60 SECTIONAL ADDRESSES. 



erosion, but how far up the slope the Trias may have extended it is 

 impossible to say. The thickness of that formation is variable, but even 

 after making allowance for a slight rise of the base of the Keuper north- 

 ward, it seems hardly likely that the thickness of the formation at that 

 point would be sufficient to carry even the youngest beds of the formation 

 to the summit of the Pennant scarp. The Triassic feature nearly coincides 

 with the existing escarpment along the south side of the coalfield, but east- 

 ward of Llanharan the latter crosses the Palaeozoic strata from the 

 Pennant Sandstone until ultimately it coincides with the base of a group 

 of sandstones and conglomerates forming the upper part of the Old Red 

 Sandstone. From the foot of this feature the summit levels of the hills 

 slope gently down towards the base of the Trias north of Cardiff, or 

 towards the Alluvial flats between Cardiff and Newport, which are believed 

 to be underlain by that formation. North-east of Cardiii the main 

 escarpment swings, however, in a northerly direction towards Pontypool, 

 and its course shows no relation to the trend of the base of the Trias. 

 Between Newport and Chepstow there is, however, a sharp rise of ground 

 which pursues a slightly undulating course across the Old Red Sandstone 

 and Carboniferous Limestone, and thence across another Old Red 

 Sandstone area between Chepstow and the southern end of the Forest of 

 Dean. This feature stands in sharp contrast to the gently sloping plain 

 to the south, and appears to be independent of the nature of the rocks. To 

 the south of it the Triassic rocks are never far away, and it is not unlikely 

 that here again, as at Llanharan, there is preserved a line of cliffs on the 

 Trias plain of denudation, though in all probability somewhat modified 

 by subsequent erosion. It may be that only a part of the feature is of 

 Triassic origin, the remainder having been developed subsequently by the 

 differential erosion of strata of varying resistances. 



Strahan appears to have held the opinion that the main escarpment 

 south and east of the coalfield was in existence in Triassic times, and that 

 the Trias deposits had covered at least that part of it which is occupied 

 by the Carboniferous Limestone. ' The main limestone throughout the 

 greater part of its outcrop west of Pontypool shows the red staining 

 characteristic of rocks which have been overspread by the Trias. In 

 other districts also both in South Wales and elsewhere this alteration of 

 limestone into iron-ore shows an obvious connection with the present or 

 past distribution of the Trias. We know that the Trias steadily overlaps 

 the Old Red and Lower Carboniferous rocks between Newport and Llan- 

 trisant, until it comes to rest on Coal Measures, and we may reasonably 

 suppose that at no distant date (geologically speaking) it extended over 

 all those parts of the limestone where the ore and the red staining occur.'* 



The view implied in the above quotation is that the iron which is 

 responsible for the reddening of the limestone and for the development of 

 ■ iron-ore bodies by the replacement of that rock was obtained from super- 

 incumbent iron-charged Triassic sediments. 



A similar view is expressed by Dr. T. F. Sibly in explanation of the 

 iron-ore deposits in the Forest of Dean and in South Wales, as the following 

 quotation shows^ : — 



* The Geology of the Country around Newport. 2nd ed., p. 23. 

 Iron Ores of the Forest of Dean and S. Wales. 2nd ed., 1927, p. 88. 



