RIGS of the Month - April
Brandon Hill, Bristol
SITE SPECIFIC INFORMATION
Location: BS1 5QT (GR = ST 577 728)
Accessibility: Municipal park (open all year round. Small amount of metered parking nearby. Limited access to wheelchair users.
Risks: Minimal
Topography: Hilly, tarmac paths.
Topography: Hilly, tarmac paths.
Google Earth Map of Brandon Hill. Numbers refer to sites described in the text. |
All site photos can be viewed in a larger format
https://picasaweb.google.com/charly.stamper/BrandonHillPhotosCabot Tower is a prominent feature in the skyline of central Bristol. It was built in 1897 to commemorate the 400th anniversary of John Cabot’s landing in the new found land (later Newfoundland!) of Canada. Set in the lush surroundings of Brandon Hill park, it also marks the spot of an intriguing geological conundrum.
There are two main lithologies exposed in Brandon Hill:
Upper Carboniferous quartzite (the eponymous Brandon Hill Grit); and Triassic
Dolomitic Conglomerate.
The quartzite is best viewed in the north-eastern sector of Brandon Hill, close to the entrance at the end of Charlotte Street (see site 1).
Within the park there is clear evidence for quarrying, and Brandon Hill Grit was
used for many local buildings such as QEH school on Jacobs Wells Road and the
Merchant Venturer’s Building at the University of Bristol (perversely, Cabot
Tower itself was built using New Red Sandstone from the Midlands).
Brandon Hill Grit is a resistant, coarsely bedded
quartzite with a strong siliceous cement indicating that deposition occurred in a shallow deltaic
environment. Although similar in appearance to the classic ‘Millstone Grit’
facies, research has shown the Brandon Hill Grit to be diachronous with the
local Carboniferous limestone. Intermittent channels in the delta sands are
filled with coarser grained material: a good example is visible at the base of the north-west wall of the old bowling green (see site 2).
Site 2: Brandon Hill Grit (channel infill). Coarse-grained sedimentary rock with sub-rounded clasts of up to 10mm in diameter and a matrix comprising 1mm rounded quartz grains. |
There is a break of over 50 million years before the next
rock unit appears during which the collision of two continents in the Variscan
orogeny led to the uplift of existing landmasses. Triassic Dolomitic
Conglomerate is exposed on the lower slopes of Brandon Hill near to Jacob’s
Wells Road and is basal to the New Red Sandstone. The rock provides
evidence that the Carboniferous Brandon Hill Grit was subject to strong erosional forces and
that the Triassic conglomerate is the product of rapid deposition of the newly
formed upland areas.
Although the 1930’s landscaping makes it hard to determine
which rocks are in-situ, more tranquil Triassic deposits can be found
elsewhere in the park. Sub-horizontally bedded Triassic sandstone are present in
amongst the rockery around Cabot Tower. These exposures are authochthonous (in their original place of deposition) though they have been reinforced with concrete between some bedding planes, presumably in an effort to stop the beds crumbling into the path.
Further to the north-west at the edge of the pond near to the public toilets, the bedrock is once more exposed as Brandon Hill Grit, which leads to a puzzling conundrum:
if Upper Carboniferous quartzite is exposed on the west and east flanks of the
hill, but Triassic sandstone is found on the summit, where is the contact
between the two different rock units? Logic dictates that the unconformity must be hidden beneath the rocky flowerbeds beneath Cabot Tower, but as yet, the exact location is unknown.
Charly Stamper
Thanks to Andrew Mathieson for sharing his local geological
expertise. This post references from an article on Brandon Hill in the paper
version of Outcrop (Issue 17) – many thanks to the author, Eileen Stonebridge.
The human history of Brandon Hill park is equally
fascinating: it is one of the country’s oldest municipal open spaces and is the
site of the only remaining Civil War defences in Bristol. For more details visit
the Parks & Gardens UK website (http://tinyurl.com/6uw9yk8).