e2 SECTIONAL ADDRESSES. 



was formed. Similar waters entering the area in which the Trias was 

 deposited would furnish the necessary iron content to those sediments. 



If the only interest of the relation of the escarpment bordering the 

 South Wales Coalfield to the former distribution of the Trias, and to 

 Triassic erosion, was its bearing upon the mode of origin of the iron-ores 

 of the region, I would not have referred to it on this occasion. But a 

 much wider question is involved, affecting possibly the origin of the 

 physical features of a large part of Wales and the south-west of England. 

 If the escarpments bordering the south side of the South Wales Coalfield 

 and the Forest of Dean are to be regarded as in great part of Triassic age, 

 may not other comparable features in Wales be attributed to the same age 

 and conditions of erosion ? 



The former extension of the Trias to the west of the Vale of Glamorgan 

 is proved by a small area which occurs near Port Eynon on the south coast 

 of Gower. This peninsula is in the main a plateau at an elevation of about 

 400 feet, and it is clear that in places at any rate the pre-existing rocks 

 had been eroded almost to their present level before the close of the Trias. 

 The escarpment which is so prominent on the borders of the Vale of 

 Glamorgan is, however, not readily identifiable in Gower, though it may 

 be represented there. Still more remarkable evidence comes from South 

 Pembrokeshire, where Mr. B. E. L. Dixon has described deposits having 

 the aspect of typical Keuper marls which occur in great masses of breccia 

 in the Carboniferous Limestone cliffs.® These Gash breccias, as they have 

 been called, appear to be masses of limestone which collapsed into sub- 

 terranean caves during the Trias (probably Keuper) period. The region 

 in which they occur forms at the present time part of an extensive plateau, 

 but it is clear that Triassic deposits formerly occurred at some unknown 

 distance above the present surface. From the fact that the Gash breccias 

 are truncated at the top of the cliff in such a way as to indicate that they 

 formerly continued to a higher level, Mr. Dixon is of the opinion that the 

 old land surface during the formation of the breccia gashes lay at a con- 

 siderable height above the present, and he attributes the deposition of 

 the red marls to a slightly later period, possibly following some depression 

 of the area. It is clear, however, that the greater part of the denudation 

 which removed an enormous thickness of Carboniferous and other rocks 

 from that area subsequently to the Armorican folding had already been 

 accomplished before the close of the Triassic period. Mr. Dixon remarks 

 that although ' the Triassic floor has undergone some later planation this 

 has merely touched up the work of the earlier erosion ' {op. cit., p. 162). 



In South Pembrokeshire the plateau rises on the whole northward ; in 

 that direction various formations ranging from the Old Ked Sandstone to 

 pre-Cambrian gneisses and volcanic rocks occupy its surface. The whole 

 plateau appears to form one continuous feature, and it is not improbable 

 that, as in the southern part of the county, most of the degradation which 

 the greatly folded rocks have suffered occurred as the result of erosion 

 during the New Red Sandstone period. 



In North Pembrokeshire certain hill masses, such as the Prescelly 

 Range, stand conspicuously above the general level of the plateau, and 



^ The Geology of the Country around Pembroke and Tenby. Mem. Oeol. Survey, 

 1921, p. 158. 



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