72 ME. H. LAPWOETH ON THE [Feb. I90O, 



country is devoid of agricultural value, except in the low-lying 

 areas ; but is utilized as an immense expanse of sheep-runs, some of 

 which are many thousands of acres in extent. Inside the valley, 

 however, in the immediate neighbourhood of Rhayader, the greater 

 part of the land is cultivated ; but the soil is poor, and does not 

 seem well able to support the growth of corn. 



The rocks of the Rhayader District may be described briefly as 

 consisting of conglomerates, grits, shales, slates, and 

 mud stones. The ridges and caps of the hills of Gwastaden, 

 Corn Gafallt, Cnwch, and Allt Goch are formed chiefly of 

 grits and conglomerates. These graduate first into flag- 

 stones, and eventually into shales and mudstones. Through 

 these softer rocks the rivers have cut their way, carving out the 

 valleys and low-lying areas. The Rhayader Valley proper is 

 floored mainly by shales and mudstones ; and from a point as far as 

 6 miles north of Rhayader to the base of Gwastaden the Wye may 

 be followed without once crossing a bed that may be strictly 

 termed arenaceous. One or two very thin limestones occur 

 among the shales in the river-course south of Rhayader, but they 

 appear to be of merely local occurrence, and with these exceptions 

 limestones may be said to be absent from the district. 



The strike of the rocks is roughly north-east and south-west. 

 The dip of the strata varies : the average inclination of the beds, 

 however, may be taken as from 20° to 30° north-westward. The 

 argillaceous rocks are all highly cleaved: even those that 

 occur between thick bands of grit and conglomerate have suffered. 

 Indeed, so intense has been the cleavage in certain areas that it 

 affects many of the thickly-bedded arenaceous rocks. The strike 

 of the cleavage is parallel to the strike of the bedding. 



Through the central portion of the Rhayader District the beds 

 run fairly evenly along their lines of strike, but as we approach 

 the western limits folding commences. At Caban Coch, crumplings 

 and bucklings in the Caban Conglomerates on both sides of the 

 gorge can be well seen from the high road. 



A fault of some considerable downthrow occurs in the neigh- 

 bourhood of Caban Coch, accompanied by others of less importance. 

 This main fault was detected by the officers of the Geological Survey 

 about the year 1850, and is shown upon the Horizontal Sections. 

 It is usually referred to as the Abernant Fault by the engineers 

 on the Elan Valley Works, and to this term I propose to adhere. 

 A long strike -fault runs along the Rhayader Valley from north- 

 east to south-west, with several branching dip -faults. These, 

 however, will be mentioned on a subsequent page, when the detailed 

 structure of the various rock -groups is dealt with. 



(b) Historical Summary. 



The earliest reference to the Rhayader District is found in 

 Murchison's ' Silurian System,' 1 in which a short description is 



1 London, 1839, pp. 316, 317. 



