Yd. 56.] THE SILURIAN SEQUENCE OF RUAYADEK. HI 



The Mono g r aptu s - S edgw icTcii Grits (Bb } ), which form a 

 prominent outcrop on the north side of the river, should, by virtue 

 of their dip, appear in an almost symmetrical form on the south. No 

 exposures but those of the next higher division are to be found in this 

 position ; a fault, therefore, is required along the valley to separate 

 the two groups. This, so far as I have been able to make out, runs 

 at the foot of the sloping ground along the south side of the Elan. 

 One end is apparently limited by the cross-fault already mentioned ; 

 the other by a north-easterly-and-south-westerly dislocation passing 

 through Cnwch Farm. The shales crop out along the crest of 

 Cnwch Wood and in the wood above the Elan Village Mission Hall. 

 They are readily recognizable by their thin bandings of soft rotten 

 felspathic and ferruginous grits, and the presence of worm-trails. 

 Some of the seams appear to be calcareous, yielding casts of 

 Rhynchonella and Ortliis sp, Monograptus Sedgwicldi, M. lobi- 

 ferus (Clingani-type), and M. Proteus also occur. The dip is from 

 15° to 20° northward 



Crossing the Cnwch Fault and ascending the eastern slopes of Corn 

 Gafallt Hill, we enter ground that has already been described. The 

 section, however, shows the gradual tilting-up of the beds as they 

 are followed eastward, with the usual accompaniment of faulting, 

 as they commence to swing round. 



It is now evident that between Caban Coch and Allt-y-bont the 

 rocks are folded into a gentle syncline, the true axis of which may 

 be said to lie along a line from Cnwch Hill to Llanfadog-isaf. In 

 order, therefore, to realize the maximum amount of folding that has 

 taken place, a section should be shown at right angles to this axis. 

 The typical section portrayed to some extent the western pro- 

 longation of the syndine in Cnwch Hill. A parallel line from Corn 

 Gafallt Cairn to Careg Bica, about | mile east, however, displays the 

 fold to greater advantage (fig. 16, p. 110). 



At Corn Gafallt Cairn the beds, as we have seen, dip north-north- 

 eastwards These should, through their inclination, reappear on the 

 north-western face of the hill in Allt Ddu wood. No exposures 

 unfortunately, are visible at this point ; but, from the relief of the 

 ground, it may be gathered that they run continuously from the 

 cairn to the low ground. The nose of the faulted outlier is crossed 

 by the section, but no rock is exhibited. 



Once across the river, we are in new ground. The mass of Cefu 

 Llanfadog rises almost precipitously before us, with terrace upon 

 terrace up to the crest of the cliff, simulating an enormous thickness 

 of rock. This is largely the effect of the impression that the beds 

 are dipping northward, but it will be found on approaching the 

 various outcrops that the dip is decidedly to the southward. 



The Monograptus-Sedgwickii Grits (Bb x ) have been laid 

 open in the cutting for the new road above the railway, and west of 

 the outlet-end of the Foe] Tunnel. For a study of this group in 

 its unweathered condition it is one of the finest sections to be seen in 

 the district. The beds are much crumpled and folded, and the true 

 dip is often difficult to ascertain : it varies from 10° to 30° east- 

 north-eastward and north-eastward. 



