RIGS of the Month [September ] – Redcliffe Caves, Bristol
RIGS
of the Month – September 2012.
Recliffe
Caves – Bristol
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.
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
Cave web site now greatly updated and changed to
ReplyDeletewww.redcliffecaves.org.uk