﻿Vol. 64.] BALA AND LLANDOYEEY ROCKS OF GLYN CETRIOG. 555 



IV. Detailed Description of the Subdivisions. 



(1) Cwm-Clwyd Ash. — On the l-inch Geological-Survey map 

 this is marked as a continuous belt extending across the whole of the 

 district. It is, indeed, exposed, west of our area, in the upper part 

 of the Ceiriog ; but between that point and the valley of the Ceiriog 

 at Pandy it is scarcely ever visible, presumably on account of the 

 covering of peat and heather that conceals the underlying rocks. 

 The westernmost point in our map (PL LIII) at which it actually 

 appears at the surface is in the woods of Erw-gerrig. Even here the 

 exposures are very poor, and it is not until we cross to the east side 

 of the Ceiriog tbat the band can be studied to advantage. It forms 

 a series of fine crags near Cwm Clwyd, where the bedding is 

 well displayed, some of the beds being very compact and others 

 of medium grain. A remarkable nodular rock which occurs here 

 presents a somewhat deceptive resemblance to an amygdaloidal 

 lava, but under the microscope the material of the apparent nodules 

 is seen to be similar to that of the matrix in which they lie.^ 



From Cwm Clwyd the band may be traced towards Spring Hill. 

 For a considerable distance it lies at the top of Springhill Wood 

 and in the fields above, gradually sinking down into the wood towards 

 the east. About 450 yards west of the farm it is dislocated by a 

 small fault. Beyond this it lies just within the northern margin of 

 the wood, which it leaves to cross the stream immediately south of 

 Spring Hill. Here it is very thin, compared with its great develop- 

 ment at Cwm Clwyd, and is underlain by slate with sandstone-bands 

 and overlain by sandstone. Beyond the stream it can be traced 

 for some distance by the line of blocks on the hillside, but after 

 700 or 800 yards even these disappear beneath the heather, and 

 there is no satisfactory evidence of the course that it takes. Our 

 boundary is, therefore, drawn on the assumption that there has 

 been no great change in the strike. 



Near Llechrhydau the ash again comes into sight, at first as a 

 small patch about 150 yards south-west of the farm, and again in 

 full strength at Llechrhydau itself. At this point there has evidently 

 been a very considerable amount of disturbance, and the exact 

 relations of the beds are not very" clear. The ash forms a broad 

 band running nearly due south from the farm, and appears to lie 

 conformably upon the sandstone and slates to the west of it, but to 

 be separated from the beds on the east by a fault. About 300 yards 

 south of the farm the ash is thrown into a small anticline faulted 

 on one side, which is well exposed on the western bank of the 

 stream. 



A little farther south, the band crosses the Morda, and forms a line 

 of picturesque crags along the crest of its eastern bank, where the 

 character of the beds is well displayed. The greater part of the 

 band consists of platy beds of fine or medium grain, but there are 

 two or three beds of very coarse tuff containing more or less rounded 



1 See T. H. Waller, ' Midland Naturalist ' vol. xvi (1893) p. 201. 

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



