132 ME. F. DIXEY AXD PEOF. T. F. SIBLT OX THE [vol. Ixxiii, 



West of the Taff. 



In the district west of the Taff valley the Lower Limestone 

 Shales are, to a great extent, concealed bv Boulder Clay and 

 G-lacial s^ravel, but the three divisions established east of the 

 Taf£ can be recognized. The development of shale in the lowest 

 division appears to increase considerably as Ave pass westwards. 

 This phenomenon continues the change already noticed in the 

 outcrop east of the Taff : it was recognized by the officers of the 

 Greologieal Surve}" on the evidence of exposures near Bolgoed.^ 

 west of the Ely valley, and expressed on the Survey map (Bridgend 

 sheet) bv the representation of three divisions in the Lower 

 Limestone Shales Avest of Grroes-faen, namely, lower and upper 

 shale-divisions separated by a band of limestone. The thickness 

 of the Avhole series appears to increase westwards, but the limestones 

 which form the middle member do not contribute to this increase ; 

 probably their lowest beds are split up bv shales in a westerly 

 direction, and so become merged in the underlying group of shales 

 and limestones. 



Between the Taff and the Ely, a distance of about 5 miles, 

 outcrops of the Lower Limestone Shales are practically confined 

 to two strips, each about a mile and a quarter long ; one 

 lying south and south-west of PentATch, the other extending 

 through G-roes-faen to the neighboui'hood of Croftau. At Creigiau, 

 a coA^er of Mesozoic rocks conceals the beds, but elsewhere the 

 maskino- is due to Glacial drift. The beds do not crop out along 

 the margin of Gartli Wood, on the Avestern side of the Taff A^alley, 

 as represented on the Geological Survey map (Cardiff" sheet) : 

 they lie buried under drift immediately to the south, and the scarp- 

 face of Garth Wood belongs Avholly to the Main Limestone. At 

 Groes-faen, the middle and upper members of the Lower 

 Limestone Shales produce landscape features similar to those 

 Avhieh are so conspicuous in the country east of the Taff,. but 

 less pronounced. The limestone grouj) giA^es rise to a Ioav, Avooded 

 ridge, to the north of Avhich lies a shallow strike- A^alley, or a 

 grassy fiat, marking the outcrop of the shales. The outcrops 

 and their accompanying features are displaced by dip-faults 

 immediately Avest of Groes-faen : farther Avest, the features die out 

 as the beds pass under a coA^ering of drift. 



West of the Ely the drift-coA-er is CA'en more extensive. The 

 beds crop out for a short distance at Caer-gAA'anaf-isaf, immediately 

 AA'est of the Ely valley and north of Hensol Park, and are seen 

 again over a small area betAA^een YstradoAven and Llansannor ; 

 between these two localities, their extent is almost entirely 

 conjectural." 



1. Limestones and shales. — The lower group is noAvhere 

 exposed between the Taff and the Ely. West of the Ely, and 



^ Bridg-end Memoir, p. 9. 



- Ibid. pp. 7 & 9. and Bridgend map. drift edition. 



