﻿584 DK. T. GEOOai AND ME. P. LAKE OX THE [NoV. I908, 



The remaining faults, so far as we are aware, are nearly vertical 

 planes, and (with the exception of the Cae-mawr Fault) appear to be 

 of the normal type. There are two principal series : — the one set 

 running nearly due north and south, or, in the western portion of 

 the map, north-north-west and south-south-east ; and the other set 

 running approximately from west-north-west to east-south-east. 



The North-and-South Series of Faults. 



In the western part of the map there are two long faults running 

 from north-north-west to south-south-east, both of which have 

 their downthrow on the east. The most conspicuous of these runs 

 from near Tomen-y-meirw to Ty-isaf, and is very easily detected 

 because it throws both the Glyn Grit and the ash-band some 

 200 yards to the south on its eastern side. It crosses 'Nant 

 Tyn-y-twmpath a little north of the road from Glyndyfrdwy to 

 Glyn Ceiriog. The absence of the higher zones of the Pen-y- 

 glog Slates noted by IVIiss Elles ^ is probably due to this fault. 

 Near the northern margin of our map (PI. LIII) it is met by a fault 

 running nearly due north and south, and likewise having a down- 

 throw on the eastern side. 



The second of the north-north-west to south-south-east faults 

 extends from the uppermost of the Glyn slate -quarries through 

 Aber-chwil, and probably into the valley of Cwm Clwyd. At one 

 time it was distinctly visible in the slate-quarry, where it brought 

 the green slates on the west into contact with the Denbighshire 

 Series on the east ; but the exposure is now concealed by a fall of drift 

 and soil. The effect of the fault on the Glyn Grit and on the Craig- 

 y-Pandy Ash is small, although the greater part of its course is 

 marked by a very distinct depression. 



South of the ash-band its course is not very clearly shown ; but 

 in the old Teirw-Hill Quarry close to Pandy there is a fault that 

 lies approximately in the same line. jS'ear the ash-band the fault 

 is met by a second fault, which runs from north to south, and has 

 its downthrow on the western side. 



Near Llechrhydau a group of three faults, which run nearly 

 due north and south, will be seen on the map (PI. LIII). The 

 westernmost of these is clearly shown in the stream west of the 

 farm, where it brings slates on the west into contact with ash upon 

 the east, the direction of the fault being N. 12° W. Owing to 

 the covering of drift and heather, it is impossible to be quite certain 

 whether these slates are above or below the lowest ash-band. 



The next of the series of faults passes immediately to the east of 

 the farm, and an old trial-working appears to have been driven 

 along it for some distance at the north-eastern corner of the farm- 

 yard. It is also seen a little farther north, where its face has been 

 exposed for a short distance by the action of the weather, and its 

 direction is N. 8° W. The fault here hades steeply to the west. 



1 Quart. Journ. Geol. Soc. vol. M (1900) p. 398. 



