58 SECTIONAL ADDRESSES. 



that I should deal with this subject at a meeting of the British Associatioa 

 in the City of Bristol, since the position and prosperity of the City is s» 

 intimately dependent upon this great waterway. 



The Triassic Planation. 



Probably no single episode has contributed so materially to the shaping: 

 of the surface in this region as the intense erosion which succeeded the Post- 

 Carboniferous or Armorican earth movements. Prior to these movements 

 there had been a long period of sedimentation, which resulted in the accu- 

 mulation of thousands of feet of Carboniferous and Devonian or Old Red 

 Sandstone strata over the southern part of the British Isles. Mountain- 

 building movements beginning in late-Carboniferous times folded these 

 strata into great anticlines and synclines, the axes of which trend nearly 

 east and west from South Wales and the west of England to the south- 

 west of Ireland. Intense erosion of the uplifted areas followed in the New 

 Red Sandstone period, and it is the generally accepted opinion of geologists 

 that this erosion which furnished the materials and determined the charac- 

 ters of the Permian and Trias (or New Red Sandstone) occurred under arid 

 continental conditions. The relation of the New Red Sandstone to the 

 Palaeozoic rocks proves that along the axes of greatest upheaval the Car- 

 boniferous and Old Red Sandstone formations had been completely 

 removed before the close of that era. Indeed, in the neighbourhood of 

 Cardiff the underlying Ludlow rocks had been eroded, and New Red 

 Sandstone laid down in contact with the Wenlock formation. 



The Palaeozoic rocks are exposed in the core of an anticline which ranges 

 nearly east and west from Penylan north of Cardifi through Llandaff 

 towards the neighbourhood of Cowbridge. The thickness of the Old Red 

 Sandstone in the north flank of this anticline has been estimated at 3,500 

 feet. The succeeding Carboniferous Limestone which occurs in both flanks 

 of the fold varies in thickness from 880 feet to over 2,000 feet on the north 

 side, and reaches on the south a thickness of 2,750 feet. It is, therefore, 

 unlikely that less than 2,000 feet of limestones occurred along the axis of 

 the anticline. With regard to the Upper Carboniferous, there is some 

 uncertainty. Farther west, in the South Wales Basin, the combined 

 thickness of the Millstone Grit and Coal Measures is between 7,000 and 

 8,000 feet, but towards the east, and especially at the south-east margin 

 of the coalfield, these formations are greatly attenuated. We can reason- 

 ably assume, however, that about 2,500 feet of Upper Carboniferous strata 

 overlay the Limestone. These figures show that at least 8,000 feet of Upper 

 Palaeozoic rocks had been removed from the region before the deposition 

 of the Keuper division of the New Red Sandstone which rests upon the 

 eroded edges of the older rocks. Strahan's estimate^ that a thickness not 

 far short of 7,000 feet had been removed from the crest of the anticline 

 before the Keuper was laid down is, in my opinion, unduly conservative, 

 and it may well have been exceeded by 2,000-3,000 feet. Even the lower 

 figure is sufficient, however, to impress upon the mind the effectiveness of 

 erosion under the arid climatic conditions that prevailed during the New 

 Red Sandstone period. 



' The country around Cardiff, p. 39, Mem. Geol. Survey. 



I 



