112 A. Strahan — The Age of the Vale of Clwyd. 



were faulted down against the Silurian rocks all along tlie 

 eastern side of the Vale by a great compound fracture, to which 

 the name of "Vale of Clwyd Fault" seemed suitable, and were 

 repeatedly displaced on the western side of the Vale by a series of 

 what seemed to be branches fro,m that great fracture : — That the 

 Trias nowhere overlapped the whole of the Carboniferous, as formerly 

 supposed, but generally rested upon the purple strata; though it 

 frequently overlapped them and a considerable part of the limestone 

 also, it was never directly in contact with the Silurian rocks, except 

 where faulted against them : — That the faulting was certainly in 

 part post-Triassic, but that much of it had been accomplished in 

 pre-Triassic times. 



In the years 1897-98 Mr. Morton ^ published the results of 

 a detailed examination of the whole Vale, including those parts to 

 which the six-inch resui'vey had not extended, and added not a little 

 to what was already known in a district where the prevalence of 

 Drift and scarcity of sections makes every observation important. 

 He concluded that the faults were wholly post-Triassic, and remarks 

 that he " failed to find a satisfactory section showing the Trias 

 overlapping any portion of the Carboniferous Limestone or the 

 Purple Sandstone," and further on says that " the assumption that 

 the faults are partly of pre-Triassic age and partly post-Ti-iassio 

 seems to arise from a lingering faith in the old Map of 1850, and 



Carboniferous age, therefore, was no longer left in doubt, but there remained the 

 difficulty that they were quite unlike the rocks which lie next above the limestone in 

 East Flintshire. 



The East FKntshire Millstone Grit is of an abnormal type, for though in the 

 southern part of the county it consists of quartz-grits and conglomerates of the usual 

 character, towards the north it passes almost wholly into chert. With neither of 

 these types had the purple measures anything in common, but after some little search 

 I was able to match them exactly in some micaceous sandstones and shales which lie 

 between the Millstone Grit and Middle Coal-measures near Mold, and which may be 

 supposed to belong to the Lower Coal-measures of other regions. Subsequently, 

 when the mapping showed that the Millstone Grit locally thinned out in Mid-Flint- 

 shire (" Geology of Flint," etc., p. 53), leaving the supposed Lower Coal-measiu-es 

 in direct superposition to the limestone, I attached more importance to the lithological 

 affinities of the purple strata, and concluded that they also were probably younger 

 than the Millstone Grit (ibid., pp. 33, 69). 



Mr. Morton, however, states that "the Purple Sandstone and Shale, coloured as 

 Coal-measures on the Map, are really on the horizon of the Upper Black Limestone 

 of Prestatyn and the Arenaceous Limestone of Mold, subdivisions which do not belong 

 to the Coal-measures" (Proc. Liverpool Geol. Soc, 1897-8, p. 280). This is true 

 in the sense that they rest upon the " Upper Grey Limestone " of his classification, 

 but it is not the case that they represent or pass horizontally into the black limestones. 

 These latter are strictly local in their development, and thicken and thin rapidly in 

 East Flintshire. In South Flintshire they are represented by alternations of sandy 

 limestones and sandstones, and in a corresponding position in the south end of the 

 Vale of Clwyd strata of this character, though poorly developed, are seen dipping 

 beneath the purple beds. 



This poor development, the absence of the black limestone, and the local thinning 

 out of the Millstone Grit, may be taken with the general attenuation of the limestone 

 as indications of a Carboniferous shore-line at no great distance westwards, the only 

 evidence we possess that Snowdonia had commenced existence as an elevated region 

 in Carboniferous times. 



1 Proc. Liverpool Geol. Soc, 1897-8, pp. 32 and 381. 



