Vol. 65.] SUCCESSION AKOUND PLYNLIMON" AND PONT ERWYD. 525 



This replacement of a single fo]d by compound folds probably 

 takes place frequently ; but the general characters of sections taken 

 across the strike as illustrated by fig. 12 (p. 508) remain unchanged. 

 An excellent idea of these structures is given also by the horizontal 

 sections of the Geological Survey published about 1845. 



Disregarding these i-eplacements, which probably occur as fre- 

 quently in one direction as in the other, a prominent fold can 

 generally be recognized in several boundaries. For instance, the 

 Craig-Yspio anticline ranges for the sharp anticline which brings 

 up the Rheidol Group on the south side of the fault in Nant Meirch ; 

 and the anticline immediately to the west of it can be recognized 

 clearly in the Drosgol Grits, the Eisteddfa Group, the Castell 

 Group, and in the Devil's Bridge Group, and possibly within that 

 group on Mynydd-yr-Ychion, as mentioned above. Numerous other 

 instances might be cited. 



The influence of the pitch upon the topography is one of the 

 salient features of the district. If the hills in the Plynlimon area 

 and to the north of Pont Erwyd be viewed across the strike, one 

 is immediately reminded of dip-slopes and escarpments. On closer 

 examination this suggestion is borne out, though the slopes are not 

 dip-slopes in the ordinary sense of the expression, but may be 

 termed ' pitch-slopes,' the outline of the hills conforming closely to 

 the pitch of the folds. This is illustrated by the outline-section 

 (tig. 16, p. 524) from Plynlimon to Pont Erwyd : the southern 

 slopes, such as Plynlimon, Drum Peithnant, Drybedd, and Bryn- 

 glas, are all characteristically long and gentle, while the northern 

 slopes are short and abrupt. 



Of individual hills, however, the best examples are the prominent 

 hills of Dinas and Disgwylfa-fawr, in which the hard Castell Beds 

 project northwards as synclinal spurs far into the relatively low 

 ground occupied by the underlying softer rocks. Their form is an 

 accurate index to the pitch of the folds, and indeed its value may 

 often be determined by observing the slope of the hills. 



To the south of Pont Erwyd, and especially around Devil's 

 Bridge, the hills are almost flat-topped, or at most have a gentle 

 southerly inclination, which is in agreement with other evidence 

 of a diminishing pitch in that direction. 



The influence of the pitch upon the appearance of many of the 

 hillsides also calls for some comment. Eor various combined 

 reasons, among which the direction of the axes of folding (or 

 the average strike) and the prevailing direction of cleavage 

 are probably the most important, many of the more prominent 

 hills range a little west of south and east of north ; and, when 

 these hills are viewed across the strike, the outcrops of alter- 

 nating hard and soft beds give them a terraced appearance, and 

 whatever the direction of the dip all the terraces have 

 a gentle southerly inclination. The diagram (fig. 17, p. 526) 

 illustrates how this southerly inclination is a result of the pitching 

 of the folds. AB represents the direction of the hill-slopes which 



