174 STRATIGRAPHICAL GEOLOGY 



and these cannot be far below the horizon of the Aymestry lime- 

 stone. This zone is probably to be found in the banded beds (No. 5) 

 of the Clyvvd valley section, but has not yet been detected there. 

 In South Denbigh the highest beds occur at Dinas Bran near Llan- 

 gollen, and have yielded Orthoceras Sedgwicki, Cucullela coarctata, 

 Dayia navicula, Chonetes minima, and other fossils. 



The series is believed to be over 5000 feet thick, and is 

 nevertheless incomplete owing to the unconformable superposition 

 of the Carboniferous rocks. 



Downtonian. In the country round Kington, Clun, and 

 Eadnor Forest these graptolitic shales are succeeded by a great 

 thickness of brownish sandy shale with beds of brown flagstone and 

 sandstone, which are shown in the sections of the Geological 

 Survey to be from 1700 to 2000 feet thick. 



The highest beds in Denbighshire, i.e. the sandstones and 

 mudstones of Moel Ganol, may represent the Upper Ludlow shales, 

 but proof of such equivalence has not yet been found. That beds 

 of Ludlow age did originally extend over Denbigh is proved by 

 the occurrence of fossiliferous pebbles in the conglomerate at the 

 base of the Carboniferous Series near Abergele. 13 These are 

 pebbles of green and red sandstone containing Orthis lunata, 0. 

 filosa, Chonetes striatella, Spirifer elevatus, Pterincea retroflexa, and a 

 few other species. 



3. Westmoreland 



The Silurian sequence in the north of England does not differ 

 greatly from that of North Wales, but it is much more complete, 

 has been more fully investigated, and the beds are also rather 

 more fossiliferous, so that they are more easily subdivided. For 

 a long time the correlation of the Westmoreland Groups with those 

 of Shropshire and Radnor was very uncertain, but the recent studies 

 of graptolites by Messrs. Marr and Nicholson, 14 by Miss G. Elles, 15 

 and by Misses Watney and Welch 16 have placed the comparison on 

 a sounder basis. 



Valentian. Where the sections are most complete, as in 

 Stockdale and at Spengill, there are no arenaceous beds of the 

 Llandovery type, but a series of shaly mudstones resting directly 

 on the Ashgill shales with apparent conformity (see Fig. 37). 

 In some parts of the area, however, the lowest shales are replaced 

 by a few feet of gritty conglomerate. The ordinary facies is 

 known as the Stockdale Shales ; their total thickness is not 

 more than 250 feet, but divisible into two sub-groups : (1) the 

 Skelgill Beds, (2) the Browgill Beds. By the Geological Survey 



