part 3] lower paljeozoic of arthog-dolgelley. 267 



slightly displaced by faulting, the faults being a continuation of, 

 or branches from, the Llyn-Bodlyn Fault north of the Estuary. 

 Immediately west of the glen the beds escape from the anticline 

 with an abrupt change of strike, so that they form the rough hills 

 above Arthog Terrace, and thence, resuming the normal south- 

 westerly strike, continue for half a mile parallel with, and alongside, 

 the main road as far as Capel Horeb (about half a mile south- 

 east of Barmouth- Junction Station). The beds are here affected 

 by a shallow north-and-south synclinal fold, complementary to the 

 anticline previously mentioned. The gentle folding is clearly 

 displa} r ed in the run of the ridges as viewed from Barmouth 

 Bridge. Along the above-mentioned half-mile stretch the Base- 

 ment Series is bordered by the alluvium of the estuary, and the 

 base of the Ordovician is here only 1| miles distant from the 

 outcrop of the Lower Cambrian rocks at Barmouth. 1 



Near Capel Horeb the south-westerly strike carries the outcrop 

 inland once more towards Cyfanydd. 3 The further continuation of 

 this line is shown on the 1-inch Geological Survey map 3 passing 

 about lg miles east of Llwyngwril, and gradually swinging round 

 to an almost north-and-south direction until it finally disappears 

 under the alluvium of the Dysynni Valley near Llanegryn, 3§ miles 

 north-north-east of Towyn, and 8 or 9 miles south-south-west of 

 Llyn Wylfa. 



On both sides of the Arthog Valley the strata of the Basement 

 Series are intimately associated with diabase intrusions which 

 generally obscure the junction with the Tremadoc Slates below. 

 Grits and diabases, together with the overlying rhyolitic rocks, give 

 rise to rough and often densely-wooded ground, scarred by numerous 

 crags and screes. Such ground offers a marked contrast to the 

 smoother ground occupied by the higher part of the Tremadoc 

 Slates. The difference is well seen from near Barmouth Junction, 

 whence the course of the Upper Tremadoc Slates south-west- 

 wards can be readily pieked out, by reason of the line of green fields 

 below the rough rock-strewn and wooded slopes occupied by the 

 Ordovician strata. 



West of Llyn Wylfa the Basement Series can be followed, as 

 described above, without interruption for a distance of over 9 miles. 

 Immediately east of the lake, however, the series disappears under 

 the Cae-Einion diabase, and is cut out by the Derwas Fault 

 (p. 311), so that it does not appear in the Gwynant Valley, and it 

 is not again seen for a distance of 1| miles, until the neighbour- 

 hood of Maes Angharad is reached. Here the characteristic 

 banded flags emerge from the alluvium for a short space. The 

 band probably continues eastwards past Llwyn Iarth, where, 

 however, it becomes concealed owing to the rapid spreading of the 

 diabase intrusion. 



- l A. E. Andrew, Geol. Mag. 1910, p. 167. 



; - 2 About 1 mile east of Fairbourne Station, beyond the western boundary of 



the map, PI. XX. 



:1 Old Series, 1-inch, Sheet 59 N.E. 



