Fig. 13. — Vertical section through the Ystwyth Stage, showing the 

 lithological and palceontological succession. 



Regular "alternations of dark-grey 



Rhuddnant p* 

 Grits (Cc. 2 ). 



Shales 

 (Ce,). 



Rlaen 



Myherin 



Mudstones 



(Cb 2 ). 



Dolwen 

 Mudstones 



(Cb,). 



GrROUP 



(Ca). 



evenly- bedded grits with smooth 

 blue shales. 



Monograptus marri, M. holmi, 

 M. priodon. 



Monograptus turriculatus. 



Dark-blue mudstones, with frequent 

 shale-bands. Monograptus exiguus 

 common, with M. holmi, M. marri, 

 M. nudut, M. galaensis, M, cf. pri- 

 odon, etc. 



Blue-grey mudstones, with shale- 

 bands. Monograptus cf. exiguus 

 abundant, with M. galaensis, 

 M. marri, M. cf. holmi, M. nudus, 

 etc. 



Blue-grey compact mudstones, with 

 Monograptus resurgens, M. becki, 

 M. marri, M. nudus, and Bastriles 

 linncei. 



Pale blue-grey mudstones, with thin 

 gritty bands. Monograptus nudus 

 var. variabilis common. 



Monograptus exiguus appears, to- 

 gether with Eastrites linnai, 

 Monograptus resurgens, M. marri, 

 etc. 



Alternations of blue-grey mud- 

 stones and shales with gritty 

 bands. 



Monograptus nudus var. variabilis 

 abundant, with MI. cf. involutes, 

 M: becki, M. runcinatus, M. crassus, 

 M, turriculatus, etc. (Bodcoll 

 Quarry. 



Pale greenish-blue shales and mud- 

 stones, with regular alternations of 

 thin grits with black laminae. 



Abundant Monograptus turricu- 

 latus, with M. becki, M. runci- 

 natus, M. nudus var. variabilis, 

 etc., and dendroid graptolites. 



[Scale : 1 inch = 500 feet.] 



