Yol. 65.] SUCCESSION AKOTTND PLYNLIMON AND PONT ERWTD. 4S3 



appear to correspond with, those in the stream-section on the 

 east. 



Although the base-line hereabouts seems to follow a fairly 

 straight course from east to west, if it could be observed in detail 

 it would probably be found to be an undulating line, since the 

 Eisteddfa Beds on the south and the Bryn-glas Mudstones on the 

 north are traversed by numerous shallow folds which have a steady 

 southward pitch of about 12°. A section showing the general order 

 of superposition of the strata may be obtained along the slopes of 

 Banc Bryn-chwith by following the direction of the pitch of the 

 folds from the base to the summit of the hill (see fig. 6). As a 

 section of this kind is likely to be free from strike-faulting, it 

 furnishes a reliable estimate of thicknesses — provided that the 

 pitch be known. 



Fig. G. — Section along the pitch of the folds, from the Afon Castell over Banc 

 Bryn-clnvitli, showing the general relations of the rock-groups. 

 (Section No. Von the Map, PI. XXIV.) 



N.N.E. S.S.W, 



Afon Castell Road Banc Bryn-cHwith 



j ! Be 



[Scale: 6 inches=l mile.] 

 [Ac = Bryn-glas Mudstones. Ba= Eisteddfa Group. 



Bb = Kheidol Group. Bc = Castell Group.] 



The Bryn-glas Mudstones (Ac) are exposed in the bed of the Afon 

 Castell, and in small bosses on the peaty ground between that river 

 and the Devil's Bridge road. The red-stained flags and shales (Ba), 

 with abundant Climacograptus and occasional Diplograptus, form 

 the steep slope south of the road, and are succeeded near the summit 

 by dark-blue shales (Bb) with Monograpti ; while the hill is capped 

 by pale greenish-blue mudstones {Be), striped with seams of sandy 

 material. The same order is, therefore, preserved as in the former 

 sections ; the thickness of the Pont Erwyd Stage below the pale 

 mudstones is about 660 feet. 



T may now pass on to the description of the magnificent sec- 

 tion exposed in the deep gorge of the Bheidol below the village 

 of Pont Erwyd (see PI. XXV). This is by far the most important 

 section in the district, and the succession can be studied in great 

 detail : consequently, although the strata are in several places highly 



