part 2] PALiEOZOIC EOCKS of the LLAlSTGOLLElSr DISTEICT. 179 



the Vale of Clwyd forms a great sunken grab en nosing south- 

 wards into the upland. 



The chief structural units of the district are shown in the out- 

 line-map (fig. 1), and the general arrangement of the formations 

 in the map forming PL V. 



(B) Ordovician. — The Bala Formation. 



In the three anticlinal areas : namely, the Berwyns, Cyrn-y- 

 Brain, and Mynydd-Cricor, rocks referable to the Ashgillian Series 

 of the Bala Formation are found, while in the Berwyns the under- 

 lying Caradocian Series also comes to the surface. 



The sequence of the complete Bala Formation in the Glyn- 

 Ceiriog district at the eastern end, and in the Moel-Fferna district 

 at the western end, of the Berwyn outcrop is set out in Table I, 

 pp. 180-81. The Ashgillian development of the Bala in the two 

 northern anticlines is also shown. 



Where it has been found practicable, the names instituted by 

 Dr. Grroom & Mr. Lake (op. cit. 1908) have been adopted ; but it 

 was found, on mapping still farther Avest, that their ' Graptolitic 

 Slates ' embraced two different formations of importance, to which 

 the names ' Blaen-y-cwm Beds ' and ' Ty'n-y-t\vmpath Beds ' have 

 been assigned. 



Caradocian Series. 



(1 & 2) Teirw Beds and Cwm-Clwyd Ash. — In considering 

 the Caradocian Series in the Northern Berwyns, it is unnecessary 

 to redescribe the Teirw Beds in the Glyn area, so fully are they 

 dealt with by Dr. Groom & Mr. Lake ; but farther west appears a 

 thick ash-band which, on the Old Series map, was correlated with 

 the Pandy Ash. It lies, howevei", in the middle of the Teirw Beds 

 and may be called the Swch -Gorge Ash, since it is best exposed 

 in the gorge of the Ceiriog near Swch-cae-rhiw. 



The following is the sequence here : — 



r Pen-y-graig Ash. 

 Bryn Beds. < Fossiliferous mudstones. 



I Pandy Ash, seen at the top of the gorge. 



fMudstones and sandstones, with two thin ashes or agglo- 

 { merates. 



Teirw Bed s.<{ Swch- Gorge Ash in two bands, the lower containing rolled 



I pebbles of hard shale. 



1 



i^Sandstones and sandy shales, with a thin ash-band. 



Cwm-clwyd Ash in two bands. 



The age of the Teirw Beds is in doubt, because they appear to 

 contain a fauna intermediate between Llandeilian and Caradocian, 

 a fact hinted at by Dr. Groom & Mr. Lake {op. cif. 1908, pp. 589- 

 90). To the foi'ms enumerated by them {ibid. p. 559) we can 



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