Vol. 6 5. J SUCCESSION AROUND PLYNLIHON AND PONT ERWTD. 479 



Another section may be taken, about a mile south of the last, 

 'from Craig Yspio towards Cwm Ergyr, passing about 300 yards 

 aiorth of that farmhouse (fig. 4, p. 476). For a part of the way 

 it runs nearly parallel with the main road to Llanidloes, as it sweeps 

 round the side of the hill between the loth and 16th milestones 

 from Aberystwyth. The rocks are well-exposed on the steep slopes 

 north and west of the road, and also in a large number of road- 

 cuttings. 



Craig Yspio stands on a spur which projects southwards along the 

 axis of the anticline mentioned on p. 474. The abrupt fall of 

 the ground at the end and round the flanks of the spur is due to 

 the oncoming of the red-stained Eisteddfa Beds above the harder 

 mudstones which form the higher ground. 



The lowest strata, which are exposed in the line of section, are 

 dark-blue sheared-looking mudstones, belonging probably to the 

 Bryn-glas Group. They are followed by nags with thin grits and 

 containing an occasional specimen of Climacograptus scalaris var. 

 normalis ; these pass up into flags and shales with thin siliceous 

 seams, weathering in brilliant red and orange colours which render 

 them remarkably conspicuous from a great distance. Though the 

 rocks are highly cleaved, graptolites of the genera Climacograptus 

 and Diplograptus can be obtained in several places ; but the best 

 specimens were found on the west side of the well-defined syncline 

 which lies to the west of the Craig Yspio anticline. The following 

 specimens were collected near the base of the group, in a strike- 

 gully lying high up on the steep slope west of Pont Nant Lladron, 

 and plainly visible from the main road (F. 5) : — Mesograptus 

 modestus and its variety parvulus, GlyptograpUis persculptus (?), 

 Gl. sp., and Climacograptus scalaris, vars. normalis & miserahilis. 



North of the 15th milestone from Aberystwyth, the red-stained 

 strata form prominent crags by the side of a small stream which 

 crosses the road near the milestone. What are probably the same 

 rocks are exposed in the road-cutting 250 yards to the east, 

 where the section shows a succession of shallow anticlines and 

 synclines, the effect of which is gradually to introduce higher beds 

 towards the east. All the rocks are cleaved, and the difference 

 between this structure in the soft shales and in the interbedded 

 flags is strikingly exhibited, especially on weathering. The shales 

 break up into small pieces as thin as paper, while the flags (having 

 developed a much coarser structure) part into rhomboidal prisms. 

 Strata of this character extend for about a quarter of a mile ; 

 but, for some part of this distance, the road runs nearly along 

 the strike. 



After crossing a small stream, which flows past some old 

 spoil-heaps, soft orange-weathering shales predominate for about 

 100 yards, then giving way to hard blue sandy shales with rather 

 thick flags which persist to the point where the road sweeps 

 round to the north towards Eisteddfa Gurig. These higher shales 

 weather to a dull bronze-brown, in this respect and in their more 



Q.J.G. S. No. 260. 2 



