c. ystwytu 

 Staqk. 



Geological Society* 



/ I'alo iiui<lstoHc» with iiu 

 i Iniiiiiiiitetl ^rit-bHiidx. 



339 



Valkhtian . J. p 



190 feet. 



.Stage. 



6. ifonf of Monograptut " 

 lain. 



4. Zone of Monograptut 

 tedgwicki. 



h CJrmu? i ^- ^'"'" "^ ifonograptut 



Cwniere 

 (iroup. 

 334 feet. 



Hartfkll rA. Pltn- f 

 (Obdo- J, 



VICIA5). l^ 



regi 



2. Zone of JfotmgraplM 



leptothcca. 



1. Zone of Mesrtgraptus 

 L viagnus. 



3. Monnqraptus spp. 



IJed.-.; 



2. Zone of Diplograptut 

 ammhiatus. 



I 1. Zone of Glifptograp- 

 I. tus persculptus. 



I.I.MOX -^ Ty'n-y-maen 

 Staob. (^ Group 



J Dark iniulKtone.'?, grits, and some 

 j conglomerates. 



Pale-blue and green- 

 ish mAdstones 

 with bands of dark 

 "graptoliferous shales, 

 and some thin 

 green flags. 



Dark rusty- 

 weathering 

 shales and 

 limestones. 



' ^fottlod Beds ' and 

 blue mudstones. 



The distribution and character!? of these beds are described. The 

 'Mottled Beds' form the ba.se of the Silurian and rest sharply on the 

 underlying beds, and there i.s evidence of complete di.scontinuity at 

 this level ; they have proved of gi"eat service in elucidating the 

 structure. The Monograptut spp. Beds contain graptolites which 

 elsewhere pertain to the zones of Monor/raptus triangidaius, 

 M. cyphus, and M. acinaces ; but another zone, that of M. ataius, 

 lias not been proved, althougli it probal)ly occurs. Tlie I)erwen 

 Group consists of a regular alternation of mudstones and shale-bands 

 with graptolites, which have al.>-o proved of service in mapping. 

 Only a small thickness of the Vstwyth Stage occurs, and no sub- 

 divisions are attempted. 



The rocks are sharjjly folded, and sometimes oVerfolded, towards 

 the ea.st. Their axes range approximately north-north-east and 

 south-south-west ; the folds in the cent^•al area pitch northwards-, 

 but north of the Dovey a southerly jntch sets in. Each large 

 fold is composed of a number of smaller folds having parallel axes, 

 and changing in pitch more frequently than the larger folds-. 

 Strike-faults of considerable magnitude range nearly parallel with 

 the folding axes, and are in all cases ovorthrusts towards the east. 



Of greater interest are the transvei-se faidts ranging nearly ea.st- 

 north-eaist and west-south-west. Most of these are small, but their 

 course acro.ss the higher ground is indicated by well-defined notches 

 in the ridges that they cross. Two of these faults, the Pennal 

 and LlA'fnant Faults, are .shatter- belts. The Llyfnant Fault dis- 

 places .several folding axes, and overthrusts to the east on the north 

 side. Its vertical displacement is on an average about 300 feet, 



