80 



SECTIONAL ADDRESSES. 



Swaledale Wensleydale 



Kettlewell 



Trollet's Gill 

 near BurnsaM 



Mirk Fell ■ 

 Main Li' ■■ 



Sitnonstone ■ 



HardrawScar 

 Girvanella Bed 



Fig. 4. 



Diagram showing Overstep of the Millstone Grit in the Yorkshire Dale and Craven 

 Country. (From information supplied by Mr. R. G. Hudson.) 



on the very base of Sj (Oefn On). This is uot, however, due to the coming 

 in of grit conditions at successively lower levels, but, as suggested by 

 Dixon ^^ and confirmed by Dixey and Sibly,** to overstep. Farther to 

 the north-east the overstep continues to still further reduce the exposed 

 thickness of the Carboniferous Limestone, till at the north-east corner of 

 the coalfield little, if anything, more than the K beds remains between 

 Millstone G-rit and Old Red Sandstone. This is the limit of the overstep, 

 as, when followed westward along the north crop, the Millstone Grit rapidly 

 retrogresses, with the result that all the zones up to S.^ reappear in a distance 

 of six miles, and farther west the D beds are seen. Still farther west, 

 however, overstep again sets in, Mr. T. N. George informing me that at 

 Penwyllt (Tawe Valley) the Upper Limestone Shales or rottenstones (D.^.^) 

 are cut out by overstep and the Millstone Grit rests directly on D.^. Finally 

 at a point about three miles east of Kidwelly it oversteps down to about 

 the base of D,^. 



At Pendine, to the west of Carmarthen Bay, it again rests on D,„ but 

 at Haverfordwest it rests on S.,. 



In the Forest of Dean there is no Millstone Grit seen, owing to the 

 overlap of the Coal Measures, which may overstep all the Avonian rocks 

 till they come to rest on the Old Red Sandstone.^^ The Coal Measures 

 play in the Forest of Dean the part which is played by the Millstone Grit 

 in the South Wales coalfield. 



North of England. 



In the Yorkshire dales it has long been known that the beds of the 

 Yoredale series immediately below the Millstone Grit disappear in 

 successioii southwards, the Grit thus appearing to become estalilished at 

 lower and lower horizons in that direction. The officers of the Survey, 

 especially Dakyns and Clough, realised this, but Goodchild*^ was the first 

 to show that it was due to unconformable overstep of the Millstone Grit, 

 and this fact has been fully established by later workers,*' by whom it has 

 been shown that the Millstone Grit of North- West Yorkshire rests in 



*^ Newport Memoir, 2nd ed., p. 20. 



*^ Q.J.G.S., Ixxiii. (1917), p. 167. 



« Sibly, Oeol. Mag., Dec, v., vol. ix. (1912), p. 421. 



** Victoria County History, Cumberland, Geology, 1901, p. 28. 



*' See L. J. Chubb and R. G. S. Hudson, ' The Nature of the Junction between the 

 Lower Carboniferous and the MiUstone Grit of North West Yorkshire,' Proc. Yorks 

 Oeol. Sac, n.s., vol. xx., pt. 2, 1925, p. 257 ; and L. H. Tonks, ' The Millstone Grit 

 and Yoredale Rocks of Nidderdale,' ibid. p. 226. 



