]):irt L* cAinjoMi Kijuis i.i.MKsinM; or s. walks coamielI). 117 



Itsendo-concretioiuiry strueturos. ami sumo Vtamls of rliiniistone-limt'stone. the 

 latter containing' seams erowdeil with Seinintihi Jioiules. 



In the disturbed ground adjacent to the Creigiau Fault, the Seniiniiln Oolite 

 is fotuid to be much ironstained and almost wholly vein-dolomitized. as shown 

 a little west of the fault by a railway-rutting a third of a mile north of 

 C'reig'iau Station, and on the easteni side of the fault bj- cmgs on the steep 

 hillside lielow Coed-y-Creigiau. The highest beds of Creigiau C^tiarry, much 

 fault-shattere<l and dolomitized, probably l>elong to the bas(f of the St^niiindn 

 Oolite, 



Half a mile north-east of Creigiau an old (juan-y ((Uam. '.U\ SE \ 7) lying 

 loo yards north-north-west of Pen-Llw\ni exposes highly f(tssiliferous beds (jf 

 grey oolite and oolitic limestone, overlain by dark limestones with liciiutifully 

 ileveloped pseudo-concretioiuiry stnictiuvs. This section appear- to lie 

 within the Settnniila Oolite. 



The Modinia phase of S., is evidencetl by numy small exposures along the 

 outcnip extending from the Creigiau Fault to Pen-y-garn and farther east. The 

 most easterly exposures occur at the roadside 500 to 90(> yar<ls east-north-east 

 of Capel-Horeb. Pen-y-garn: they show (politic and compact noilular lime- 

 sttmes. with aV»undant Semiiinhi. 



In the Taff valley. Cwane (ilas (Cilam. 30 SE X 3), on the steep north- 

 eastern slope of Garth Wood (PI, XIII. tig. 1). is a long-disused, pictiu'esijue 

 tjuarry in the Seininidii Oolite: vein-dolomitization is very extensive here. 



An old quarry in the woods 450 yards south-west of Cwarre (tIjis shows 

 dark-grey tinely -oolitic limestones overlying coarse oolite. This probably 

 represents the toj) of the Semintiht Oolite. 



Below the Setttiinilii Oolite of Cwarre Glas lies the monotonous series of 

 crystalline «lolomites which form the bulk of the Main Limestone in the 

 Taff valley. On the e^^<lence of fossils fountl on the opposite side of the 

 gorge (p. 150) the topmost portion of this dohjmite .series is of S_. age. 



Losvcr l)ihuii()j)hiilliiin Zone : D,. 



Tlu' 1k'(Is assigned U) this siil)/.oiii' contain a clianiotcristic 

 asseinhhii^c of corals witli Dihitintphi/llifin 6, Carcliioitlnfllinn 

 V(fi((/Ji(itu\ and ('i/dfliophj/lliini iiiiiri'hisnni as es.st'ntial nicnilHTs. 



The I)ihuin>i,lii/llinii iH'ds are aj))»arently conlined to tliat part 

 of tlie district whii-li lies west of the Klv river. They have not 

 been traced farther east than the u'l'oond innnediately nortli of 

 Brynsa<ldl«'r. in the Kly valley, and in all probability thrir base is 

 overstej)]ied by the Millstone (Jrit in tin- drift-covered area north 

 of Hrynsiddler. 



Aronnd lintbin. in tlw extreme west oF the district here described, 

 j)ersistent northi-rly dips over a wide outcrop indicat*' a thit-kni'ss of 

 about (KM) feet of linu'stone above the Sriniunln Zone, before the 

 Carl»)niferous Limestone finally dis^ippears beneath the Keu)>er on 

 the north. Of this thickness, about l'>0 feet can be assit>:ned to 1)^ 

 on conclusive faunal evidence : hii^her be<is in the Kuthin ouU"ro|> 

 may bclouLT either to D^ or to 1),, but the fossils which have been 

 yii'ldcd by the nn-agre exposiires of these l>ed.>N are of no zonal value. 



Lithol«»i;y. Chit'tly ijfrey crinoidal and shelly limestones, 

 often highly crystallini". and occasionally «)olitic. A characteristic 

 feature in this district, as in (iowtr and elsewhere, is the extoiisUe 

 development of pseudo-brecciate<l structures ^ in ti»e linu'stones. 



' First describefl by Mr. K. H. i'iddeman, • The Country around Swansea' 

 Mem. G<>ol. Siirv. I!»n7. p. lo. 



