212 DE. ^vriLLS A>'D ME. SMITH OX THE LO^EE >ol. Ixxviii, 



plexus wliicli joins the Llanelidan Fault at Llandegla (see map, 

 fig. 5, p. 220). The faulting has locally produced considerable 



shearing, the rocks being almost converted into mica-schists. 



A broad outcrop of Lower Yalentian rocks occurs on the 

 southern side of Cyrn-T-Brain in which the rocks have fairlv- 

 unif oiTu southward dips ; but the structure may not be so simple 

 as it seems, for strike -faultinsr mav be easilv overlooked in so 

 monotonous a series. We know, in fact, that near Plas-uchaf the 

 "World*s-End Fault cuts out a large pai-t of the Lower Yalentian 

 in the eastern part of its com-se, and passing westwards it elimi- 

 nates much of the Tarannon Series. 



The Tarannon outcrop belongs more naturally to the complex 

 northern half of the Llans^ollen Svnclinorium, and will be dealt 

 with under that head (see p. 213). It may be suggested, there- 

 fore, that the core of Ordovician and Lower Yalentian rocks acted 

 as the head of a ram that was thinist into the syncline of newer 

 strata, and produced the concertina-folding in them. 



As in the case of Mynydd-Cricor. so on Cvrn-y-Brain, a number 

 of ti'ansverse adjustment-faults (probably partaking of the nature 

 of tear-faults) comply with the demands of torsional movement. 

 These faults have little effect on the tough Ordovician greywackes, 

 but become more conspicuous in the Yalentian. Their general 

 direction is usually more or less north and south. 



Llangollen Svnclinorium (see Pis. lY A: Y). — This sti-uc- 

 tural element may be defined as embracing the broad outcrop of 

 Tapper Silm-ian rocks southwards fi'om Cym-y-Brain. across the 

 Dee Yalley, to the northern flank of the Ber^vyn anticlme with its 

 north-western butti-ess that projects towards Corwen. The syn- 

 clinorium is obhquely truncated on its north-western side by the 

 Brvneglwys Fault. 



The synclinorium as a whole pitches eastwards, and its axis 

 sti'ikes between 10~ and 20' south of east roughly along the line 

 of the Dee Yalley.^ Xorth of this lies a complex limb in which 

 concertina -folding has taken place, the axes of the minor folds 

 having the same sti'ike as that of the whole synclinorium. South 

 of the axial line the sti'ucture is extremely simple, the rocks being 

 gently folded into a single, shallow, pitching fold. The average 

 angle of the pitch varies between d~ and 10" (PL lY). 



The two stfuctm-al units conti-asted in the last paragi*aph 

 merge in reahty more or less one into the other in the axial region ; 

 but a convenient boundary may be found in the Llangollen Fault, 



^ Tlie stmetiLre will be more readily understood if reference be made to 

 the map ("PL Y) and serial sections (PL IV). It must be pointed ont again 

 that, owing to the absence of easily traceable horizons in the monotonons 

 Denbighshire Series, and to the complexity of the stmctnre. both map and 

 sections are to some extent generalized. The generalization is, however, 

 based npon an enormons number of observBtions made during the re-snrrey 

 of the area. Even so, the boundaries of the subdivisions suggest a degree of 

 certainty that is unjustifiable. 



I 



