part 2] PALiEOzoic eocks of the llangollen disteict. 221 



The exact nature of the movement which produced these apparent 

 displacements is not quite clear. Where the Bryneglwys and 

 Llanelidan Faults become one, there seems evidence of a southward 

 crushing from the north, producing a sort of thrust, but not 

 necessarily an overthrust ; for, concurrently with this, there was 

 relative movement of the block of country south of the fault in 

 an eastward direction. A similar lateral movement doubtless 

 occurred along the Bryneglwys Fault south-west of Llandegla. 



Despite its Caledonian trend, the Bryneglwys Fault owes its 

 conspicuous displacement to post-Carboniferous movements ; and 

 it is by no means easy to decide the function Avhich it played in 

 the Devonian dislocations. There is no doubt, however, that it 

 was even then one of the master-faults of the region. 



Cleavage. 



The Devonian movements subjected all the Lower Palaeozoic 

 rocks of this district to forces that produced cleavage in them, in 

 greater or less degree according to their nature and their position 

 in relation to the folds. 



Over the greater part of the area, there is a marked agreement 

 in the strike of the cleavage with the strike of the folds and 

 buckling, so that a direction between east 10° and 20° south is 

 normal. The dip is northwards and, except south of the Llan- 

 gollen Fault where it is consistently about 30°, it may be said 

 elsewhere to average 60° to 70°, although occasionally it is as low 

 as 30° or as high as 85°. The northward dip suggests that the 

 cleavage-forces (and therefore also the forces producing the folding) 

 acted from north to south. The movement was, in our opinion, a 

 superficial southward displacement relative to a more powerful and 

 deeper-seated northward or northwestward Caledonian movement 

 in areas not far removed from, or even underlying, this tract of 

 countr}^. The presence of a thorough, though low-dipping cleavage 

 in the very gently folded rocks south of the Llangollen Fault and on 

 the flanks of the Berwyn Anticline may, perhaps, be cited in favour 

 of such an origin for the cleavage-forces. Be this as it m^j, the 

 occurrence of good roofing-slates in beds in which the true dip 

 is 10° to 15° and the cleavage-dip about 30°, both northwards, is 

 remarkable and difficult to explain. Yet these conditions hold 

 throughout the outcrop of the Pen-y-glog Slates from Carrog to 

 Glyn-Ceiriog. 



Post-cleavage faulting has upset the general direction of cleav- 

 age-strike and dip in a pronounced manner in the area between the 

 Llanelidan and Bryneglwys Faults, eastwards from Llanelidan and 

 Bryneglwys ; in many cases the strike has been rotated from east- 

 and-west to about north-and-south, so that the dip is now west- 

 wards. In such cases the true dip and strike of the folded rocks 

 have undergone a similar alteration, and it is obvious that the 

 whole block defined by the faults has rotated, probably in post- 

 Carboniferous times. This is well illustrated in fig. 5. 



