64 SECTIONAL ADDRESSES. 



extensive deposits of irregular strata whose composition and want of 

 fossils indicate their desert origin. . . . Various additional details are given, 

 with the conclusion as above quoted ; these rock-floored plains are not 

 uplifted peneplains, but are the product of desert erosion unrelated to 

 normal base-level in which occasional water action has co-operated with 

 more persistent wind action.' 



The great plateau of Wales is not, however, horizontal at the present 

 time, nor is the base of the masses, which might be regarded as residual, 

 everywhere at the same level. The possible reasons for these difEerences 

 will be dealt with later. In other respects, however, the features of the 

 region conform to a remarkable degree with Passarge's description. 



When we turn to the Mendip region we find features similar to those 

 of the Vale of Glamorgan — clifis eroded in Carboniferous Limestone against 

 which the Trias is banked up ; the cliffs were subsequently overwhelmed 

 by the sea and covered by Liassic deposits. South of the Channel also 

 many of the abrupt slopes around the Quantock Hills and south of Porlock 

 and Watchet are in most cases fringed, at no great distance, by Triassic 

 deposits, and it is not improbable that some of these steep slopes mark the 

 edge of a former desert plain out of which residual hills rose abruptly. As 

 in Wales, so in Devon an enormous amount of erosion of the Paleeozoic 

 rocks had occurred before the New Eed Sandstone was deposited. 



Whether or not it be the case that the great plateau of Wales is in the 

 main the product of erosion under the arid conditions of the New Red 

 Sandstone period, it is at least certain that on both sides of the Bristol 

 Channel the pre-existing surface was eroded into a broad depression 

 bounded by scarps trending generally in the direction of the present 

 channel. Against these scarps some depth of Triassic deposits was 

 banked up, but how far up the slopes the highest beds of the Trias may 

 have reached is at present uncertain. From the fact, however, that the 

 products of erosion during the earlier stages of the New Red Sandstone 

 were removed from the area to regions more favourable for the accumula- 

 tion of deposits, it may be inferred that the level of the surface was then 

 relatively higher than the basins of deposition. There is, therefore, no 

 evidence of a tectonic basin in the channel region at that period, but only 

 of a depression caused by erosion between the uplands which bounded it 

 on the north and on the south. 



The Formation of the Mesozoic Cover. 



At the close of the Triassic period there was a general subsidence of 

 the British region, and the sea invaded the area of Triassic sedimentation. 

 The enormous extent of the Rhaetic formation, considering its small thick- fl 

 ness, proves that before the marine invasion that area was almost a dead " 

 level plain, and it is probably safe to assume that marine sediments were 

 deposited where Triassic rocks are now found, as well as in those areas from 

 which the Trias has been removed by subsequent erosion. 



The problem of the former extent of the Mesozoic rocks has exercised 

 the minds of many geologists in the past, notably Ramsay, Hull, Strahan 

 and Lamplugh. Strahan and Ramsay in particular regarded the problem 

 from the point of view of the origin of the drainage systems of England 



