sheer from the edge of the moorland to the bed 

 of the stream 600 or 700 feet below. Numerous anticlines and 

 synclines may be detected, and the course of the strata as they 

 sweep up and down the side of the ravine can be followed very 

 clearly. Graptolites were obtained at several places along the 

 valley in the bands of ironstained shale which occur in the mud- 

 stones : these are of the same species as those recorded from the 

 Myherin Valley above Nant-y-Creiau. As in that section, the 

 mudstones become darker in ascending the sequence, and their 

 peculiar mode of weathering in brilliant concentric rings becomes 

 conspicuous. These changes set in about 2 miles up the valley, 

 near the right-angled bend which the stream takes after cascading 

 down from the moorland above; and the blue-black mudstones 

 and shales are beautifully exposed between that point and the 

 head of the ravine, although they are not readily accessible. 



When the edge of the moorland is reached, where the stream 

 begins to plunge into the ravine, the mudstones give place gradually 

 to soft blue shales weathering in rusty-bronze tints. A few yards 

 higher up stream thin greyish-white siliceous streaks make their 

 appearance in the shales, and are seen to increase in number in 

 ascending the section. These striped rocks are followed immediately 

 by several feet of coarse, grey, speckly grit in rather thick beds : 

 this sequence appears to correspond with that observed at the head 

 of the Myherin Valley. In the section now described, however, 

 higher strata are seen to a thickness of 400 or 500 feet ; they 

 consist, in the main, of soft, smooth, blue shales with thin gritty 

 bands and an occasional bed of massive, coarse, speckly grit. Near 

 the point where the stream emerges from the tarn known as Llyn 

 Rhuddnant, a fresh lithological type makes its appearance ; the blue 

 shales there contain a series of hard, dark-blue, fine-grained grits, 



