Yol. 65.] SUCCESSION AROUND PUYNLIMON AND PONT EEWYD. 



B. The Pont Erwyd Stage. 



The rocks succeeding the Bryn-glas Mudstones are well-exposed 

 in many natural sections around the village of Pont Erwyd, which 

 stands near the confluence of the Rheidol and an important 

 tributary, Afon Castell, flowing in from the north-east. They are 

 distributed in two narrow belts, which diverge towards the north 

 in the form of an open V having its apex a little to the south of 

 the village. The eastern belt follows in the main the valley of the 

 Castell, while the western belt skirts the Rheidol valley for about 

 a mile and then gradually bears to the left of that valley. These 

 soft rocks have given rise to the tract of relatively low ground 

 which partly surrounds the higher central tract extending from 

 Bryn-glas to Plynlimon. 



From their splendid development around the village I propose to 

 include them in the Pont Erwyd Stage (B). 



In order to study the relations of these rocks to those underlying 

 or overlying them one cannot do better than select a few sections 

 across the Castell and Eheidol valleys, beginning at the eastern 

 edge of the district near Eisteddfa (or 'Steddfa) Grurig, where the 

 order of superposition of the various rock-types is clear and 

 unmistakable. 



At Eisteddfa Gurig the Tarenig stream, a tributary of the Wye, 

 ■crosses the main road from Aberystwyth to Llanidloes. It runs 

 there nearly due east and west ; but about a quarter of a mile west 

 of the bridge is a right-angled bend, the course of the stream from 

 there to its source on Plynlimon being nearly north and south. At 

 the bend it is joined from the west by a small feeder, ]S"ant Nod, 

 which has cut a shallow trench through the rocks, thus exposing a 

 fairly continuous section (fig. 3, p. 476). At the junction with 

 Afon Tarenig there is a small waterfall over blue flags, among 

 which are intercalated thin bands of dark-blue pyritous shale ; one 

 of these contained an abundance of ill-preserved specimens of 

 Climacograptus. Below them are somewhat massive flags and 

 mudstones, which pass down into a few feet of hard gritty mud- 

 stones ; an occasional specimen of Climacograptus was found, but 

 they became exceedingly rare before the gritty beds were reached. 

 The lowest exposed strata are soft, massive, blue-black mudstones, 

 weathering with a light orange-coloured stain. No fossils were 

 found in them, but they resemble those beds which succeed the 

 ' black-leaded ' mudstones of the Bryn-glas Group on the west side 

 of Drosgol. Only a few feet are exposed, the strata then rolling 

 over to the east ; and the section along the Tarenig soon comes 

 to an end. The ascending section in Nant JNod from the sharp 

 bend of Afon Tarenig shows about 300 feet of cleaved dark-blue 

 flags and shales, with occasional hard gritty flags of a paler 

 colour. About 90 yards up stream, at the foot of a small cascade, 

 a graptolitic band (F. 2) yielded the following species in some 



