214 DE. WILLS AXD ME. SMITH 0^^ THE LOWEE [vol. Ixxviil, 



Berwvn, on a large scale ; wlnle a miniature reproduction of the 

 sharp buckle superimposed on a gently pitching fold may be 

 studied on a few square yards of rock laid bare in the bed of the 

 Dee when the river is low, at a point a quarter of a mile below 

 the ' Chain Bridge.' 



It is natiu'al to expect a considerable amount of faulting and 

 thrusting to accompany this concertina-folding ; but, although 

 there is doubtless far more strike-faulting than has been discovered, 

 it is probable that the materials were extremely plastic at the time 

 of the movement. In some cases it may be seen in quarry- 

 sections that the beds can pass from a horizontal into a vertical 

 position within a few yards by folding. ^ Yet we feel that the 

 sections in PI. IV would more nearly conform to the truth if they 

 showed more faulting in the steep limbs of the folds. The faults 

 that are shown were either observed in the field, or else were 

 inferred with considerable certainty fi"om a study of the strata 

 mapped. 



A small area round Moel-y-faen may be chosen to illustrate the 

 structui'e of the whole northern limb of the synclinorium. The 

 map (PL III) shows that the surface-expression of the concertina- 

 folding is, at first sight, puzzling in the field, owing to the 

 singularly rapid changes in the amount and direction of dip that is 

 observable. But, when carefully studied, the variability resolves 

 itself thus : — Belts of country varying in width from 100 yards 

 to about half-a-mile, and often with nearly parallel boundaries, run 

 across country in a dii-ection 10^ to 20" south of east. In alter- 

 native belts very steep dips (stippled on the map) and very gentle 

 dips (plain) are found. "Within the belts of steep dip the strike 

 is consistently east and west, while in the belts of low dip the 

 strike varies, but the dip usually has an easterly component. The 

 former represent the steep flanks of the tightly-packed and 

 sometimes isoclinal folds and overfolds, the latter their crests and 

 troughs. In the belts of low dip, the easterly comp)onent is im- 

 parted by the pitch (partly original and partly due to a general 

 tilt of the whole region that finds expression in the eastward dip of 

 the Carboniferous). In the steep flanks of the folds this eastward 

 pitch is masked, although it accounts for the difference between 

 the strike of the individual beds (east and Avest) and the run 

 of the belt of high dip (east 10° to 20° south). 



It is perhaps unnecessary to describe in detail the map 

 (PI. III). It should be read in conjunction with the sections 

 in PI. lY (3rd, 4th, & 5th from the top). Xor is it proposed to 

 describe the rest of the complex limb of the Llangollen Syn- 

 clinorium, which is suiBciently illustrated by Pis. lY & Y. with the 



^ Tke plasticity of the rocks as a wliole has foitnd expression in a variety 

 of minor structures, such as slickensides and grooving along the bedding- 

 planes, movement- surfaces bet"^een cleaved shale and almost uncleaved silty 

 sandstone, internal contortion of sandstone-bands, the drawing-out of sand- 

 stone-bands into sausage-like lenticles, etc. 



