472 MR. 0. T. JONES ON THE HARTFELL-VALENTIAN [Nov. I9O9, 



From this section we learn, therefore, that four groups of rocks, 

 each distinct from the others in its lithological characters, follow 

 one another in regular ascending order from east to west, namely : 



(1) the Nant-y-M6ch Flags, containing Dicellograptus anceps ; 



(2) the grits, conglomerates, and mudstones, known collectively as 

 the Drosgol Grits ; (3) the blue-black mudstones, for which the 

 name Bryn-glas Mudstones is suggested ; and (4) the red-stained 

 nags and shales, which, as will be shown later, lie at the base of 

 the overlying Pont Erwyd Stage. The first three of these I shall 

 bring together under the name of the Plynlimon Group, to 

 which is therefore given a somewhat more precise significance than 

 was assigned to it by Sedgwick and Keeping. 



No other single section gives the relationships of these rock- 

 groups in so clear a manner, but abundant confirmation of the 

 general order here established has been obtained in many parts of 

 the Plynlimon area. 



The thickness of the Plynlimon Group is very great, although it 

 is difficult to calculate it accurately ; the following estimates of 

 the thickness of the individual members of the group may be of 

 interest : — 



Feet. 

 Bryn-glas Mudstones = 900 (this can be determined accurately.) 

 Drosgol Grits = about 1500 (fair approximation.) 



Nant-y-M6eh Flags = about 1000 (doubtful estimate; may be only 



half as much.) 



Total 3400 



To the east of Nant-y-Moch a long undulating slope, on which 

 the underlying geology is almost wholly concealed, rises to within 

 200 feet of the top of Plynlimon. The western face of the 

 mountain is formed by an escarpment of blue-black mudstones, 

 protected by massive beds of a grey felspathic grit which occupies 

 the summit, and thence falls in gentle undulations towards the 

 east. On the higher ground long, low, wind-swept ridges ranging 

 in a north-and-south direction alternate with peat-covered hollows, 

 in which large blocks of grit tend to accumulate, leaving the in- 

 tervening ridges bare. Here and there a diminutive tarn occupies 

 the centre of the hollow. 



To the north of the cairns, which occur on the summit of the 

 mountain, the ground descends in a succession of rocky escarpments 

 and grass-covered platforms to a small corrie-lake which nestles 

 at the foot about 800 feet below. This lake is one of the sources 

 of the River Rheidol, and bears the name of Llyn Llygad Rheidol 

 (the lake of the eye, or source, of the Rheidol). It is fed by the 

 numerous springs that break out on the face of the escarpment ; 

 and the water, being almost free from organic matter, is of a deep 

 blue colour. The town of Aberystwyth, which lies some 14 miles 

 to the westward, obtains most of its water from this lake. 



The rocks of Plynlimon may be studied in detail along the 

 steep face overlooking the lake, but their relations are somewhat 



