part -1] DISTRICT AROUND CORRIS AND ABERLLEFENNI. 531 



Ystwyth Stage. 



So far as they have been investigated in this area, the beds of 

 the Ystwyth Stage consist of pale greyish-blue slat}' mudstones, 

 with numerous irregularly laminated grit-bands measuring up to 

 1 or 3 inches in thickness. The laminated grit-bands have usually 

 a false-bedded appearance. 



Immediately above the base at Taren y Gesail are disused 

 quarries in a slaty group belonging to this stage. The greyish- 

 blue slaty mudstones which have been quarried are precisely 

 similar to those that have been quarried farther south at Cwm y 

 Gof and Cwm Rhaiadr (described in the Machynlleth paper). 

 They contain paler mudstone -bands and some bands of grit. 



At a somewhat higher horizon, a similar slaty group in the 

 Ystwyth Stage has been extensively quarried south of Corns, as, 

 for example, in the Era and Rhiw'r-gwreiddyn Slate-Quarries near 

 Esgairgeiliog. Here again the lithological type is a greyish-blue 

 slaty mudstone, with paler bands and some laminated gritty seams. 



The transition from the Derwen Beds to the Ystwyth Stage is 

 marked by the gradual incoming of siliceous and gritty bands. 

 They rapidly increase in number, and within a few feet become 

 well-marked grit-bands. The base of the Ystwyth Stage has been 

 taken, for convenience in mapping, where the gritty bands become 

 characteristic. Actually, this is some distance above the band 

 which yields Monograptus sedgwicJci. It should be pointed out 

 that the boundary is, in many ways, an unsatisfactory one. 



Note on the Outcrop of the Valentian Rocks. 



Throughout the Corris-Aberllefenni area, the outcrops of the 

 various members of the Valentian Series make the characteristic 

 topographical features described farther south. 1 The compara- 

 tively soft shales of the Cwmere Group form a hollow flanked on 

 one side by the rough craggy ground of the Garnedd-wen Mud- 

 stones, and on the other by the well-marked escarpment of the 

 Derwen Group, capped in most cases by the basal beds of the 

 Ystwyth Stage. This feature is particularly well displayed when 

 the rocks cross a spur or ridge between tAvo valleys. 



V. Detailed Description oe the Structure. 

 Folding. 



The area lies on the south-eastern flank of the Harlech Dome, 

 and the general trend of the strata is determined by that major 

 structure. As a result, the general strike of the beds is from south- 

 west to north-east. The beds dip south-eastwards, the dip varying 

 from about 40° to practically vertical. The lowest dips are usually 

 found along the margin of the volcanic rocks — the average clip 

 there being about l-"> . South-eastwards the dip gradually steepens, 



1 0. T. Jones & W. J. Pugh, Q. J. G. S. vol. Ixxi (1915-16) fig. 1, p. 348. 



