Stannington Ruffs

Highlights

Stannington Ruffs is the best exposure of mouth-bar deposits in the Crawshaw Sandstone Formation.

Introduction

Steep crags [SK 305 891] overlooking the River Loxley at Stannington, on the west side of Sheffield, South Yorkshire, provide an extensive outcrop of basal Westphalian sandstones in the southern part of the Pennine Basin. The geology has been documented by Guion (1971).

Description

Exposed here are about 12 m of the Crawshaw Sandstone. Very patchy outcrops of siltstone occur below the main exposure, but there is no evidence of the Subcrenatum Marine Band which lies just a short distance below the Crawshaw Sandstone.

The sandstones are fine-grained and micaceous, and contain abundant, comminuted plant debris. There is also some evidence of bivalve burrows (Pelecypodichnus). Trough cross-bedding is well developed, forming sets about 1 m thick, with gently curved basal erosive surfaces. There is also some ripple cross-lamination, particularly in the upper part of the beds. The sequence is thought to represent mouth-bar deposits of a major distributary system, and palaeocurrents suggest an overall flow from the north-east.

Interpretation

This is the most extensive exposure of the delta-front facies of the Crawshaw Sandstone Formation, a major fluvio-deltaic unit in the basal Langsettian of the southern Pennine Basin (Figure 10.6). Most exposures of the formation (e.g. Ridgeway Quarry) are in quite a different facies, formed by transverse bars in low-sinuosity rivers; the sandstones are much coarser and show mainly planar cross-bedding. Mouth-bar deposits such as seen at Stannington Ruffs are not normally preserved, being removed either by wave-action, or by the subsequent progradation of the delta. However, the Crawshaw delta appears not have migrated further west than this part of South Yorkshire, and wave-action in the basin into which it prograded (the Edale Gulf) was minimal. Together, these factors allowed the preservation of the mouth-bar deposits.

Conclusions

Stannington Ruffs is an important exposure of sandstones known as the Crawshaw Sandstone Formation at the base of the Coal Measures in the Pennine Basin. The particular rocks seen here represent mouth-bar deposits formed in a river delta, some 315 million years ago.

References