partli' ( AKHOMI Kl{t»rs I.IMKSTONK (»!• S. WALKS ( OA JJI K I.D. I •"• 1 



Tjie passafe'e int«. the uverlying <»<»lit<'^ i^ well exposed in the name euttiiiK. 

 No definite division-line can be drawn, because dolcmiitization has affected 

 the basal portion of the <K)lite very extensively, spreading not only ni veins 

 along joints, but also in nuisses which involve the body of the rock. 



The Bjvrrv Kailwav also affords a good section «.f the <.olites : but these 

 beds are ovt-n better displaved in Portobello (-iuiirry. which lies between the 

 two milwavs. and in the cuttings on the Cardiff Railway. They exhibit the 

 typical litholog^' and fauna of the S^niinnh, Oolite, although everywhere 

 veine.1 bv dolomite and calcite. In the northern portion of each railway- 

 secti.m tlie beds are much shatter- jointed and traversal by occasional 

 faults, iirobably thrusts. The following crals and bmchiopods have been 

 reconletl : 



Sijriityopora .sp. Ctmiiiia sp. ; rare. 



Utho^imiion martini. Pmdurtiig mrnigntu -ht^mtsjiherir,,^ ; 



Lithonf ration (yematnphylhnn) common. 



minns (lower beds only) ; rare. Papilionaceous Chrmetex ; common. 



Cn,Tim>i>hijUum sp. St-minnla ficoulcs; abundant. 



Northwards, the section (»f Carboniferous Limesttmc terminates in the 

 Snuinula Oolite on both railways. On the Cardiff Railway, limestone 

 extends to the end of the cutting, where the ground falls abruptly to an 

 alluvial terrace. On the Barry Railway, the limestone-section terminates 

 within the cutting, at the point where a bridge crosses, and the rest of the 

 cutting lies in Glacial gravel. Glacial «lrift probably covers about lOO feet of 

 S. beds at the top of the Main Limestone in this locality. The northernmost 

 exposure of limestcme is a small outcrop in a lane immediately east of the Barry 

 Railway, 65 yards south of Ty-rhiw.' The rock is an oolitic limestone containing 

 commiuuted shell. 



The Main Limestone cast of tlie Tiitl' -oi-e. 



As explained in the t^eneral aeeount of the sueees.sion (§ H , 

 pp. 122 2:i I and iUusti-iited hy tlie vertieal seetiniis (in^. 2, ]). 1 19). 

 the Main Limestone, when tnieed nortli-east wards from the Tuff 

 vallev, underjrous three important elianu^es, namely {I) the loss 

 of H] in conse(iuenee of overstej) by tlie Millstone (irit, (2) the 

 attenuation of the surviving zones, and (3) the i^'reat develo])nn'nt 

 of Mo(f/oht-\)h'diio deiK)sits in C-S,. 



Ahundanee of exi)osures enables us to establish the suceessiim of 

 l:>eds in eonsiderable detail at Thornhill, and again in the vicinity 

 of Cefn-On Farm, 1 \ miles east-north-east of Thornhill. There is 

 little ehani^e between thesr jMjints. The Cefn-On secjuenee, which 

 is illustrated bv a horizontal .section (hg. L p. 120) and by a vertical 

 section in tig. 2 ( j). 11!) ), may be tiibulated to ].rovide a basis for the 

 descrijition of the Mam LimWone between thoTaff and the Kbbw. 



The Main Limestone of Cefn-On. 



bWt. 

 (iii) S . (basal beds of ). Dohmiites with a Imnd of oolite 70 



r * rM(M/i«»^f phase (S,) ^^ 



Crvstalline dolomite and dolomitio limestone 



(")i^^+«.- i avs.) 100 



I^C, (part of) iModiola phase (C,-<J«) '^^^ 



(i) I ^' (P^^ *" ) \ Dolomites, with some ehert near the bas«' 45(> 



- Total i>20 



' There is no exposure at the |M)int, 25 yards south-west of Ty-rhiw. 

 marked by a dip-arrow on the tJ-inch Geological Survey map (Glamorgan, 

 'M SW). 



