206 DE. WILLS AjS^D MR. SMITH ON THE LOWER [vol. Ixxvili, 



appear to have the following thickness, although the lowest member 

 is in part cut out by the Grljai-Ceiriog Fault : — 



Feet. 



Uppermost group 600 



Slab horizon 270 



Lowest group (part) 240 P 



The thickness is not so easily measured in the disturbed northern 

 part of the synclinorium, but may be approximately 1600 to 

 1700 feet. 



(ii) Nant-y-Bache Group. — In this group the rocks are pre- 

 dominantly uncleaved silt}^ sandstones and roughl3^-cleaved sandy 

 shales. They are best studied south of the Llangollen Fault, where 

 they occupy most of the upland between the Dee and Ceiriog 

 drainages, east of Gljai-Dyfrdwy. North of the fault, they are 

 involved in the severe folding, and therefore, although well deve- 

 loped, they are more difficult to investigate. The group appears 

 to be approximately 900 feet thick. 



The base of the group has been taken to be the lower limit of 

 somewhat calcareous and (as a rule) internally-crumpled ^ silty sand- 

 stones, which form a bold escarpment from near Foel, past Glyn- 

 Ceiriog and Dolywern, to the Ceiriog valley near Pont-fadog, where 

 they cross to the eastern side and. forni the nameless hill 1330. 



Near Dolywern and Glyn- Ceiriog a calcareous conglomei-ate of 

 rounded blue sandstone-pebbles, with a few shale-fragments and 

 many small blackish pebbles, occurs in the lowest part of the 

 group. It varies from 2| feet to about 1 inch in thickness. 

 A search for fossils in the pebbles revealed only one OrfJioceras, 

 though their lithology is suggestive of the Teirw Beds. The signi- 

 ficance of the conglomerate is not known, but it appears to be an 

 indication of renewed uplift along the Berwyn axis (see p. 183). 



Associated Avith crumpled sandstones are fossiliferous laj^ers, 

 often crowded with Monograptus cMmcera, M. duhius, and 31. 

 tumescens (?), while occasionally small brachiopods are very 

 abundant in calcareous bands that weather to rottenstone. North 

 of Dolywern, the following forms Avere found : — Atrypa reticu- 

 laris Linnaeus, Dayia navicula J. de C. Sowerby, and OrtJiis 

 (Dahnanella) sp. In slightly higher beds in the same area 

 Gardiola (Slava) interrupta J. de C. Sowerby is fairly common. 



There are extensive exposures of the Nant-y-Bache Grou]) in the 

 stream from which it takes its name and in the other branches of 

 the Cyflymen Brook, which descends towards Llangollen, although 

 here no fossils have been found. The rocks are predominantly 

 sandy ; but there is also some sandy, poorly cleaved shale. 



^ We term the structure 'internal crumpling,' because it can be seen in 

 many cases that a bed, say, a foot thick, preserves its upper and lower surfaces 

 sub-parallel and seemingly undisturbed, while within there is folding and 

 even overfolding. The more shaly bands interstratified with the sandstones 

 are roughly cleaved. Less conspicuous internal crumpling occurs in the 

 sandstones of the uppermost Glyn-Dyfrdwy Beds. 



