Vol. 65.] HARTFELL-VALENTIAN SUCCESSION AROUND PLYNLIMON. 463 



29. The Hartfell-Valentian Succession in the District around 

 Plynlimon and Pont Erwtd (North Cardiganshire). By 

 Owen Thomas Jones, M.A., B.Sc, F.G.S. (Read May 12th, 

 1909.) 



, [Plates XXIV & XXV- Maps.] 



Contents. 



Page 



I. Introduction 463 



II. Historical Eeview 464 



III. The Stratigraphical Succession 467 



IV. Detailed Description of the Beds 468 



V. The Western Limb of the Bheidol Anticlinorium . 519 



VI. The Geological Structure of the District 521 



VII. Comparison with the Deposits of other British 



Areas 527 



VIII. Description of Two New Species of Graptolites ... 53] 



I. Introduction. 



The area described in this paper forms part of the hilly country 

 some 12 to 16 miles to the east of Aberystwyth. It extends for a 

 distance of about 9 miles, from the Plynlimon Mountains near the 

 northern margin, to the Ystwyth Valley which forms the southern 

 boundary ; the eastern limit lies on the elevated tract which forms 

 the divide between the coastal drainage and that of the Biver Wye 

 and its tributaries, while its western margin is roughly defined by 

 the western slope of the Bheidol Valley. 



The area of 40 square miles or so included within these 

 boundaries is given over almost entirely to sheep-grazing, and only 

 small tracts along some of the valleys are under cultivation. In 

 former times mining operations were vigorously carried on, but that 

 industry is now decadent. 



The small village of Pont Erwyd stands in the Bheidol Valley 

 near the western margin of the district, and on the main road from 

 Aberystwyth to Llanidloes which divides the area into two almost 

 equal portions. 



Begarded broadly, the district forms part of the elevated 

 * plateau ' which is characteristic of so much of Central Wales ; 

 south of the Aberystwyth road it reaches a nearly uniform level 

 of about 1900 feet, but north of the road it rises gradually to 

 2468 feet on Pen Plynlimon Fawr — the culminating point of the 

 plateau. The numerous valleys which intersect the tract are deep, 

 steep-sided, and characteristically devoid of trees, which are almost 

 confined to the gorge-like valleys of the Bheidol and the Ystwyth. 

 This general absence of trees and also, over large areas, of gorse 



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