﻿Vol. 64.] BALA AITD LLANDOVEEY EOCKS OF GLYN CEIRIOG. 57^ 



(10) Ty-draw Slates.— The rron-Frys Slates are followed 

 by homogeneous pale-green slates, free from grit-bands, very 

 similar in character to the Tarannon Shales of other areas. 

 They are well exposed at Tomen-y-meirw ; on the northern slopes 

 of Cefn-coch ; in Nant Llafar ; on Mynydd Fron Erys ; and along 

 the old road from Glyn to Oswestry in the neighbourhood of Plas-on 

 and PwU-hir. On Mynydd Fron Frys they are about 90 feet thick, 

 and have been extensively quarried. 



There does not seem to be any unconformity between them and 

 the Fron-Frys Slates below ; but a fairly definite line can be 

 drawn between the two, although near the boundary it is not 

 always easy to say whether the rock belongs to the one series or to 

 the other. 



Wherever the junction between the Ty-draw Slates and the 

 Denbighshire Series is well exposed, the two appear to be inter- 

 bedded. This is particularly well shown in the stream which flows 

 past Tomen-y-meirw. Towards the top of the Ty-draw Beds bands 

 of dark slate are interbedded, and at the base of the Denbighshire 

 Series bands of pale-greeu slate are included. Similar relations are 

 visible at the junction near the farm of Cefn-isaf, but here the beds 

 are not quite so well exposed. It is, therefore, impossible to draw 

 an absolutely sharp line between the two series. 



In genera], the Ty-draw Slates appear to be quite unfossiliferous. 

 In a dark band, however, in the midst of the series close to the 

 farm of Cae-mawr, we found a somewhat badly-preserved graptolite 

 which Miss Files has kindly identified for us as Monograjptus Marri, 

 a species belonging to the Tarannon Series. 



(11) The Denbighshire Series. — The Denbighshire Series 

 covers a very large area in the Glyn Valley and the neighbouring 

 district ; and, although we do not propose in the present communi- 

 cation to deal with these beds in detail, it is necessary, in order to 

 complete the account of the structure of the district, to describe 

 their distribution. 



In the whole of the western portion of our map (PI. LIII) they 

 lie immediately to the north of the Ty-draw Slates, and are parti- 

 cularly well exposed near Tomen-y-meirw and in the slate-quarries 

 in Nant Llafar, as also at Glyn Ceiriog itself. 



To the east of Glyn, where the valley of the Ceiriog assumes an 

 easterly direction, they occupy the whole of the northern side of 

 the valley. But, excepting for a small patch which extends across 

 the river towards the cemetery, they do not occur south of the 

 Ceiriog until we reach Dol-y-wern. Near that village, however, 

 owing to the Cae-mawr Fault, which has already been mentioned, 

 they cross the river and extend past Penllwyn to the quarry of 

 Nant Gwryd-uchaf. They have not, however, been found on the 

 south side of the little valley in which Plas-on lies. 



