part 4] DISTRICT AROUKD CORRIS AND ADERLLEEEIvNt. 509 



extensive slate-quarrying region which extends from near Towyn 

 in the west as far as Dinas Mawddwy in the north-east. 



Except for slate-quarrying, the main occupation is sheep-farming, 

 there being comparatively little ground capable of tillage. 



Physical Features. 



The area forms part of a deeply dissected plateau, which attains 

 an altitude of 1200 to 1S00 feet ; but locally greater heights are 

 reached, as, for example, at Taren y Gesail, south-west of Corris 

 (1901 feet), and Waen Oer, north-east of Aberllefenni (over 

 2000 feet). 



Many of the streams show well-marked features of rejuvenation, 

 a characteristic which is common to many of the streams comprised 

 within the Dyfi drainage-area. The rejuvenation is well seen in 

 some of the tributary valleys of the River Hulas, and particularly 

 those lying south of Corris. The upper part of these tributary 

 valleys is wide and floored with boulder-clay, while, lower down, 

 the stream suddenly plunges into a steep-sided and wooded gorge. 

 Such characteristics, to cite one example, are well shown in the 

 small stream called Nant y Daren, where the change in character 

 occurs about the 600-foot contour-line, and may be recognized at 

 once by examination of a large-scale map. In the held this 

 change in topography is well marked. 



The area exhibits many of the characteristic features resulting 

 from glaciation ; for instance, the broad, straight valley of the 

 Llefenni (above Aberllefenni), the small cwms of Taren y Gesail, 

 etc. ; but the district is comparatively free from glacial drift. 

 Where glacial deposits do occur, they are almost entirely confined 

 to the floor of the larger valleys and the upper parts of the tributary 

 or smaller valle3 r s. As a result, rock-outcrops are numerous over 

 practically the whole area. This, coupled with the numerous 

 quarries and trials for slate, makes it possible to survey the district 

 with a considerable degree of accuracy. 



II. History of Previous Research. 



There are comparatively few references in geological literature 

 to this area, and little detailed work appears to have been attempted 

 previously. 



In 1817, Adam Sedgwick 1 described a section from a point on 

 ' the south side of the Barmouth estuary over the top of Cader 

 Idris, and thence over the ridges of Arran y Gesail 3 to the valley 

 above Machynlledd.' He states that such a section ' will be nearly 

 transverse to the general strike of the country. 1 Sedgwick divided 

 the rocks along this line of section into a number of groups, based 

 principally upon their lithological characters; hut it is interesting 

 to observe that he recognized the two great groups of rocks in this 



1 Q. J. G. S. vol. iii (1847) pp. 147-48. 



• Now spelt on the Ordnanue Survey Maps ' Taren y Gesail.' 



