RIGS of the Month [September ] – Redcliffe Caves, Bristol

RIGS of the Month – September 2012.

Recliffe Caves – Bristol

Entrance to caves on Phoenix Wharf.

There are more pictures here.

Location: Phoenix Wharf, near Redcliffe Parade ST 589 723

This area has been a designated RIGS since 1986.

Access: The caves are owned by Bristol Council but access is controlled by the Axbridge Caving Group, who open the caves for tours annually during the Bristol Open Doors Day each September. They can also be approached for Group tours at other times, see here for details:

Risks: Low roofs, rough ground. Hard hats, stout footwear and torches advised. 

Friends Burial Ground showing cliff near hermitage.

The caves are not natural caves but excavated tunnels into the Redcliffe Sandstone. They were originally excavated to provide sand for glass making and have been used over the years for many different storage purposes.

The foundations of what is now the Mercure Hotel can be seen underground, in the caves.

The sandstone underground is massive with little jointing and no cross- bedding can seen although examples can be seen in this formation at the exposure in the Friends Burial Ground adjacent to the hermitage entrance.

Efflorescence from roof surface

There is surface efflorescence in several areas from the roof, which is assumed to be the decalcification referred to in the Bristol memoir.

The headroom varies around a norm of 2m and there is estimated to be some 6m of sandstone between the cave roof and the ground surface above.

The floor is mostly composed of dumped ash and slag – from the old lead works – that have consolidated over time.

Tree roots have penetrated in some areas.

Cross bedding near hermitage

The caves are cut into the Triassic Redcliffe Sandstone Member.

Redcliffe Sandstone Member ( RESA )

Lithological description:

Sandstone, distinctive fine to medium grained, deep red, calcareous and ferruginous. Commonly decalcified at shallow depths below the surface, giving rise to an uncemented sand.

BGS Computer code:

RESA

Definition of Lower Boundary:

Unconformable, at the abrupt base of the red sandstones of the Redcliffe Sandstone Member overlying sandstones and mudstones of Late Carboniferous age. 

Definition of Upper Boundary:

Conformable and gradational, with interdigitation between red sandstone of the Redcliffe Sandstone and reddish-brown mudstone of the Sidmouth Mudstone Formation (undivided).

Thickness:

Up to 65m

Geographical limits:

Crops out in the Bristol area between Bedminster and Winterbourne.

Parent Unit:

Sidmouth Mudstone Formation

Group:

Mercia Mudstone Group

Age:

Triassic

Stratigraphic setting:

Following the Variscan orogeny at the end of the Carboniferous Period, erosion stripped off some of the Coal Measures during the Permian so the RESA was laid down unconformably on the irregular remnant Carboniferous landscape, diachronous with the marginal facies of the Mercia Mudstone Group. RESA was deposited in an elongate depression between Bedminster and Winterbourne and locally exceeds 50m in thickness. The RESA passes laterally into red mudstones and is locally interdigitated with Mercias Mudstone Marginal Facies ( MMMF ). The best exposures of RESA are found in cliffs along the Avon in Redcliffe, the eponymous district of Bristol, and in the New Cut along Coronation Road, Southville.

Applied geology:

RESA was used for glassmaking, the glass being used to make bottles for the thermal water from the five Bristol hot springs which were then largely exported.

The RESA is an aquifer which was an important source of water for Bristol in the past until contamination from local cess pits and burial grounds resulted in several epidemics.

Ground conditions:

In some areas the RESA has become decalcified, losing its cement and becoming friable and difficult to handle when wet. Changes in groundwater regimes can be responsible for this decalcification process, which in turn can cause local subsidence. Weathered RESA , when used for fill material has also been known to provide poor support for buildings.

References:

BGS Lexicon           -           http://www.bgs.ac.uk/lexicon/

BGS map      2004   -           England and Wales sheet 264 – Bristol – Solid and Drift

BGS Memoir 2002   -           Geology of the Bristol District

Memoir 1949 / 2000 -           Geology & scenery of the West of England – AE Turner & N Chidlaw                         

Cave web site         -            http://www.bristoltours.com/Redcliffe.htm

Photo credits          -            Richard Kefford

Richard Kefford