﻿Vol. 66.~] GEOLOGY OF THE DISTRICT AROUND LLANSAWEL. 407 



direction of the dip. To the north-east, east, and south, on the 

 contrary, the structure is complicated by a considerable amount of 

 faulting ; and we believe that these two regions are separated by 

 one great fault (or a series of faults), the exact position of which 

 is masked by the superficial deposits of the Cothi Valley. To the 

 north the position of the break can be located to within a short 

 distance, as the Clynmarch Grits are there seen striking directly 

 into the Llathige Mudstones ; and farther south its course probably 

 follows closely that of the Cothi, ranging to the west of the high 

 ground in Pen-y-ddinas and Banc Beili Tew. 



In general, the direction of the faults is that of the axes of the 

 folds, that is, north-east to south-west, but there are departures 

 from this rule. The faulting chiefly occurs in the neighbourhood 

 of the massive grits and conglomerates of the Caio Group. These 

 have acted as resistant rigid bodies, and have first been folded and 

 then broken under pressure from the north-west, the result being 

 a series of faults, throwing alternately in opposite directions ; and 

 in every case the north-western limb of the fold appears to be 

 faulted out. 



IV. Detailed Description op the Beds. 



(A) The Beili Tew Group. 



The grits and shales characteristic of this group are well exposed 

 in small quarries on the south-west side of Banc Beili Tew. Here 

 the grits are typical ; they are never massive, but alternate with 

 the shales, which are often crushed up between them. This grit is 

 easily distinguished from those of higher horizons. It is far less 

 quartzitic, more sandy, and much more ferruginous, weathering 

 very pale with red speckling; it contains unidentifiable organic 

 fragments. The shales are very tough, and the bedding is in most 

 cases obscured by concentric nodular banding. There is no sign of 

 fossils, but from their position and lithological characters we believe 

 the beds to be of pre-Birkhill, probably Hartfell, age. 



(B) The Caio Group. 



(B x ) The Shon Nicholas Conglomerate is well exposed 

 ^xlong the crest of the ridge of that name, 3 miles east-south-east 

 of Llansawel. Its conglomeratic nature is here very evident, the 

 pebbles ranging in size from small grains to boulders 6 to 10 inches 

 in diameter. The matrix is a tough grey grit, and the pebbles are 

 mainly white quartz. These characters are not constant, as, on the 

 next hill to the north (Banc Bwlch Cefn Sarth), the matrix is much 

 more shaly ; and, on Banc Goleugoed still farther north, the rock is 

 virtually a grit, and the large pebbles are no longer evident. A 

 few fragmentary fossils were obtained, but collecting was extremely 

 difficult, owing to the toughness of the rock. On the hill-tops the 

 conglomerate stands out as rugged bluffs, but where the weathered 

 product collects, it forms a good red gravel. 



