72 SECTIONAL ADDRESSES. 



Between Cowbridge and Bridgend, however, there are large tracts where 

 Lias rests directly upon Old Red Sandstone or Carboniferous Limestone. 

 I have attempted to estimate at numerous points the approxiniate present 

 level of the Lias-Rhaetic junction, but where the Lias overlaps the Rhaetic 

 and comes to rest upon the Palaeozoic rocks it is no longer possible to do so, 

 since in all probability there is overlap in the Lias itself, in which case the 

 level corresponding to the base of the Lias must be lower than the altitude 

 at which that formation rests upon the Palaeozoic. Thus, supposing the 

 Lias rests upon the Carboniferous Limestone at a height of 300 feet above 

 O.D., and that there is an overlap of 100 feet in the Lias ; the level of the 

 plane of the Lias-Rhaetic junction, if no overlap had occurred, would at 

 that point be 100 feet lower. 



On the other hand, it may, I think, be assumed that wherever Rhaetic beds 

 were deposited they were followed by Lias. We can therefore make an 

 estimate of the level of the base of the Lias in j^laces where that formation 

 has been removed, but the Rhaetic remains, and a less close approximation 

 to the former level can be obtained in places where the Keuper alone 

 survives. In the area west of Cowbridge there is too much uncertainty as to 

 the amount of overlap in the Lias to make the attempt worth while. Around 

 Cowbridge, the Lias-Rhaetic jimction rises in places to over 400 feet, and 

 then falls northward to about 100 feet, while southward this junction must 

 be well below sea level. In a section drawn northward from the coast 

 through the Cowbridge area, the base of the Lias thus rises to a crest line 

 near the latter place, and then descends northward about 300 feet. Still 

 farther north the Lias has been eroded away, but the Keuper surface rises 

 in that direction, and it is certain that before the present northern edge 

 of the Mesozoic is reached the base of the Lias must have attained an eleva- 

 tion of well over 400 feet. The anticlinal axis can be traced from Cow- 

 bridge eastward in the direction of the Leckwith Hills, north-west of Cardiff, 

 where the maximum elevation is about 200 feet, but the anticlinal structure 

 is not evident in those hills. Westward the continuation of the axis is 

 indefinite, but it appears to range towards Bridgend, where the Rhaetic- 

 Lias junction stands at over 300 feet. The anticline is probably not a 

 simple structure but consists of two, if not of three, short axes lying en 

 echelon, each axis going west being situated a little to the north of the 

 one to the east. The depressed area to the north runs somewhat indefinitely 

 from Llandaff to north of Bridgend, i.e. roughly parallel to the anticline 

 and from two to four miles to the northward of it (see Plate I). 



If, by applying the accurate zonal work of Trueman on the Liassic 

 rocks of the region more precise determinations were made of the horizon 

 of the Lias in contact with the Palaeozoic rocks, it would be possible to 

 trace the structures westward in greater detail than can be done with the 

 available data. 



The Mendip Region. — In the Mendips the Lias-Rhaetic junction 

 reaches a height of at least 850 feet above O.D. south of East Hampshire, 

 whence it descends both to the north and to the south. It is true that 

 the local base of the Lias attains even greater elevations, but at such 

 points the Rhaetic has been overstepped, and it is probable that owing to 

 overlap the beds resting on the Palaeozoic rocks are not those of the basal 

 zone. A careful survey of the Lias should afford evidence of the extent 



