part 2] PALEOZOIC rocks of the llangollen district. 219 



structure is the fault which ranges obliquely across the Central 

 Wales Syncline from Llandderfel to Corwen, and thence (as shown 

 in PI. V) to Llandegla. The part lying within the district described 

 in this paper has been named the Bryneglwys Fault, although, 

 according to the Old Series maps, it is merely part of the great 

 Bala Fault. Until this interpretation has been confirmed, a local 

 name appears preferable. At Llandegla it joins the Llanelidan 

 Fault, and the two pass as one across the northern side of 

 Cyrn-y-Brain, and so into the Carboniferous country on the 

 north-east. 



The Brjmeglwys Fault is in reality a plexus of subparallel 

 fractures that enclose lenticular strips of various rocks. The 

 nature of the fault can be best studied near Llandegla, and is 

 illustrated in fig. 5 (p. 220). Here we find in the angle between 

 the converging faults a triangular patch of Tarannon Shales, 

 Probably the rocks north of the Llanelidan Fault moved west- 

 wards, and those south of the Bryneglwys Fault eastwards, relative 

 to the block in between. The Tarannon Shales, squeezed between 

 the two faults, have been tilted towards the Bryneglwys Fault, and 

 thrust westwards over the lowest Wenlock zones, which either dip 

 towards, or strike against, the junction. Farther west and south- 

 west these Wenlock rocks form a (probably convex) syncline, the 

 axis of which runs nearly due north and south, but is slightly 

 curved so as to be convex westwards. Its axial plane is inclined 

 eastwards. In the centre of the syncline Ludlow rocks crop out. 

 This area seems to have been subjected to a clockwise movement 

 which has twisted round the cleavage nearl}^ 90° from the normal 

 direction (see p. 221), so that it now strikes about due north and 

 south and dips westwards. 



The triangular patch of Tarannon Shales, which we have just 

 shown to be bounded by a thrust on the west, is faulted on the 

 north by the Llanelidan Fault against a wedge of high-zone 

 Carboniferous Limestone, which itself is separated by a secondary 

 fracture on its north side from the Cefn-y-fedw Sandstone, south 

 of Llandegla. 



The multiple nature of the Bryneglwys Fault-fracture is well 

 seen on the south-eastern side of the triangle of Tarannon Shales. 

 Here we find strips of Middle or ' White ' Limestone and of Cefn- 

 y-fedw Cherts dropped down between the above-mentioned Tarannon 

 and the Ashgillian of Cyrn-y-Brain. The main branch of the 

 Bryneglwys Fault between the Limestone at Craig and the Ash- 

 gillian is concealed by Boulder Clay, though indicated by a fine 

 series of swallow-holes. 



The south-easternmost branch of the Bryneglwys Fault can be 

 traced subparallel to the main fault across the north-M^estern flank 

 of Cyrn-y-Brain. It cuts off a corner south-east of Llandegla, 

 and produces the shatter-belt along which the Hafod-dafalog 

 stream has excavated its deep valley. 



For long distances south-westwards the line of the Bryneglwys 

 Fault is masked by Grlacial drift that obscures the flat-bottomed 

 hollow in which it lies. 



