C— GEOLOGY. 65 



and Wales. Some indirect evidence of the probable former extent of those 

 formations can be gathered from a study of their lithological characters, 

 and more particularly from the variation in their thickness which takes 

 place from the outcrop eastward. When Ramsay and Hull investigated 

 the problem, the only evidence available was that obtained from the study 

 of the rocks at their outcrop, but within recent years it has been supple- 

 mented by the information obtained from various deep boreholes which 

 have penetrated a part or all of the Mesozoic cover. Much yet remains 

 to be done before the evidence afforded by the Mesozoic rocks in regard 

 to their former extension can be fully utilised. The work of Buckman, 

 Trueman, Richardson and others on the zonal development of the early 

 Jurassic rocks points the way to further studies in the same direction. 



In the Vale of Glamorgan the Lias in places follows conformably on the 

 Rhaetic ; in other places it overlaps on to the Carbomferous Limestone or 

 older rocks, and then assumes a littoral type consisting either of conglomer- 

 ates with limestone pebbles or of oolites. It is remarkable, however, that 

 no pebbles of Coal Measure rocks occur in the conglomerates, although 

 during the Trias the escarpment composed of these strata overlooked the 

 area of sedimentation at a distance of only a few miles to the north. No 

 evidence of littoral conditions attributable to that escarpment has been 

 obtained in the Lias outcrops nearest to it, and Strahan' remarks that 

 ' the absence of marginal types of limestone (in the northern part of 

 the vale) suggests that the shore lay a good deal farther north, though at 

 the same time it is hardly probable that the Lias overlapped the Pennant 

 scarp.' This conclusion depends, however, on the extent to which the 

 upland rose at that time above the base of the escarpment, and upon the 

 thickness of the Trias in that region. If the escarpment was no higher 

 than the existing feature, then taking into account the rise of the base of 

 the Lias along the north side of the Vale, and the probability of some con- 

 siderable thickness of Keuper having been removed from that area, it is 

 possible that at least some of the Lias deposits would overtop the escarp- 

 ment and come to lie upon the rocks within the coalfield. On the other 

 side of the Bristol Channel, Woodward observed that although Palseozoic 

 rocks rise at present to a great height above the level of the Lias, the latter 

 shows no signs of littoral conditions, and he argues that the shore line must 

 have been some distance away. This could only have been possible if the 

 escarpment against which the Trias terminated was wholly or in great part 

 buried beneath that formation. A former extension of the Lias westward 

 into the Gower peninsula is indicated by the finding of ' several Liassic 

 oysters, allied to Ostrcea irregularis, from stalactites in the Carboniferous 

 Limestone at Mumbles.'^" 



In the underlying Rhaetic beds, however, definite changes of lithology 

 occur in a westerly direction. The normal shales and limestones of the 

 Cardiff district pass, near Bridgend and Pyle at the western end of the Vale 

 of Glamorgan, into coarse, impure sandstones, very similar to Millstone 

 Grit. In places intercalations of red and mottled marls, only distinguish- 

 able from Keuper marls in containing fossils, occur a few feet above the 



^ Mem. Oeol. Survey, ' The Country around Bridgend,' p. 59. 

 '" A. E. Trueman, The Liassic Rocks of Glamorgan. Proc. Oeol. Assoc., vol. xxxiii., 

 p. 278 (1922). 



1930 F 



