part 2] CAKHOMFKHOrs LIMKSTONE OF S. WALKS COALKILI.D. 1 1 •'> 



>tnu'tiirt',* s;ive f(»r tho proUibly significant fact that both the Taft' 

 and the Ely oross the Carboniferous Liniestune on lines of dip- 



The liniest«»ne ridi^'e attains an altitude of more than DOU feet 

 between the Ebbw and the Hhynmey, althougli it is there no 

 more than a feature on the south-eastern slope of Mynydd Maehen 

 (PI. XII, tig. 8). Strongly developed all the way from the 

 Hhymney to the Taff, it rises above SOU feet in Cefn-(3n ( PI. XII, 

 MiTs. I iV: 2) and Cefn Carnau. West of the Taff, it remains 

 sharply dertned. witli a steej) scarp-face, through Garth Wood 

 f PI. XIII, tig. 2) as far as Pentyrch; but it dies down at Creigiau, 

 and in the outcrop west of that place never approaches its eastern 

 development. 



* The striking development of the Main Limestone escarpment in 

 the eastern i)art of the district here described results from a remark- 

 able adjustment of minor drainage-lines to geologieal >trueture 

 in that ])art of the eoaUield margin. Tril)utaries of the Khynmey 

 and tlie Taff. selecting the outcrops (»f the less resistant binds, such 

 as the Lower Limestone Shale>, together with the topmost beds of 

 the Old Red Sandstone, and the shales which form the lower part 

 of the Millstone Grit, have entrenched themselves in strike-valleys 

 along those lines (see map. PI. XVI, and i)hotographs, PI. XII, 

 tigs. 1 Si 2). /Hetween the Ebbw valley and the Taff' valley, the 

 (piartz-coiii^lomenites which lie at the base of the Tpjier Series t»f 

 the Old Red Sandstone have determined a bold escarpment which 

 runs parallel to the limestone ridge. This is magnitieently deve- 

 lo])ed in Craig Lysfaen, Craig Llanishen, and Coed-y-\Venallt, 

 between the Khymney and the Taff". The most notable of the 

 strike-valleys mentioned above is Cwm Draethen ( PI. Xll, fig. 2), 

 which separates Craig Lysfaen and Craig Ku)>erra from the lime- 

 stone ri«lge on the north. In this valley the Draethen brook 

 (Xant-y-Draethen ), rising on the dip-slope of Craig Lysfatn, 

 f(dlows the b«nmdary of the Old Red Sandstone and the Lower 



Limestone Shales for about 8 miles, in its rapid descent to the 

 Rhymney at Draethen. In the upper part of Cwm Dr.iethen, as 

 in several other portions of the outci-op, the valley is relii'ved by a 

 low ridge formed by a group of limesti»nes in the Lower Limest<»ne 

 Shales. This feature, illustnited in PI. XII, figs. 1 i^ 2, is further 

 • leseribed on pp. 125 2(5. 



Within the Main Limestone itself, variations of lithology ai-e 

 often clearly exj)ressed in the suH'ace relief. This is }»articularly 

 the case in the «>utcrop east of the Taff valley, where less ri'sistant 

 bands in thr Main Limestone are fre<|uently marke<l by dejires- 

 sions. For »'xampK', a well-detint-d dry valley east of Riulry marks 

 a group of dolomite-mudstones and shales in the middle ttf the 

 Main Limestone; and on th(.' eastern side of the Rhymney valley 

 the lithological differenci' betwi'cn tlu- lower half of the Main 



' A. Stmhan. ■ On the Orijrin <>f thf Kivfr-Systt-m nf South WuIch. .v its 

 Connexion with that of the Severn it the ThameH ' (.^ J. (t. S. vol. Iviii (lJ>i»2) 

 !.. 207. 



