part 1] IGNEOUS AND ASSOCIATED ROCKS OF LLANWRTYD. 25 



(4) Description of Typical Sections. 



By far the most important and complete section is seen on 

 •each side of the deep valley of the Nant Cerdin, which cuts right 

 across the anticline about a mile and a half north of Llanwrtyd 

 Wells. Owing to the pronounced pitch of the anticline — or rather 

 dome — at its northern and southern ends, the other two streams 

 which cut across the anticline only expose the higher beds. The 

 streams are respectively the River Irfon on the south (where the 

 main anticline has a strong southward pitch), and the Nant 

 Cwm-du on the north (where the northward pitch is equally well 

 marked). 



(«) The Valley of the Nant Cerdin. 



As mentioned above, it is only in the deep valley of the Nant 

 Cerdin that some of the lower beds of the Llanwrtyd sequence are 

 exposed. The transverse section of the anticline (fig. 3, p. 30) is 

 taken a short distance south of, and parallel to, the valley, and one 

 may regard the section as a diagrammatic view of the arrangement 

 of the beds on the southern slope of the valley. The sequence of 

 rocks in the centre of the anticline is shown in greater detail in 

 fig. 1 (p. 26). A fence l runs across the valley from north to south, 

 •coinciding almost exactly with the axis of the fold, and the section 

 has been taken very nearly along this line. On the north of the 

 stream the exposures occur in a little gully a few yards east of the 

 fence, and it is here that one sees the dark graptolitic shales 

 yielding Dicranograptus, etc. On the southern side the exposures 

 are generally a few yards west of the fence. On each side of the 

 valley, but particularly on the southern flank, the various hard 

 bands form sparsely covered features, while the intervening shales 

 are grass-covered slopes. When standing on the opposite slope 

 of the valley, one is able easily to distinguish the anticlinal 

 structure by means of these hard bands. The detailed section is 

 as follows : — 



5. Upper Ashes 



Hardened Mud- 

 stones, etc. 



Approximate thick- 

 ness in feet. 

 ( Coarse rhyolitic ash, frequently whitish by "1 

 - alteration or weathering-, forming isolated > 40 

 tumps or crags near the crest of the hill, j 



f Not exposed, presumably shales. 



Band of ashy limestone. 



Not exposed, presumably shales. 



Band of fairly coarse ashes. 

 ■{ Not exposed, presumably shales. 



Band of very fine ashes. 



Shales (20 feet). 



Band of very fine ashes. 

 I Black Shales (50 feet). 



1 



> 160 



1 A wall on the lower ground 



