Structure - Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75

From: Howells, M. F., Francis, E. H., Leveridge, B. E. and Evans, C. D. R. 1978 Capel Curig and Betws-y-Coed. Description of 1:25 000 sheet SH 75 Classical areas of British geology, Institute of Geological Sciences. (London: Her Majesty's Stationery Office.) Map: Sheet SH 75 Capel Curig and Betws-y-Coed. 1:25 000 series - Classical areas of British geology] Under construction

Figure 23 Sketch map showing the distribution and orientation of structures in the district.

Figure 24 Waterfall at Pont Cyfyng, Capel Curig, where the Mon Llugwy is deeply incised and potholed in cleaved mudstones in the upper part of the Carneddau Group.

Figure 53 Wavy bedding cut by cleavage.

Figure 54 Refraction of cleavage in layers of different composition.

Folds

The district lies within the Snowdon Synclinorium, which has a general north-easterly plunge. The component folds are gentle to isoclinal with axial planes dipping steeply to the north-west and gentle plunges variable in direction. Measurements normal to the bedding in mesoscopic folds show no marked hinge zone thickening. Trends of the major folds vary between north-north-easterly in the northern part of the district and easterly in the south. This variation is illustrated by two of the principal structures, the Capel Curig Anticline and the Dolwyddelan Syncline, which are periclines extending beyond the western margin of the district.

The Capel Curig Anticline, formed of rocks low in the sequence, occurs within, and is congruous with, the central zone of the synclinorium. It typifies the folding in the northern part of the district in being an open structure. Farther east, around Cribau, the folding is mesoscopic and similarly open. To the south, on Moel Siâbod, the folding is again open, although axial-plane traces indicate components of the E–W trend which becomes dominant farther south in the Dolwyddelan Syncline and the complementary Llcdr Valley Anticline. Coincident with the change in trend from north-easterly to easterly there is an increase in the scale and degree of closure of the folds.

The Dolwyddelan Syncline is slightly asymmetrical at its eastern end with its axial plane steeply inclined to the north. To the west the northern limb steepens, to become inverted north of Dolwyddelan. On the southern limb in this area dips are steep in the axial region, but decrease to the south. West of Dolwyddelan Castle the inverted limb dips moderately northwards and the fold is here isoclinal. The bedding-cleavage relationship is everywhere congruous with the major structure. Various levels of the Dolwyddelan structure are seen by virtue of faulting and the variable plunge. A constructed profile shows an axial plane steepening at depth. The overturning of the fold at higher levels would appear to represent a separate deformation during which it was considerably tightened with the rotation of both bedding and cleavage.

The convergence of the major fold axes towards the southwest of the district (Figure 23), together with the increasing scale and tightness of folding in that direction, probably reflects location of the area between two major regional structures–the Harlech Dome and the Snowdon Synclinorium.

The presence of later folds trending between north-west and west-north-west can be inferred from flexures in the major fold traces and associated cleavage. On the other hand, plunge culminations and depressions do not in general appear to reflect a separate phase of deformation and are more likely to be the result of inhomogeneous strain, though this has yet to be demonstrated.

Cleavage

Cleavage related to the axial planes of the folds is generally moderate to steep, though it decreases progressively southwards from Dolwyddelan. Its intensity is closely related to lithology. In mudstone it is slaty, with platy minerals showing a high degree of preferred orientation. In the coarser sandstones of greywacke-type, recrystallisation of quartz and micaceous minerals in the matrix has led to the development of a strong preferred orientation. Intergranular fracturing related to this fabric varies from continuous to discontinuous. Preferred orientation fabrics are less apparent in the finer sandstones, which have a smaller proportion of matrix, and fractures tend to be concentrated into widely spaced zones.

Recent unpublished experimental work (N. Price, personal communication, 1974) suggests that such fractures may be the product of pressure solution.

In the poorly cleaved acid volcanic rocks a tectonic fabric is common in the fine matrix, its intensity being dependent upon the mica content. In highly siliceous tuffs, which lack such a matrix, there is little sign of deformation.

In the slates and tuffites of the Dolwyddelan Syncline mineral growth along cleavage is indicated by elongate pyrite spotting and strain shadows on dispersed tuffaceous inclusions. Elsewhere such growth is ill defined, but clasts, nodules and accretionary lapilli in tuffs, and fossils in sediments, show varying degrees of flattening and extension. The long axes of mineral growth and extension indicators everywhere pitch at a high angle on cleavage planes.

Faults

Most of the faults of the district fall into one of two groups, northerly to north-easterly or easterly to east-south-easterly. Faults of the former group, particularly prominent in the east, generally form pronounced features over long distances although many show little displacement. One of the largest is the Conway Valley Fault, which follows the line of the valley south of Waterloo Bridge. Another is the Dolwyddelan Fault which has a northerly trend at Dolwyddelan, but swings to northeasterly and reverses throw farther north before terminating against the Llyn-y-Parc Fault. The trend of the latter does not fit either of the main groups, but it joins the Conway Valley Fault north-east of Betws-y-Coed and is therefore probably contemporaneous with the northerly group. Other faults of this trend bound the block of Upper Crafnant volcanics and Llanrhychwyn Slates around Llyn Goddionduon.

The easterly trending group of faults have a marked effect on the topography in certain areas; one for instance bounds a prominent scarp between Bryn-y-Fawnog and Sarnau. Others partly control the courses of the Mon Llugwy, downstream from Swallow Falls (Figure 24), and the Mon Lledr.

In general the northerly faults terminate against or are displaced by the westerly faults.

Structural history

It is probable that a regional structure approximating to the Snowdon Synclinorium was formed at an early stage in the Caledonian movements. Another early structure has been postulated by Shackleton (in Beavon, 1963), who described an arcuate syncline, crossed obliquely by the main Caledonian cleavage, which he interpreted as a rim syncline surrounding a magmatic dome in Central Snowdonia. The concept of a volcanotectonic structure was later developed by Rast (1969) and Bromley (1969), but no entirely satisfactory evidence of the existence of the rim syncline has been forthcoming (Fitch, in discussion of Bromley, 1969).

The main deformation is the first phase of Helm and others (1963). A proposition by Lynas (1970) that the low-angle main cleavage of the northern part of the Harlech Dome, just south of the district, preceded the main Snowdonian cleavage was refuted by Bromley (1971). The modification of the Dolwyddelan structure may be a reflection of the second phase of Helm and others. The major flexuring of early cleavage about north-westerly trending axes was regarded as a regional phenomenon by Shackleton (1952) and corresponds to the third phase of deformation of Helm and others (1963). Divergence of the major folds from original Caledonian trends is seen as a result of this or even later deformation.

As has been suggested by others (e.g. Roberts, 1967) it would appear that the maximum compression during the main deformation phase was between north-west and south-east. Apart from the Mon Lledr structures, none of the folds of the district have the strong asymmetry which elsewhere in North Wales (Shackleton, 1952) is taken to indicate relative transport of overlying rocks towards the north-west. The F₃ structures suggest maximum compression at right angles to that of the main deformation phase (Helm and others, 1963).

The dating of mineral deposits in the Llanrwst mining field (p. 55) indicates emplacement during the Carboniferous. This suggests that the occupied faults are pre-Hercynian structures. They post-date cleavage and appear to be undeformed by F3 folds, suggesting a late Caledonian age. Brecciation of mineral deposits shows that there has been later movement along these planes.

In the Capel Curig Anticline, the parallelism of minor faults and a cleaved dolerite dyke suggests that those faults may be early Caledonian structures.

Faults, where exposed, are seen to be steeply inclined normal faults showing no signs of lateral movement. The northwesterly dyke trend is common in North Wales and is regarded by Shackleton (1952) as indicating emplacement when maximum regional stress was NW–SE, i.e. during the main deformation phase. The fault pattern in general appears to bear no fixed relationship to the main Caledonian structures.