Vol. 65.] SUCCESSION AROUND PLTNLIMON AND PONT ERWYD. 521 



VI. The Geological Structure of the District. 



It -will be seen from the map (PI. XXIV) that the various rock- 

 groups are disposed roughly in the form of an open V, the apex of 

 which is directed southwards : and that the oldest rocks occur at 

 the northern end of the district, while successively younger strata 

 wrap around them on the east, south, and west. This disposition 

 is shown most clearly in the rocks of the Plynlimon and Pont Erwyd 

 Stages ; the area of the Ystwyth Stage surveyed is not sufficient to 

 show it clearly. 



This disposition is a consequence of the folding of the rocks 

 in a large ' anticlinal ' fold, the nose of which dips or pitches 

 southwards. Superposed on this primary fold is a large number 

 of secondary anticlines and synclines, having a similar southerly 

 pitch ; these folds are usually simple, but occasionally they are 

 complicated by smaller (tertiary) folds. Such a structure is termed 

 an anticlinorium ; its greatest elevation follows the eastern 

 slope of the Eheidol Valley from near Xant-y-Moch to Pont Erwyd, 

 and is probably coincident with the anticlinal axis which traverses 

 the Pheidol gorge west of Bryn-chwith. This line of greatest 

 elevation may be spoken of as the axis of the anticlinorium, 

 though, strictly speaking, in such a structure there are several axes. 

 It brings up the oldest beds at Nant-y-Moch, and it will be noticed 

 that the rocks of any given group project farthest in a southerly 

 direction along this line. 1 Its range from Xant-y-M6ch to Parson's 

 Bridge is S. 12° W. The axes of the secondary folds have an 

 almost constant direction of S. 5°-10° W., occasionally varying 

 from these limits by a few degrees. The pitch lies between 10° and 

 15° from Pont Erwyd to Plynlimon, but appears to be somewhat 

 less to the south of Pont Erwyd. I believe also that it shows signs 

 of diminishing to the north of Plynlimon, and so the area herein 

 described may be only the southern half of an elongated dome- 

 shaped elevation. The axial planes of the folds are, as a rule, 

 vertical, that is, the folds are of the symmetrical type, the dip in the 

 limbs varying in different localities from 30° to 60°. Occasionally 

 a steeper dip than that just mentioned, even up to verticality, may 

 be observed ; but no general rule can be given as to its direction. 

 On account of the sharpness of the folds the outcrops of their two 

 limbs do not, in general, include an angle greater than 30°, and it 

 is frequently less. The average strike is, therefore, nearly north and 

 south ; strictly speaking, it has the same average direction as the 

 folding axes, since the folds are so nearly symmetrical. 



On account of the strong pitch the average strike is, however, of 

 little importance, except as a rough indication of the direction of 

 the folding axes. It would be, for instance, quite wrong to expect 

 rocks which crop out at a given place to appear again half a mile 



1 The Plynlimon Grits west of Drybedd seem to offer an exception to this 

 statement, but it is probable that they extend farther south than is indicated 

 on tbe map. The solid geology thereabouts is largely concealed by glacial 

 deposits. 



