Vol. 56.] THE SILURIAN SEQUENCE OF RHAYADER. 71 



engineer on the building of this portion of the aqueduct, I have 

 devoted my occasional leisure-hours to a study of the geology of 

 the immediate neighbourhood, and the present paper embodies the 

 results of my researches. The area examined lies more or less 

 symmetrically about the town of Rhayader, and therefore may be 

 fittingly termed the Rhayader District. 



(2) The Rhayader District. 

 (See the topographical map on p. 70.) 



(a) General Characteristics. 



The town of Rhayader is situated near the eastern fringe of the 

 Central Wales complex, where the waters of the Wye enter a 

 somewhat lozenge-shaped valley, stretching from north-east to 

 south-west. Prom end to end this valley has a length of some 

 7 miles, and a breadth of a little over 1 mile at its widest. The 

 Wye may be said to divide it into a western area drained by the 

 Elan, and an eastern drained by the Rhyd-hir Brook : these two 

 streams empty themselves into the Wye. The little market-town 

 of Rhayader lies to the north of their confluences, nestling against 

 the northern border of the valley itself. 



The Rhayader Valley is bordered on the south by the long ridge 

 of Gwastaden. This rises to a height of some 900 feet above the 

 bed of the Wye, and shuts off like a great wall the country lying 

 to the south. The border of the Rhayader Valley crosses the Wye 

 a little below its junction with the Elan, and, passing westward 

 along the ridge of Corn Gafallt Hill, swings round in a gentle 

 curve, which terminates west of the Elan Village. At this point 

 the hilly border is cut through by the deep gorge of Caban Coch, 

 overhung by steep crags, and forming the exit from the Vale of 

 Nantgwyllt. On the south side of Corn Gafallt the hill drops 

 suddenly into the desolate valley of the Dulas, part of^which, at 

 least, must be included in the Rhayader District. On the north- 

 east, beyond Castle Hill, the Rhayader Valley may be considered to 

 end between the hills of Moel Hywel and Llan Goch. On the north 

 and north-west, between Moel Hywel and Caban Coch, the Rhayader 

 Valley is shut in by the vast plateau of Central Wales, which extends 

 in countless undulations westward for over 30 miles, to the coast- 

 line of Cardigan Bay. 



The district lying south and east of the Rhayader Valley is in 

 striking contrast with that lying north and west. From the hill 

 of Gwastaden southward to Builth run a regular series of alter- 

 nating ridges and hollows parallel to the border-lines of the sur- 

 rounding formations. Indeed, the Gwastaden range, and even the 

 Rhayader Valley, may be said to form part of this alternating 

 series. 



The physical characters of the districts lying outside the Rhayader 

 Valley, particularly on the north and west, are those of bleak moor- 

 ands ribbed by ridges of cleaved slate. As a consequence, the 



