Blackhall Rocks

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This beach photo taken from the top of the Magnesian limestone cliff outcrops laid down during the late Permian ( 300-250 mya). Formed by lagoonal deposits left by the rising and falling levels of the Zechstein sea, various formations can be seen including the Hesleden Biostrome from the beach.

A closer view of the rubble on the beach reveals a stony residue of colliery waste dumped on the beach from local mine workings. The colliery closed in the 1990’s but it was left for the tides to disperse the spoil out to sea. There is still a detectable sulphurous smell.

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The cliffs look more imposing from the beach.

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The laminations in the Biostrome are visible from close up as tightly packed thin layers. Finely laminated horizontal layers within the rock, each no more than a millimetre thick were formed from layers of filamentous blue-green algae that gradually accumulated on the shallow bed of the tropical sea. The Biostrome consists almost entirely of dolomite rock and comprises a thick and highly varied boulder conglomerate overlain by a thicker unit of algal laminites. The conglomerate is formed mainly of rolled cobbles and boulders derived by erosion of the underlying (but unexposed) reef-flat rocks of the Ford Formation.

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Stromatolites occur in the form of large domes up to 20m across and 3m high, pictured below. They are intriguingly displayed on the shore. Structures of this size are currently unique to the UK, and rare in marine Permian strata generally.

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They are more difficult to spot in the cliffs, however, this picture shows the convex dome shaped ‘Crinkly Beds’ of this upper (stromatolitic) part of the biostrome.

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Evidence of a fault is shown below in the different strata on either side of the slipped glacial till. This fault between Limekiln Gill and Cross Gill brings the Seaham Formation and Seaham Residue (left) into contact with the Roker Dolomite Formation (right). The base of the Roker dolomite rises to the cliff top beneath the boulder clay.

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A closer view of the Seaham residue, predominantly thin-bedded limestone (calcite mudstone/wackestone with some interbedded coquina , packstone, grainstone, mudstone and concretionary limestone) with some dolostone.

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Towards the south, a block of the Roker formation had fallen on the beach. Not only were mud slides dripping down the cliffs; occasional rock falls littered the beach. The area, in the longer term, is under going progressive erosion.

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The Roker formation is a cream, oolitic dolostone with subordinate thin beds of fine-grained dolomite and breccias. It rests on the Concretionary Limestone Formation (seen above at the bottom) as rounded cannonball-like strata that occurs above the Hartlepool Anhydrite Formation offshore with a dissolution residue that overlies the Ford Formation onshore.

The glacial tills are 24 m deep of southwards-thickening Quaternary (late Devensian)  deposits overlying up to 10 m of Magnesian Limestone. They form a layered sequence of two stony clays separated by a sand and gravel layer from which perennial seepages cause instability and slips.

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The site is noted for cliff top magnesian grassland and plants such as Sea Spleenwort (Asplenium marinum ). It grows in crevices and fissures in rocks, caves and walls near the sea and within the sea-spray zone.

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The beach at Black Hall Rocks contains some of the largest cave formations to be found on the Durham coastline, the magnesian limestone cliffs here measure 15–32m in height. The Gin Cave is an example of a Sea (Littoral) Cave. Colourful red algae, probably Hildenbrandia rubra decorates the walls.

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A  tantalizing view of the beach from within one of the caves. Caves are formed by the action of destructive waves hitting the cliffs with great force. The action concentrates on weak points within the rock, this may be a fault line or the bedding planes of sedimentary rocks, such as limestone.

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Caves in headlands may eventually erode right through to create an Arch in the rock. This will continue to erode and eventually the arch will collapse leaving a free-standing pillar of rock known as a Stack. The coastline here used to feature many stacks. These are no longer present through either natural causes or human intervention.

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A group of ringed plover and a few Redshank were seen patrolling the tideline.

 

 

Reference:

http://archive.jncc.gov.uk/pdf/gcrdb/GCRsiteaccount3016.pdf

https://designatedsites.naturalengland.org.uk/PDFsForWeb/Citation/1000255.pdf

https://www.geocaching.com/geocache/GC205F8_gin-cave-earthcache-black-hall-rocks

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