﻿Vol. 6 1." 



CONTIGUOUS DEPOSITS OF GLAMORGANSHIRE, 



415 



In the neighbourhood of Bridgend, a considerable deposit of sand- 

 stone was made while black sediment was being laid down around 

 Lavernock. The very different nature of these contemporaneous 

 deposits is to be accounted for, mainly by the fact that there was 

 only imperfect connection between the two areas of deposition : 

 land, composed of Palaeozoic rocks, intervened. 



There is evidence (which will be quoted shortly), however, to 

 show that near the commencement of the Upper Rhaetic ' Age ' the 

 deposits in the Lavernock area were elevated, and a stretch of water 

 separated-off to the north. Theoretically, this stretch of water may 

 be regarded as having been connected with that in which the 

 Upper Rhaetic beds now visible at Bishton, near Newport, 1 were 

 deposited. This view explains the similarity noticed between the 

 beds distinguished as 3 at the Quarella Quarry, Bridgend, and 

 Bishton, and likewise between tbe remaining similarly -notated 

 deposits. 



The effects of these earth-movements were not merely local ; by 

 the elevation of certain areas, shallow lagoons were formed in 

 others, initiating conditions suitable for the existence of Esiheria. 

 While the Upper Rhaetic Beds 3 to 1 were in the course of 

 formation, the Lavernock area was above sea-level; the marl-deposit 

 (4) was subjected to subaerial denudation and somewhat fissured 

 (fig. 4, p. 412). 



VI. On the White Lias of Glamorganshire, and on the 

 Strati graphical Position of the White Lias. 



A gradual subsidence in the Lavernock area, accompanied by the 

 reverse movement to the north, allowed again of deposition, and 

 into the fissured marls (4) gritty material was washed. The 

 relations of the White Lias and the Upper Rhaetic Beds may be 

 represented as shown in fig. 5, below. 



Pig. 5. — Diagrammatic section shoiving the relationship of the 

 White Lias to the Upper Rhcetic in the Cardiff district. 



S.W, 



Goiaciiff 



near Kfswport 



N.E . 



In the Bath district, the sea in which the well-known White 

 Lias was deposited was deeper than at Lavernock. At the latter 

 locality, the various lamellibranchs are usually found grouped 

 upon the surfaces of the limestone-bands : Plicatula intus-striata, 



1 Quart. Journ. Geol. Soc. vol. lxi (1905) pp. 377-78. 



