part 1] IGNEOUS AND ASSOCIATED ROCKS OF LLANWRTTD. 29> 



The area of igneous rocks is bounded on the west by a great 

 fault. The direction of the fault is not quite parallel to the axis. 

 of the fold, wherefore in the north it cuts off a considerable part 

 of the western limb of the fold : this is readily apparent from 

 the map (PL II). The course of the fault is well marked topo- 

 graphically. In the south it occupies a deep, narrow, straight 

 valley in the northern slope of the hill known as Banc Grlyn 

 Gyrnant. It skirts the eastern side of the valley of the Irfon near 

 Llanwrtyd, and probably determines the direction of the river at 

 this point. Near the intrusive mass it crosses higher ground (by 

 Pen-y-banc Farm), but on the north again gives rise to a deep 

 gully north-east of Pen-y-banc. The contorted rocks in this gully 

 are described and figured in ' The Silurian System ' (p. 344 & 

 figs. 62, 64). Farther north it gives rise to the little rift-valley 

 (Nant-y-Grlo) north-east of Nant-yr-odyn Farm (seen in the 

 distance in fig. 64 of 'The Silurian System'). 



The anticline of Llanwrtyd is, however, complicated by a series 

 of folds which cross it almost at right angles, and cause a pro- 

 nounced pitch in the north and south. More correctly, therefore,, 

 the igneous rocks are arranged in an elongated dome. The surface- 

 of the ground in the north follows the northward pitch in such 

 wise that the ridge of Car Cwm sinks rapidly from 1600 to about 

 900 feet. At the southern end of the ridge the southward pitch is. 

 even more marked. The outcrop of the Upper Ashes drops from 

 a level of 1300 feet at this point to about 800 (near the Well Bath- 

 Houses) in a distance of less than half a mile. It is possible that 

 this pitch is more apparent than real, as one must bear in mind 

 the possible existence of an east-and-west fault in the valley of the 

 Irfon immediately west of Llanwrtyd Wells. Some transverse 

 faults from west-north-west to east-south-east certainly do occur 

 in the district ; but they are of small throw, as shown on the map 

 (PI. II). Some minor folding and faulting north of the transverse 

 valley of the Nant Cerdin has given rise to a subsidiary dome-like 

 structure east of Nant-yr-odyn Farm. 



One of the most striking features of the district is undoubtedly 

 the deep valley of the Nant Cerdin, which cuts the range in two.. 

 The origin of this valley is not readily apparent. It does not seem 

 to be determined by a line of faulting ; it is more probable that 

 the Cerdin stream was deflected from its north-and-south course 

 by the resistant intrusive mass on the south. 



The close relationship between the present contours and the- 

 folding of the strata will be at once seen from the sections 

 (figs. 2 & 3, p. 30). 



(6) Comparison witli other Areas, 



It was at one time customary to regard the Ordovieian igneous 

 rocks of Wales as forming, for the greater part, two series : a lower 

 series ranging in age from Arenig to Llanvirn, and occasionally 

 lasting until Lower Glenkiln (Llandeilo) times ; and an upper 



