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Photos from Aust Cliff, South Gloucestershire

These are photos I took during a trip in early 2007. The first is a sight many will recognise: the old Severn crossing. The concrete structure to the right sits on rocks of Carboniferous age, the top surface that marks an unconformity with the overlying Triassic beds. The softer mudstone of the Triassic Twyning Mudstone Formation has been eroded away here.

Interestingly, when the tide drops further there's a pronounced drop of 2 or 3m height at the edge of this Carboniferous outcrop to the far side of the concrete structure. It's at about the same position as other such ledges along the Somerset coast, and I reckon it's probably related to a period of lower sea level prior to the last glaciation.

Severn Bridge founded on Carboniferous limestone

This is the cliff behind the last photograph. The red coloured material is the Twyning Mudstone Formation, the lighter grey above it the Blue Anchor Formation, and the darker grey the Westbury Formation of the Penarth Group.

Aust Cliff

Gypsum veins at the base of the cliff in the Twyning Mudstone Formation.

Gypsum veins

An annotated view of the succession. Above this, and visible at certain points, is the Lilstock formation and the base of the Blue Lias Formation.

Three of the formations visible along most of the cliff

A block of the Blue Anchor formation.

A block of the Blue Anchor Formation

The Westbury Formation, here, has several thin units of limestone or sandstone in addition to the basal bone bed, this is a fallen block from one of them.

A block of the Westbury Formation

Bioturbated mudstone, probably from the Westbury Formation.

A block of the Westbury Formation

The following is a burrow preserved below one of the limestone units in the Westbury Formation. Actually, it's upside down, but it was easier to photograph like that! The organism has burrowed down into what is now one of the finer grained shale units, and the void has subsequently filled with a better cemented material as the depositional environment changed. After this fell out of the cliff the less well cemented shale has been washed away leaving the detail of the burrow preserved.

A burrow preserved in one of the Limestone units

I suspect this is part of the Cotham Member, from the Lilstock Formation, or base of the Lias. I need to find out more about this section of the succession, unfortunately it's not actually accessible to examine and find out what goes where. You just have to put the pieces together from what you observe that's fallen from the cliff. The literature says that the Langport Member is absent here, so that narrows things down, a little. This block contains flakes of a different lithology to the rest of it on one face.

Flakes in the Cotham member

A block from the Pre-planorbis beds at the base of the Lias with the characteristic fossils of Liostrea hisingeri.

A block from the Pre-planorbis beds