Vol. 65.] SUCCESSION AROUND PLYNLIMON AND PONT ERWYD. 52S 



closely with the observed and calculated pitch of the individual 

 folds of which it is built up. 



The relation of the ' trend-lines ' to the actual outcrop and to 

 the average strike of the beds has an important bearing on im- 

 perfectly exposed folded districts, where ' trend-lines ' only (or an 



approximation to them) 

 -— — can be mapped, and 



where therefore the 

 boundary - lines fre - 

 quently cross the ap- 

 parent strike at a large 

 angle. In such cases 

 it may be inferred that 

 the structure is one of 

 pitching folds, and that 

 the actual boundaries 

 are probably zigzag 

 lines. 



In certain parts of 

 the present district the 

 ' trend-lines ' range for 

 short distances at right 

 angles to the average 

 strike ; the best ex- 

 amples are seen in the 

 outcrop of the grit- 

 bed in the Rhuddnant 

 Group south of Llyn 

 Rhuddnant, and in 

 the base of the Cas- 

 tell Group near Pont 

 Erwyd; the latter case 

 deserves brief mention. 

 The base of the Cas- 

 tell Group, for about 

 half a mile to the north 

 of Pont Erwyd, runs 

 below the road on the 

 west side of the valley 

 at about the 800-foot 

 contour (see plan & 

 section, figs. 14 & 15). It then meets with a succession of sharp 

 folds, and trends at right angles to its previous course for about a 

 furlong ; when it resumes its north-and-south direction, it is far 

 above the level of the road (nearly at 1100 feet). The effect of 

 the folded belt has been, therefore, not only to shift the outcrop 

 laterally, but to produce a marked change in its level on both sides 

 of the belt. In fact, it simulates closely the effect of a normal 

 fault across the strike ; but, as the course of the strata can be 



Fig-.i4.-Plan of folding-, 

 about half a mile north 



of Pont Erwyd. 

 Scale: 6 inches = I mile.. 



Fig. 15. — Section along the line AB in Jig. 

 14, looking southwards. (Section No. 

 VIII on the Map, PI. XXIV.) 



[Scale : 12 inches = 1 mile.] 



