RIGS of the Month [August] - Blaise Castle Estate

RIGS of the Month - August
Blaise Castle Estate

Geological Time Travel in a Land of Giants

A view for all seasons southwest over Henbury gorge from Castle Hill towards Failand Ridge on the horizon. 

Please follow the Geologist's Code.
http://www.brerc.org.uk/rigs_site/geologists_code.htm

SITE SPECIFIC INFORMATION

Location: Henbury, Bristol.

Accessibility: Various access points, see map. From Coombe Dingle car park, on metaled paths through steel gates. Maximum gradient 12%. Some points off main paths are steep and over rough ground. Contact duty ranger on 07795 445999 for car park opening times and gate access for wheelchair users.

Risks: Beware of cliff edges.
Topography: Steep-sided, incised plateau with shallow streams.

All photos from this post can be viewed in a larger format - http://tinyurl.com/blaise-rigs

Blaise Castle Estate is a magnificent Grade II listed parkland.  Jane Austen wrote in Northangar Abbey: “Kingsweston! Aye and Blaize Castle too…The finest place in England – worth going fifty miles at any time to see.”  Here though, Thorpe is somewhat deceitful in his persuasion of the reluctant Catherine to join the excursion from Bath. Even so, Blaise is justly renowned for its picturesque landscape, making it a popular attraction to this day.

18th Century Blaise Castle House and Museum.

Blaise is set mainly onto the Carboniferous Limestone ridge of Kings Weston Hill and Castle Hill, on the western outskirts of Bristol. This dramatic and scenic incised plateau area is a microcosm of the solid geology of the Bristol district. The changing environment and evolution of the landscape can be traced from its sedimentary rock units in the section dating from the late Devonian period, c 360 Ma. Tectonic forces and thrust faults that shaped the region are evident from the folded strata of the Westbury Anticline along with steeply-dipping, overturned and shattered beds in the northern limb of the anticline. Finally, a drainage system etched its channels through the covering sediments and into the Carboniferous rocks to form the spectacular, steep-sided, densely-wooded Henbury gorge: a recent sculpting of the landscape, also seen in similar features across the region, from the nearby Avon Gorge to Burrington Coombe  and Cheddar Gorge in the Mendip Hills.

The Blaise Castle Estate/Kings Weston Ridge RIGS extends from Henbury, ST 562 788 westward along the northern limb of the Westbury Anticline as far as Shirehampton, ST 530 774 and southwest to Coombe Dingle, ST 558 773 including the River Trym valley from Coombe Farm, ST 563 776 to Sea Mills Lane, ST 555 766.  The site is important for education and research into the palaeoenvironment of the region, periglacial landscapes and stream action.  Its rich variety of rock units along with their twisted and contorted structures make for a challenging lithology and stratigraphy.  It’s an area of extraordinary natural beauty, complex geological structure, with some remarkable exposures.

Click to switch map, change view and read bedrock descriptions. 

A walk through geological time starts in Coombe Dingle by the River Trym, see gorge walk, at the base of the succession in the late Devonian, c 360 Ma. From the confluence of the Trym and Hazel Brook the course of Hazel Brook is followed into the shady gorge and a gentle ascent to the top of the early Carboniferous, c 330 Ma and the giant’s footprints on Castle Hill.  From nearby Lover’s Leap, and then from the legendary giant Goram’s Chair opposite, the full panorama of Blaise and the gorge can be admired.

The massive and thick-bedded river cliffs of conglomerate at the Coombe Dingle entrance rest on the Upper Old Red Sandstone of the Portishead Formation.  These 10m high cliffs of Triassic age were formed c 206 to 248 Ma from the debris of shattered and scoured uplands in Carboniferous and Permian times.  Debris collecting in the valleys and wadis below were cemented into the Mercia Mudstone Marginal Facies (dolomitic conglomerate) along shorelines, wedged between and against the limbs of the Westbury Anticline, forming the present day bedrock. 

Outcrops of Triassic dolomitic conglomerate can be seen where the dry stream tributary meets the Trym at Stoke Bishop (above) and also by Hazel Brook near Henbury Church.

The angular unconformity where the Triassic breccia overlies what appear to be vertical beds of Devonian old red sandstone is exposed at ground level to the left of the path c 150m from the Coombe Dingle entrance. A similar unconformity, but with a conglomerate of different origin, is exposed on a far greater scale at Kilkenny Bay, Portishead. At Blaise, the Upper Old Red Sandstone continues along the wooded slopes beyond the Trym floodplain, on the south side of the Trym valley, heading east to Westbury-on-Trym. These rocks were formed by rivers depositing sand and gravels into river channels, c 354 to 364 Ma.

Angular unconformity where the Triassic dolomitic conglomerate overlies vertical beds of Devonian upper old red sandstone.

After a few metres along the path the base of the Avon Group is reached marking a change from fluvial and deltaic to marine environments, c 344 to 354 Ma. Down-slope to the river confluence outlines a small area for the Shirehampton Formation. Surface rock samples show light-grey, medium-grain, crinoidal limestone, possibly formed in a warm shallow sea or lagoons. Across the Hazel Brook weir footbridge and close to the river bank marks the boundary with a thin bed above the Shirehampton Formation. Rock samples show a distinct reddening of finer grained, sparsely fossiliferous limestone and red-brown, siltstone that together may reveal the Bryozoa Bed.

From the main path above the footbridge, an unmade path forks left and leads after c 100m into woodland and to an outcrop of the Maesbury Mudstone Formation, at the top of the Avon Group. 

Maesbury Mudstone Formation beds formed in a warm shallow lime-mud sea with fluctuating levels.

Penny Well has its source c 20m farther along the path, ST 558 778. The spring also marks a termination of a 1 km fault line SW - NE, to where Hazel Brook enters the gorge near Henbury Church. Penny Well feeds into Hazel Brook a few metres from the bridge south of the lily pond. At this point of the succession the base of the Pembroke Limestone Group has been crossed and the main path passes the pond through the Black Rock Limestone Sub-Group, rocks again formed in a warm, shallow, carbonate sea.

A small disused quarry to the right of the path next to the lily pond has been used to extract limestone. North of the pond, by the next bridge, the path meets a shear rock face of Black Rock Limestone beds dipping 86^o NE, climbing away to the east, disappearing into dark woods, upwards to the precipitous Potter’s Point.

As the path winds its way towards the Beech Cathedral the succession passes up through the Black Rock Dolomitic Limestone.  A few metres up the slope, between the beeches, the distinctive white weathered rock of the Gully Oolite Formation crops out. 

The majestic towering columns of the Beech Cathedral is one of the finest sights on the estate.

Coming off the bend, and overlying the Gully Oolite in the succession, the brown mudstones found in the bank prove the presence of the Clifton Down Mudstone Formation.

The appearance along the path of scree deposits marks the boundary with the Clifton Down Limestone Formation and a change in the palaeoenvironment to more open seas. A vertical limestone cliff butts up against the path by the next pond, Tarn Lake (the Giant’s Soap Dish). Above the tree canopy and to the right are the soaring limestone twin towers of Goram’s Chair. On the opposite side of the gorge the limestone cliffs, their beds dipping 30^o E, line up to buttress Castle Hill, screes piled high at their bases.

The uppermost formations of succession are reached at the bridge near Stratford Mill. The paths to Blaise Castle and Rhododendron Walk lead to good exposures of the Oxwich Head Formation (previously the Hotwells Limestone Formation) and the Cromhall Sandstone Formation.

Cromhall Sandstone Formation beds on the path to Blaise Castle.  Limestone, mudstone, siltstone and sandstone may be present suggesting a change in the environment from warm, shallow seas to rivers, marginal marine and estuarine environments, c 327 to 341 Ma. The red colours are caused by oxidation of pyrite. 

A flight of steps leads up to the giant’s footprints, in an outcrop of the Oxwich Head Limestone Formation.  The feature has the appearance of a limestone pavement.  These are usually formed on horizontal beds by slightly acidic conditions dissolving the limestone around cracks, such as bedding planes and joints. They are sometimes associated with glaciation. It’s unusual to find them in southern Britain. 

Limestone pavement on the lower eastern slope of Castle Hill.

Proof of the top of the succession is more reliable in some exciting outcrops along Rhododendron Walk. Rocks more recognisable of the Cromhall Sandstone Formation crop out in places up to the Rustic Lodge.  Oxwich Head Limestone crops out along the path to the gatehouse at Henbury Hill, and before Henbury Lodge the bank leads up to the site of the former Henbury Hill Quarry, reclaimed for the 14^th Green at Henbury Golf Course on Coombe Hill.

Rhododendron Walk makes a dogleg at the Rustic Lodge where rocks of the Oxwich Head Limestone crop out in the bank.  They show recognisable coarse, grey, ooidal and crinoidal limestones that represent high-energy marine deposits expected in open shelf environments.

The path through the rhododendrons runs along the rim of the gorge on Coombe Hill and leads to Goram’s Chair. From one of the cliff towers there is a commanding view across the gorge to Kings Weston Hill, Echo Gate and Castle Hill.  It overlooks the steepest part of the gorge, plunging 60m to the bottom. The Severn Estuary and South Wales can be seen over the col at Echo Gate on a clear day.

View from Goram's Chair northwest across the gorge to Kings Weston Hill, Echo Gate col and Castle Hill to the right.

A local legend says that the giant brothers Goram and Vincent diverted the River Avon, each digging a ravine, Goram at Henbury and Vincent at Clifton. More natural explanations for the origin of the Henbury gorge and the Avon Gorge have been hotly debated from the early 19^th century almost until the present day (Bradshaw 1965).

Hazel Brook makes a curious c 90^o turn south in the Crow Lane Open Space, ST 568 796 and flows in a straight line across the Triassic dolomitic conglomerate plain to where it enters the gorge near to Henbury Church. It follows the straight line of the Henbury Fault along the strike through beds of the Cromhall Sandstone Formation to beyond the bridge at Stratford Mill. From here the brook diverges from the fault line, taking a more westerly course before turning towards the south again below Goram’s Chair, following the gorge, crossing the Henbury Fault near Penny Well.

An alternative course for a river would have been the more direct and easier westerly route across softer Triassic and Jurassic rocks to Hallen. It has been suggested that glacial diversion of the river created a new route that breached the Carboniferous rocks at Henbury, (Hawkins 1977). Evidence in the form of gravels and till found in the Bristol district has been interpreted as glacial deposits (Colborne et al 1973). But there has been no evidence found so far that ice reached as far as the Hallen gap, ST 559 795.

The plateau of Kings Weston Hill and Coombe Hill, c 100m OD, is the result of river or marine erosion, when the steeply-dipping and faulted Carboniferous strata were planed flat. An erosion surface at a similar height in Pembrokeshire is estimated to be Pliocene in age, c 5 – 2 Ma. This has led to the proposal, if the Kings Weston Hill erosion surface is Pliocene, it would have been more extensive, cutting across the hard Carboniferous rocks and adjacent softer Triassic and Jurassic strata that had buried the earlier landscape. During the following 2 million years of the Quaternary, this c 100m hard Carboniferous surface would have been gradually incised by its drainage system, and softer rocks worn down faster.  The Carboniferous rocks being more resistant remained, producing the typical Carboniferous Limestone landscape seen today.

Cheddar Gorge is thought to have been formed in the periglacial, tundra environment of the last glaciation by seasonal meltwater flowing off snowfields and over the surface of the bedrock made impermeable by permafrost.  In the last glacial period during the Devensian stage, c 118,000 to 10,000 years BP, the ice sheet advanced to South Wales. The Bristol district was in the grip of a permafrost and it’s reasonable to suppose that frost action and seasonal melts over frozen ground could have rapidly enlarged the Henbury gorge, in a similar process to Cheddar.

To conclude, it has been possible to map the succession and trace the changing palaeoenvironment from the rocks of Henbury gorge. The precise origin of the gorge remains uncertain. However, the balance of evidence found so far suggests that it's the result  of processes observed in the formation of similar features in the region.

Acknowledgements

Thanks to Eileen Stonebridge and Nick Chidlaw for helping me to identify and understand the processes and possible origins of Henbury gorge.

References

Bradshaw, R. 1965. The Avon Gorge, Proceedings of the Bristol Naturalists' Society, Vol. 31, Part 2, pp 203-220.
Colborne, G.J., Gilbertson, D.D., Hawkins, A.B. 1973. Temporary Drift Exposures on the Failand Ridge, Proceedings of the Bristol Naturalists' Society, Vol. 33, pp 91-97.
Hawkins, A.B. 1971. Some Gorges of the Bristol District, Proceedings of the Bristol Naturalists' Society, Vol. 32, Part 2, pp 167-185.
Hawkins, A.B. 1977. The Quaternary of the North Somerset Area, Geological Excursions in the Bristol District.

Maps

England & Wales, Sheet 264, Bristol, Solid & Drift Geology Map, British Geological Survey, 1:50 000.
Bristol & Bath, Sheet 172, Ordnance Survey, 1:50 000.
Bristol & Bath, Sheet 155, Explorer Map, Ordnance Survey, 1:25 000.


John Byles