80 SECTIONAL ADDRESSES. 



infers from it that the limit of the Eocene basin was not far distant [from 

 Dorset], and that it seemed scarcely probable that Eocene sediments 

 extended over South Wales. The rapid change eastward in the characters- 

 of the Eocene strata of the London basin and the Hampshire basin suggests 

 that they were deposited off a land area lying to the west. 



There appears to be independent evidence of a tilt in a southerly or 

 south-westerly direction in Central Wales. In the northern part of 

 Cardiganshire a well-defined ' Coastal Plateau ' slopes gently from about 

 400 feet at the coast to about 800-900 feet at its inner edge, where the 

 surface rises rapidly to a height of about 1,900 feet in the ' High Plateau.' 

 The elevations of Plynlimon, Cader Idris and others to which I have 

 preAaously referred stand sharply out of the higher plateau. Farther 

 south in Central Wales the Coastal Plateau persists at about the same 

 level, but the step between it and the High Plateau diminishes in height, 

 since the surface of that plateau declines southwards. In South Cardigan- 

 shire, Carmarthenshire and North Pembrokeshire, where the two plateaus 

 appear to have become merged into one, the Prescelly Range and other 

 hills rise above its surface in the same manner as the hill masses farther 

 north. Comparing the features of South-west Wales with those of North 

 Central Wales, it would apjaear that the Prescelly range and other 

 eminences in Pembrokeshire should be correlated with the summit 

 masses of Cader Idris and Plynlimon, and the surrounding plateau with 

 the High Plateau of Central Wales. Such a correlation implies that the 

 surface of the High Plateau has been warped down in a southerly 

 direction, so that it has descended to the level of the Coastal Plateau. 

 The mutual relations of the two features suggests that this warping 

 had occurred before the latter was eroded. The amount of warping is 

 about 1,900 feet in a distance of 80 miles, or about 24 feet per mile on 

 the average, but the slope appears to be relatively greater in the south 

 than in the north. 



A deformation of this amount would increase profoundly the erosive 

 power of any stream that flowed in the direction of the tilt, and I believe 

 it affords the most reasonable explanation of the great development of 

 the south-westerly flowing streams in parts of South Wales. In the 

 eastern part of the Principality, where the valleys traverse the upland 

 area of the coalfield which stands well above the surface of the High 

 Plateau, the effect of the south-westerly tilt is imperceptible, but it appears 

 to become more effective as the level of the upland in which the valleys 

 are carved diminishes in height to the south and west. It may be that 

 in the eastern region the streams were too deeply incised in the surface 

 to suffer diversion when the tilting movement occurred. 



On the assumption that the High Plateau has been warped since its 

 formation, its present form indicates that it was domed along a broad 

 area extending eastwards from North Central Wales. If the direction of 

 this dome is prolonged it meets the axis of elevation that has caused the 

 great swing in the strike of the Cretaceous rocks in the Fen country. 

 Moreover the transverse watershed in the Midlands that divides the 

 southerly streams of the Warwickshire Avon drainage from the northerly 

 streams of the Trent system lies approximately on the same line. 



It is not impossible, however, that the present slope of the surface is 



