The Pipe Rock Member

The Pipe Rock is the uppermost part of the Eriboll Sandstone Formation and is found at both around Sangobeg and also nearby the main road by Keoldale. Cross-bedding implies that these are ancient beach deposits. These beaches were formed during the Cambrian period c.500 Ma during a worldwide sea-level rise that occurred at that time. The gradual change into heavily bioturbated burrows is likely from rising sea levels allowing ‘worms’ to become abundant and secrete linings within the burrows or 'pipes'. Later metamorphism has then recrystallised the sands with the resultant porosity from weathering.

Cross-bedding is much rarer than in the Basal Quartzite and the first appearance of skolithos trace fossils are found. Here vertically orientated, circular pipe-like burrows up to 15cm in length and 2cm in diameter are seen thinning down from the bed surfaces. At the bottom of the sequence the beds are a pale beige colour almost identical to the top of the Basal Member, indicating a gradual change into skolithos rich beds.

Road cuttings exposing younger beds to the north show deep red-purple staining before the youngest section at Sangobeg Beach shows light beige to almost colourless beds, with graded bedding and some cross-lamination still present in certain horizons. Here skolithos burrows are much more visible as they have a distinctive brown-purple staining against the much lighter host-rock, most likely due to Fe/Mn oxide staining. ‘Trumpet-shaped’ monocrateron trace fossils are also found in association with skolithos but appear to be less abundant. Both burrow types are easily visible on weathered bed-surfaces as abundant circular pits that are commonplace where poorly cemented burrow infill has been weathered. Alternatively, some beds show a very ‘knobbly’ surface texture where burrow infill has been cemented better than the surrounding rock, showing differences in burrow sedimentation with differering beds. Some show a ringed pattern in plan-view, indicating possible gradual infill or lining of the burrows, often cut by fractures.