Jeffrey's Mount, Tebay

[NY 607 017]–[NY 610 026]

F. Moseley

Highlights

Clean, continuous roadside exposures here allow uninterrupted observations to be made on the D₁ folds in the Ludlow Series Coniston Grit Formation sandstones. These typify much of the structural style of the southern Lake District. Together with the Shap site, the locality permits a view of the larger-scale folds of the late-Caledonian deformation, produced during the Early Devonian, overprinted by numerous D₂ folds and fractures.

Introduction

This section is situated on the A685 about one mile south of Tebay village (Figure 3.16) and (Figure 3.19). It comprises the predominantly greywacke sequences of the Coniston Grit Formation. In this region, the Bannisdale Slate Formation and the Coniston Grit Formation were folded during the main Caledonian deformation (D₁) to form the major Bannisdale Syncline (F₁), and were affected by associated cleavage (S₁), minor folds (F₁) and faults.

The site shows many structural features similar to those seen at Shap Fell, but complements that site by its excellent exposures of structures on the opposing limb of the major, D₁ Bannisdale Syncline at a similar structural level. Further details on the general stratigraphy and structure of this area are given in the site description for Shap Fell. Mention of the site is made by Moseley (1972).

Description

At this section the Silurian Coniston Formation Grit consists of greywacke beds from 0.05 m-3 m thick, with subsidiary laminated greywackes, silt-stones and mudstones. There are nine minor folds present, all essentially concentric and with virtual zero plunge, but with complications where thicker mudstone units are involved. There are other complexities, which include small (~1 m wavelength) recumbent folds, low-angle shears, kink bands which deform the cleavage and high-angle wrench faults (Moseley, 1972, 1986). Cleavage is poorly developed.

Folds

The folds and the poorly developed, axial-plane cleavage are the product of early Devonian main phase (D₁) deformation. Local D₂ phase structures (Table 3.1) according to Soper et al. (1987) postdate the underlying Shap Granite (394 Ma BP). The faults belong to the subsequent north-east-trending wrench fractures, some of which pre-date the nearby Carboniferous succession. The folds (see (Figure 3.19)) are open and, being on the south-east limb of the synclinorium, there is a general younging of the sequence towards the north-west and vergence to the south-east. The preponderance of greywacke sandstone in both thick and thin beds, as well as resulting in an absence of cleavage, also results in an absence of minor structures, although fold 4 exhibits a recumbent buckle on its northern limb. This could represent the late-Caledonian recumbent fold phase (F₂), which has been noted elsewhere in descriptions of the Silurian of the Lake District (Moseley, 1972), but it is more likely to be a local reaction to irregular stress conditions. The stereogram (Figure 3.19, inset) summarizes the orientations of bedding and fold plunge.

Faults

The most prominent faults are near-vertical wrench faults, with north and north-west trends, the former sinistral and the latter dextral. The actual displacements of the faults cannot be determined from the section. Both fault sets, however, intersect the roadside section where they have resulted in thin shatter zones (up to 1 m wide). There are also well-developed joints parallel to the faults. Small thrusts or 'thrust joints' are also fairly common (Figure 3.19), all inclined north, and generally with displacements of only a few centimetres.

Interpretation

The structures at Tebay are a product of main phase, early Devonian deformations (D₁). Comparing them with those on the north-west limb of the Bannisdale Syncline at Shap Fell, it is evident that the folding is less intense here. This is attributed to the competence of the greywackes at Tebay compared with that of the mudstones of the A6 section, which also explains the poor cleavage development.

Since these exposures are at the same structural level as those at Shap Fell they provide an interesting contrast in fold style, in the context of variations in the multi-layer sequence and the development of cleavage. They also provide an opportunity to demonstrate the presence and geometry of a major D₁ fold, the Bannisdale Syncline, seen from the opposing vergence as well as from the calculation of sheet-dip and the demonstration of younging. Minor faults are particularly well seen here. Sections such as this, as well as that at Shap, have provided important data for the study of the late-Caledonian deformation and will do so in the future.

Conclusions

Jeffery's Mount provides an excellent cross-sectional view of the style of folding in the rocks of the Windermere Group (which are here of late Silurian age), when it is dominated by sandstones. It allows a comparison of the minor structures on the southeastern limb of one of the largest folds in the Lake District, the Bannisdale Syncline, as well as enabling the geometry and position of the major fold to be established. This major fold was the product of folding during the main Caledonian mountain building phase. Numerous later, flat folds, angular folds (kink bands), and low-angle fractures deform the main structures. These are known to be later than the granite intrusion at Shap, which is dated at 394 million years before the present, which gives an age to this final episode of Caledonian deformation.

References