Vol. 67.] ASSOCIATED ROCKS OF NORTH-EAST MONTGOMERYSHIRE. 425 



(2) Pwll-y-glo Beds. — These are best exposed in the sharp 

 ridge of Pwll-y-glo, halt a mile west of the Welshpool Dyke. They 

 consist of shales, mudstones, and flags, with some bands of hard 

 grit. More or less regular alternations occur — consisting of 2 or 

 3 feet of shale, with seams of hard, greenish, micaceous grit 

 varying in thickness from 3 to 12 inches. The beds which form 

 the ridge are folded into an asymmetrical syncline, along an east- 

 north-easterly and west-south-westerly axis. The most fossi- 

 liferous exposure is the second old quarry to the south-west of 

 Pwll-y-glo Farm. About 100 feet of shales and flags are here 

 exposed. 



The beds strike north-eastwards along the ridge towards the 

 Bron-y-Buckley Fault. Behind Groes-pluen Farm the series is 

 seen to be folding over towards the north-west, whereas elsewhere 

 along the ridge the dip is southerly or south-easterly. 



On the south-west, the Pwll-y-glo Beds continue towards Y 

 Frochas, where the strata are somewhat contorted, much jointed, 

 and to some extent faulted. At Y Frochas, the outcrop turns round 

 sharply to the north-east again, and can be traced along the 

 flanks of Harriets Hill to Cloddiau. The valley between Pwll- 

 y-glo and Harriets Hill is cut in the crest of an anticline in the 

 Pwll-y-glo Beds. 



Another faulted syncline occurs at Cloddiau, and the strike 

 swings right round to the north-west, the dip being south-westerly. 

 A good selection of fossils can be obtained in a quarry on the road- 

 side between Cloddiau and Penbryn. 



This series can now be traced round the eastern flanks of Moel-} - 

 garth Hill, until it is cut out by the Moel-y-garth Fault. 



North of Guilsfield this group is largely covered by drift. 

 It begins to crop out again from under the drift at the foot of 

 Trawscoed Bough, and is again well exposed in Gwern-y-brain 

 and the eastern foot of Gaerfawr. 



The fossils obtained from these beds clearly indicate a very low 

 position in the Bala Series. Because of the transitional characters 

 of the beds, I have considered them as a separate group. They 

 represent a period of change between the deposition of the lower 

 shales and the shallower -water types above. Their fauna is 

 somewhat remarkable, since they contain few of the graptolites 

 so abundant in the shales below ; while brachiopods, which swarm 

 in the beds above, are practically absent. (See List on p. 426.) 



(3) Gaerfawr Group. — This constitutes the most important 

 series of beds in the district. Wherever these beds crop out, they 

 form sharp tree-covered ridges, usually crested by a crown of pines. 

 Their escarpments are particularly steep and abrupt, and, since the 

 strata invariably dip at somewhat high angles, the dip-slope is also 

 fairly steep. This has probably led to their outcrops being almost 

 invariably given over to woodland. The highest ridges in the area, 

 Moel-y-garth and Gaerfawr, are so formed. 



U. J. G. S. No. 267. 2 g 



