The Group's aim is to identify, survey, protect and promote geological and geomorphological sites in the former County of Avon - the modern unitary authorities of Bath and North East Somerset, Bristol, North Somerset and South Gloucestershire. RIGS are selected for their educational, research, historical and aesthetic value.

Tuesday, 10 April 2012

RIGS of the Month [April] - Brandon Hill

RIGS of the Month - April
Brandon Hill, Bristol

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.

Google Earth Map of Brandon Hill. Numbers refer to sites described in the text.
All site photos can be viewed in a larger format

Cabot 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).
Site 1: Brandon Hill Grit. The beds are tilted steeply towards the north-east having been deformed at the end of the Carboniferous during the Variscan orogeny, when two continents collided and formed the supercontinent of Pangea.

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. 

Site 3: Triassic Dolomitic Conglomerate. Coarse-grained sedimentary rock with rounded clasts of up to 20cm in diameter, including large chunks of the Brandon Hill Grit. The unit is extensively iron stained and the haematite cement gives the rocks its distinctive red colour: it has been suggested this is a result of oxidation of pyrites in the underlying Coal Measures.

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.
Site 4: Triassic New Red Sandstone.  Laid down in a desert environment, its red colour testifies to the rock forming in an aerobic subaerial environment. In contrast to the Brandon Hill Grit at Site 1, it is sub-horizontally bedded.

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).