ON MANOD AND TDE MOELWYNS, 300 



three sections, through Moelwyn Mawr, Foel li ydd *, and Craig 

 NyLh-y-Gigfran. 



II. Topography of the District. 



The area under consideration is bounded on the west hy the 

 Moelwyn ridge, the strata of which strike 8.W. to N.E. from the 

 slopes overlooking Llantrothea to the Cwm-Orthin valley, and are 

 continued on Craig Nyth-y-Gigfran to the broken country behind 

 Blaenau Ffestiniog. Here the series is fractured by several great 

 faults with a downthrow to the south and east, so that strata once 

 continuous with those of Moelwyn now form first the black and 

 broken mass of Garcg Ddu, and then the rounded hills of Manod 

 Mawr and Manod Bach on the east. On the latter side the strike 

 is more nearly S.E. to X.W., the outcrops meeting those of the 

 ^loelwyn side near Tal-y-waenudd. The general strike for North 

 Merionethshire is east and west, so that in the disturbed region 

 round Pfestiniog we have a sharp local deflection to the north. 



Filling up the triangular area thus formed, lies the long rounded. 

 Moel Tan-y-grisiau, a mass of crystalline rock, to the intrusion of 

 which the peculiarities of the district seem largely due. 



Ill, Succession of Strata on the Moelavyns. 



For a geologist wishing to study the rocks of this district in con- 

 secutive order, the best plan is certainly to begin with the Moelwyn 

 range itself, where the strata are least disturbed. 



Looking at the mountain from some position on Moel Tan y-grisiau, 

 the east face of it is seen to consist above of upper and lower tiers 

 of steep, rugged, cliff- like escarpments of the Arenig igneous rocks, 

 and below of a series of step-like ridges and grassy slopes due to the 

 unequal weathering of the Tremadoc beds. 



At the southern end, Moelwyn Mawr, the highest point, stands 

 back behind the general escarpment, overlooking the deep hollow 

 of Trwstyllog with its lonely lake. From it, the ridge descends 

 for a time, but reascends to Foel Eydd at the northern extremity, 

 overlooking the valley of Cwm Orthin. 



The first section accompanying this paper runs through Moelwyn 

 Mawr, the second just to the north of Foel Kydd. Along either line 

 the ascent may be made without difficulty and the order of the strata 

 observed. 



(a) The Tremadoc Series lies partly below but mainly above the 

 intrusive mass of Moel Tan-y-grisiau. While the black slates which 

 commonly constitute the lower part of the series may be seen unal- 

 tered about Dduallt and near Pengwern, the upper part is in the 

 Ffestiniog neighbourhood changed into hard spotted schistose or 

 flaggv rocks. Immediately at the margin of the granite there occurs 

 a hard compact rock showing no trace of bedding or cleavage. Un- 

 der the microscope it is seen to consist of finely crystalline particles, 



* Spelt ' Moel y rhudd ' on the Survey map. 



