160 DE. G. L. ELLES ON THE BALA COUNTKY : [vol. Ixxviii, 



any distance across the dense grass- or heather-covered moorland. 

 The differential movements of the rocks, however, seem to necessi- 

 tate the existence of such lines. 



The Llyn-Tegid Line was first noticed by P. Lake (Fault B)i 

 as running along the north-western shore of the lake ; it seems 

 to emerge at the northern end of Bala Lake, and, acquiring a 

 slightly more northerly trend, runs up to the Nant-Hafhesp valley, 

 being then apparently bent round into one of the east-north-east 

 and west-south-west lines of displacement of the Moel-Emoel 

 area. Its chief effect is to cause concealment of portions of the 

 higher beds, so far as the northern part of the district is concerned. 

 It appears to be more affected by the topography than the Bala- 

 Lake Fault and, therefore, is probably of lower inclination. 



The Bala-Lake Line is a more important displacement; it 

 seems to run fairly near the south-eastern shore-line of the lake, 

 probably just outside the Llangower peninsula, and, emerging at 

 the northern end of the lake, runs up into the high ground on the 

 north approximating to the line of the Nant Cwm-da. Every- 

 where along it the rocks on its south-eastern side are driven over 

 those on the north-west ; in the extreme south the volcanic rocks 

 forming Aran Ben Llyn and the beds beneath them are brought 

 over the Dicranograptus Shales, while farther north the Caradocian 

 beds are thrust over the Ashgillian in all the country north of the 

 lake as far as Moel-Emoel. Its inclination is fairly high. 



The Llangower Displacement, with the subsidiary Cefn- 

 ddwy-graig branch, is perhaps the most striking thrust-line of the 

 district ; for there has been considerable differential movement of 

 the rocks above this Llangower thrust, and consequently a great 

 development of tear-faults is associated with it, especially in the 

 area between Llangower and Mj^nydd Cefn-ddwy-graig, where the 

 structure of the country is clearly brought out by the outcrops of 

 the Frondderw Ash. The tearing becomes definitel}^ less with the 

 development of the Cefn-ddwy-graig Displacement, which seems to 

 have effected compensation 'en bloc' The beds, however, above 

 this line of thrust are very sharply faulted and folded near Pont- 

 y-Ceunant, where the IJangower, Cefn-ddwy-graig, and Moel-fryn 

 lines all come closer together ; the rocks above the Cefn-ddwy-graig 

 thrust and beneath that of Moel-fryn show a series of small sub- 

 sidiary thrusts affecting the anticlinal lines of the small folds so 

 as to bring about an effect of imbrication, and this, combined with 

 tearing, has shattered the rocks to j^ieces, 



The chief effect of the Llangower thrust is to bring the lower 

 beds of the Allt-Ddu Mudstones, close to their junction with the 

 Frondderw Ash, over the higher Allt-Ddu Mudstones ; while 

 along the Cefn-ddwy-graig line the Glj^n-G-ower Sandstones, the 

 Frondderw Ash, and the Lower Allt-Ddu Mudstones are all in turn 



1 Geol. Mag. 1900, p. 212. 



