﻿Vol. 64.] BALA AND LLANDOVERY EOCKS OF GLYN CEIRIOG. 587 



fixed in position, it may be calculated that the movement upon the 

 eastern side had a vertical component of 4690 feet upwards, a 

 horizontal component of 14,830 feet southwards, and an inclination 

 upwards and southwards of 17° 32'. Such a movement would 

 bring the Craig-y-Pandy Ash to a point 140 yards north of the actual 

 outcrop A, which is as near as the degree of accuracy obtainable 

 in such observations can lead us to expect. 



It will be seen in the section that the Cwm-Clwyd Ash is but 

 little affected by this fault, and the movement must therefore 

 have taken place npon a surface between the Cwm-Clwyd Ash and 

 the Craig- y-Pandy Ash A. We have accordingly placed another 

 thrust-plane T' T\ inclined at an angle of 17° 32', at the base of the 

 Craig-y-Pandy Ash A, where we had long suspected the existence 

 of a fault.^ 



It should be observed that, in the foregoing calculation, we have 

 taken no account of the small fault/. This was formed after the 

 Cae-mawr Eault, and we have assumed that it was the block north 

 of this fault that remained fixed, while the block on the south 

 of it sank. If, on the other hand, it is the block on the south of / 

 that retains its original position (relatively to the eastern side 

 of the fault), the vertical component of the displacement along 

 the Cae-mawr Fault must have been somewhat greater, and the 

 angle of inclination slightly steeper ; but the difference involved 

 is very small. 



Finally, it may be remarked that, owing to the very oblique angle 

 at which the lines tt and gg meet, a slight error in observation will 

 considerably affect the position of the points o and ; but the 

 angle that the line oO makes with the horizontal will not be greatly 

 affected. Therefore, although our results may not be mathe- 

 matically exact, we think that there can be little doubt that the 

 movement along the eastern face of the Cae-mawr Fault was upwards 

 and southwards at a low angle approximating to 17|°. On no other 

 hypothesis is it possible to explain the relations of the beds on 

 the two sides of the fault, as shown in the figure. 



Owing to the almost complete absence of exposures farther east, 

 it is impossible to determine with precision the faulting in that 

 part of the district. The lowest ash-band is well exposed in the 

 stream on the western margin of Cefn-coch Wood ; but its course 

 within the wood and towards the farm called the Springs can only 

 be determined by the abundance of ash-blocks and the contour of 

 the surface. The ash, however, is again exposed in situ at the 

 head of the little valley about 250 yards south of the Springs. 

 We conclude, then, as shown on the map (PI. LIII), that there 

 are two faults running about 10° west of north. 



^ We cannot altogether exclude the possibility that the isolated crag may be 

 a faulted block of the Owm-Olwyd Ash, In that case, the thrust T' T' must 

 lie farther north, and must be accompanied by subsidiary thrusts and faults, 

 forming a complex which the poorness of the exposures does not permit us to 

 unravel. But the total effect of the complex must be equivalent to that of the 

 single thrust shown in our section (fig. 16, p. 68G), 



Q. J. G. S. No. 256. 2 r 



