﻿Vol. 64.] BALA AND LLANDOVEEY EOCKS OF GLTN CEIEIOG. 585 



South of the farm the fault appears to curve somewhat eastwards, 

 and then to resume a more southerly course. 



But by far the most remarkable and the most important of this 

 group of faults is the one that we have termed the Cae-mawr Fault. 

 Trending in a direction a little west of north, it runs in nearly a 

 straight line from near Llechrhydau past the farm of Cae-mawr to 

 Dol-y-wern, and is probably continued northwards past Llangollen to 

 the base of the Eglwyseg Crags. Despite its extent and importance 

 it is the most elusive of the series, and has escaped the notice both 

 of D. C. Davies and of the officers of the Geological Survey. The 

 latter represent it as an unconformity at the base of the Tarannons, 

 and the former as an unconformity at the base of the Wenlock ; 

 but the strike of the beds does not curve round in the way that 

 such an explanation requires. 



The exposures in this part of the district are very poor ; but, in 

 Ty-nant Wood, the Bryn Beds are well shown striking nearly due 

 east ; and the same beds are visible in place about 450 yards south 

 by west of Cae-mawr, close to the line of the fault. East of the 

 fault the Ty-draw Slates are exposed at intervals along the Oswestry 

 Eoad, and the Pron-Frys Slates upon the hillside above, the strike 

 being approximately from east to west, directly against the fault. 



The continuation of the fault northwards can be traced in the 

 hill north of Dol-y-wern. At Ty'n-y-celyn Brongyll the slates 

 immediately on its western side contain graptolites belonging to 

 the Monograptus-riccartonensis Zone of the Wenlock Series ; while 

 350 yards to the east, Ludlow Beds with Monograjptus vulgaris occur 

 on the roadside below Bron-heulog. The Wenlock and the Ludlow 

 Beds here strike towards each other, and a fault of some magnitude 

 must therefore lie between the two exposures. 



South of Cae-mawr Wood the southerly extension of the fault is 

 concealed for more than a mile by the drift that hides the whole 

 of the solid geology of this portion of the area. But, about 600 

 yards south-east of Llechrhydau, an isolated crag of ash stands high 

 above the soil-covered slope. It has been mapped by the Geological 

 Survey as a portion of the lowest ash-band ; if this be ccfrrect, how- 

 ever, the band must for a short distance be of enormously greater 

 thickness than it is anywhere else in the neighbourhood. We 

 believe that this crag is the continuation of the Craig-y-Pandy Ash 

 (which it resembles lithologically) thrown southwards by the Cae- 

 mawr Fault. 



It is, however, clear that the fault does not extend farther south- 

 wards; or, if it does, its throw is very greatly reduced: for the lowest 

 ash-band runs right across its course, with not more than a slight 

 dislocation in the line of the fault. 



The striking feature of this fault is the remarkable apparent 

 variation of its throw. Everywhere the beds upon its eastern side 

 are newer than those with which they are in contact on the west ; 

 but if the movement has been vertically downwards, then the 

 amount of the vertical throw increases rapidly towards the south 

 as far as the crag already mentioned, beyond which the throw is 



