Building Stones of Clifton
- A Walking Trail
A thirty-minute ramble through 350 million years of geological time
The oldest rocks beneath
Clifton are Devonian Old Red
Sandstone, lower Carboniferous
limestones and sandstones,
and Upper Carboniferous Coal
Measures. These are sediments
deposited during a long period
of fluctuating sea level. In the
Permian period, formation of the
supercontinent Pangaea caused
uplift of existing landmasses
which were consequently subject
to strong erosional forces. The
resulting detritus created the next
generation of bedrock, and so the
older sediments are unconformably
overlain by Triassic conglomerates
and sandstones, and Rhaetic
limestones.
Site 1 - Clifton Hill House
Bath Stone (oolitic limestone) - Jurassic
Start the trail at Clifton Hill House at the top of Lower Clifton Hill
Built in the 1740s, this former merchant’s mansion is now
part of a hall of residence for the University of Bristol. The
front of the building is faced with cream-coloured oolitic
limestone, a rock not native to Clifton; it was extensively
quarried in (and is eponymous to) Bath when it became
fashionable in the 18th century. Bath Stone was deposited
in a tropical shallow marine environment, similar to that of
the Bahamas today. The rock comprises millimetre-sized
‘ooids’, small lithic grains coated in concentric rings of
aragonite (preserved as calcite) mud. Other features, such
as cross-bedding and calcite veining, are neatly captured
in the end stone.
Start the trail at Clifton Hill House at the top of Lower Clifton Hill
Clifton Hill House - Jurassic oolitic limestone |
Site 2 - Goldney House
Brandon Hill Grit - Upper Carboniferous
Continue Clifton Hill and cross the road at Constitution Hill [150m]
Goldney House is also part of a university hall of resi-
dence, although the main building is a modern addition to
the early 18th century coach house and other outbuildings.
The coach house wall is accessible from the pavement
and is an irregular patchwork of Brandon Hill Grit, a
coarse Upper Carboniferous quartzite sourced from nearby
Brandon Hill. The rock was laid down as a deltaic sand
coevally to the limestones of the Avon Gorge; coarser
horizons in some blocks are evidence for ephemeral stream
channels. Its distinctive pink-red colouration is staining
from the overlying Triassic sediments.
Site 3 - Caledonia Place
Pennant Sandstone - Upper Carboniferous
Continue on Lower Clifton Hill as it becomes Regent Street. Walk into Clifton Village and turn left along Royal York Crescent. To the south is Dundry Hill [600m]. Walk all the way along the terrace, turn right at the end into Wellington Terrace, and then second right into Caledonia Place [500m].
Though prevalent as a building stone in the city centre of Bristol, Pennant Sandstone is not as common in Clifton. This grey-coloured sandstone is rich in feldspar and micas, and was deposited in shallow waters in the Coal Measures. The poor cementation between individual grains made the sandstone easy to quarry; however, this is counterbalanced by its relative fragility and vulnerability to weathering. In Caledonia Place it has been employed as mounting blocks (to aid Victorian residents’ ascent into horse-drawn carriages).
Pennant Sandstone - Upper Carboniferous
Continue on Lower Clifton Hill as it becomes Regent Street. Walk into Clifton Village and turn left along Royal York Crescent. To the south is Dundry Hill [600m]. Walk all the way along the terrace, turn right at the end into Wellington Terrace, and then second right into Caledonia Place [500m].
Caledonia Place - Pennant Sandstone mounting blocks |
Though prevalent as a building stone in the city centre of Bristol, Pennant Sandstone is not as common in Clifton. This grey-coloured sandstone is rich in feldspar and micas, and was deposited in shallow waters in the Coal Measures. The poor cementation between individual grains made the sandstone easy to quarry; however, this is counterbalanced by its relative fragility and vulnerability to weathering. In Caledonia Place it has been employed as mounting blocks (to aid Victorian residents’ ascent into horse-drawn carriages).
Site 4 - Clifton Suspension Bridge
New Red Sandstone - Triassic
Retrace your steps out of Caledonia Place and continue along Wellington Terrace, then Sion Hill [300m]
Clifton Suspension Bridge is Bristol’s most iconic land- mark and was designed by Isambard Kingdom Brunel in 1831 (but completed posthumously in 1864) to span the chasm between the Carboniferous limestone cliffs of the Avon Gorge. The base of the gothic towers are attractively faced with New Red Sandstone. Its distinctive red colouration reveals its subaerial formation in the deserts of Pangaea and layering from ancient sand dunes is preserved as cross-bedding.
New Red Sandstone - Triassic
Retrace your steps out of Caledonia Place and continue along Wellington Terrace, then Sion Hill [300m]
New Red Sandstone facings at the Clifton Suspension Bridge |
Clifton Suspension Bridge is Bristol’s most iconic land- mark and was designed by Isambard Kingdom Brunel in 1831 (but completed posthumously in 1864) to span the chasm between the Carboniferous limestone cliffs of the Avon Gorge. The base of the gothic towers are attractively faced with New Red Sandstone. Its distinctive red colouration reveals its subaerial formation in the deserts of Pangaea and layering from ancient sand dunes is preserved as cross-bedding.
Site 5 - The Observatory
Carboniferous Limestone - Lower Carboniferous
Follow the short footpath up the hill from the Bristol-side toll booth
[200m]
Originally built as a mill in the late 18th century,
Observatory Tower was purchased over fifty years later
by a local artist who installed a telescope and camera
obscura (to project panoramic exterior views onto a screen).
The rounded rubble walls comprise fossiliferous blocks of
Carboniferous Limestone hued from the gorge, and provide
a reminder of a time when the Avon region was submerged
beneath a balmy tropical ocean. Descend to ‘Giant’s Cave’
beneath The Observatory to further explore the strata of
the Gorge.
Site 6 [optional] - The Cumberland Basin
Cornish granite - Lower Permian
For a longer addition to your excursion, retrace your steps towards the
Avon Gorge Hotel and take the Zig Zag footpath down to The Portway.
Turn left and walk towards Bristol City Centre. Take care when crossing
the busy road - it is best to walk over the pedestrian footbridge which
begins in Granby Hill [∼1km].
The Cumberland Basin was excavated in 1809 when
the River Avon was diverted to form a floating harbour
and granite is used as capping material on the channel
walls. Petrolographic analysis has shown it to be Bodmin
Granite, part of the Cornubian batholith that is exposed
throughout Cornwall and the Channel Island. This igneous
rock formed a result of a huge mass of magma intruding
into the crust during Variscan orogeny (∼275Ma). Though
the surface has weathered to a smooth finish, individual
crystals of grey quartz, white plagioclase and pinky-
orange orthoclase feldspars, and dark-coloured biotite
mica can still be identified.
Charly Stamper
References
- Jones D (1992) A History of Clifton. Phillimore, Chichester.
- Mowl T (1991) To build the second city: Arcitects and craftsmen of Georgian Bristol. Redcliffe Press Ltd, UK.
- Savage RJG (1988) Buildling Stones of Clifton. Proceedings of the Bristol Naturalists’ Society, 48: 85-104.
- Mowl T (1991) To build the second city: Arcitects and craftsmen of Georgian Bristol. Redcliffe Press Ltd, UK.
- Savage RJG (1988) Buildling Stones of Clifton. Proceedings of the Bristol Naturalists’ Society, 48: 85-104.
No comments:
Post a Comment