Weathering on Boulders at Chapmans Pool

Many of the large boulders that have fallen from the crumbing cliffs down to the shores of Chapmans Pool in Dorset, England, show signs of being weathered by exposure to the elements. Over long periods of time, rain, sun, frost, chemical weathering from acid water and from biogenic elements,  mechanical weathering by wind-blown sand, and abrasion by wave-transported gravel and pebbles, have removed the surface of the rocks and revealed an amazing array of textures and patterns.

Softer rock matrices have been eroded to reveal harder structures normally hidden from sight within the boulders. These structures include networks of crystalline veins, as well as trace or ichno-fossils (where the hollows of burrows excavated by crustaceans in the sediments of ancient shores have been preserved, as the sediments were consolidated into rock, and eventually the spaces have been in-filled with harder and more resistant materials). It is interesting to see how small periwinkles frequently taken advantage of the dips and hollows of these eroding rocks to find shelter at low tide.

REFERENCES

Barton, C. M.; Woods, M. A.; Bristow, C. R.; Newell, A. J.; Westhead, R. K.; Evans, D. J.; Kirby, G. A.; Warrington, G. (2011) Geology of south Dorset and south-east Devon and its World Heritage Coast, Special Memoir for 1:50 000 geological sheets 328 Dorchester, 341/342 west Fleet and weymouth and 342/343 Swanage, and parts of sheets 326/340 Sidmouth, 327 Bridport, 329 Bournemouth and 339 Newton Abbot, Compiled by M. A. Woods, British Geological Survey, Keyworth, Nottingham.

Levin, H. L. (1990) Contemporary Physical Geology, Third Edition, Saunders College Publishing, Chapter 5: Weathering and Soils, pp 109 – 120 & 128, ISBN 0-03-031139-x.

Nichols, G. (2009) Sedimentology and Stratigraphy, Second Edition, Wiley-Blackwell, 6.4 Weathering Processes, pp 89 – 92, ISBN 978-1-4051-3592-4.

West, Ian M. 2013. Chapman’s Pool (Chapmans Pool), Houns-tout and Egmont Bight, Kimmeridge region, Dorset; Geology of the Wessex Coast (Jurassic Coast, World Heritage Site) of southern England. Internet site. http://www.southampton.ac.uk/~imw/Chapmans-Pool.htm. Ian M. West, Romsey, Hampshire. Version: 14th December 2013.

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Rock wall at Lunenburg

Natural rock colours, patterns, and textures in a wall at Lunenberg

You don’t have to be that adventurous to find out about the geology in a given area. You can discover a lot by exploring the roadsides and urban environments. Very often the local rocks are used for buildings and sea defences; or quarries and cuttings can be seen from the car as you travel along the highway.

In Lunenburg, Nova Scotia, most of the buildings in the old part of town are wooden. However, the odd garden boundary wall is made of stone, using the local rock type. The photographs in this post, though taken on a very dull wet day, show a wall of rocks that are particularly colourful with rusty stainings from the iron minerals they contain; and they also display patterns of the thin layers or beds in which the fine sediments were laid down. The rocks are most likely to belong to the Halifax Formation, dating from the late Cambrian to middle Ordovician Periods, i.e. 499 – 470 million years ago. The original deposits of sedimentary rock have undergone metamorphosis and been converted into slates and metasandstones.

Natural rock colours, patterns, and textures in a wall at Lunenberg

Natural rock colours, patterns, and textures in a wall at Lunenberg

Natural rock colours, patterns, and textures in a wall at Lunenberg

Natural rock colours, patterns, and textures in a wall at Lunenberg

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Kimmeridge Rocks 1-3

Wet rock colour and texture

Rock textures at Kimmeridge Bay on Dorset’s Jurassic Coast where water runs down the cliff face. Deep orange-red iron deposits on the surface of the grey limestone are revealed by recent rock falls; and granular calcium precipitation coats rock where water falls persistently.

Wet rock colour and texture

Wet rock colour and texture

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Stony Ripples from Ancient Seabeds

 Rock with preserved seabed ripples

There are many strange and interesting shapes and textures in the rocks on the beach at Mewslade Bay on the Gower Peninsula in South Wales. Most of them seem to be the result of weathering and erosion but these photographs show something different, unique, for that location. They appear to be preserved (fossilised if you like) ripple marks from the ancient seabed sediments of which the rocks are composed and date very approximately to about 350 million years ago. They have a distinct patterning which is very familiar from the sand and mud of present day seashores in the same area.

The rock itself is High Tor Limestone from the Carboniferous Period. Actually, It’s a bit old fashioned now to say just Carboniferous Period. Everything has changed. To be more accurate, I should say that the High Tor Limestone Formation is part of the Pembroke Limestone Group, which originated in the Visean division of the Dinantian, which in turn is part of the Mississippian sub-division of the Carboniferous Period.

What were at one time horizontal seabed surfaces have become near vertical over many millions of years of earth movements. The now-exposed surfaces of the old bedding planes are revealed in the entrances to caves at Mewslade Bay. The photographs show them encrusted with recent colonies of living acorn barnacles and occasional limpets.

Reference

Howells, M. F. (2007) Wales, British Regional Geology, British Geological Survey, Keysworth, Nottingham, UK, ISBN 978-085272584-9, pp 112 – 125.

Rock with preserved seabed ripples

Rock with preserved seabed ripples

Rock with preserved seabed ripples

Rock with preserved seabed ripples

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Rock around Chillagoe Part 4 – Chillagoe marble

Quarries excavating marble are a feature of the old mining town of Chillagoe. Marble quarrying is one of the few visibly active mineral exploitation industries in the area – compared with the way it was in the mining boom of the 19th century.

Earlier posts have described aspects of the limestone karst scenery in the Chillagoe District of Queensland, Australia. Limestone is a sedimentary rock formed from the shells of minute marine organisms which accumulated on the sea bed millions of years ago. The layers of sediment become compressed, compacted, hardened, and eventually folded by earth movements, and raised above sea level to their current location.

The Chillagoe limestone has in places been transformed to marble, baked by the extreme heat generated during ancient volcanic activity that has forced molten lava through, and between, the layers of limestone before reaching the surface. The calcium carbonate in the limestone has been melted and re-crystallised. Pure calcium carbonate is white but there are many other minerals in the original limestone and these give the newly formed marble its characteristic streaks and patterns.

Pink and cream colourations are caused by iron oxide inclusions. Impurities such as organic matter or globules of oil, are responsible for blue, grey, and black marble.

I was lucky enough to have a guided tour to see some of the marble, and evidence for the marble industry, while staying at Chillagoe Observatory and Eco Lodge. Rhondda, the manageress of the hotel, took us to see where massive outcrop rock faces, isolated natural boulders, and individual blocks cut out of the rock pavement, had been sliced open to examine the quality of the stone. I could compare the natural rough outer surfaces of the rock with the detailed colours and patterns revealed by the smooth cut surfaces. Many of these natural designs could be better seen when water was poured on them to bring out the detail – although we needed most of the water to drink as it was fiercely hot out in the open with no shade.

Where quarrying was still being carried out from the flatter, weathered, and smooth rock pavement, massive cutting equipment could be seen in position to excavate not far below the surface. The marble bedrock is cut into rows of huge regular blocks with geometric precision by vertical cuts at right angles to each other – and finally separated from the ground by a horizontal cut below.

A useful reference book to the geology of Queensland is:

Rocks, Landscapes & Resources of the Wet Tropics by Bernd G Lottermoser et al, edited by Warwick Willmott & bernd G Lottermoser, Geological Society of Australia, Queensland division, 2008, ISBN 9780975789483, pp 36-37.









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Rhossili beach boulders (2)

Rhossili rock texture and colours: A boulder of pink and white quartz conglomerate, from the Old Red Devonian Sandstone strata of Rhossili Down, lying on pebbles at Rhossili Beach, Gower, South Wales, UK (6)

This post presents photographs of three different Old Red Devonian Sandstone boulders on the beach at Rhossili, with separate close-up images of their texture, showing that the composition varies within individual boulders, with gradations of texture from fine to coarse constituents and stratifications known as graded bedding.

The quartz conglomerate rock strata of the Upper Old Red Sandstone (UORS) – characterised by their undivided red and white stones in a sandstone matrix – were deposited on top of the Brownstones (BRS) which are brown sandstones with thin mudstones and conglomerates. These in turn were deposited on the Lower Old Red Sandstone (LORS) comprising undivided red mudstone.

What we see in these photographs could be a range of the intermediary rock types from the Upper conglomerates to the Lower brownstones. I need to investigate further to understand what they represent. There’s always something more to learn and understand. 

Rhossili rock texture: Detail of the structure and texture a boulder of quartz conglomerate, from the Old Red Devonian Sandstone strata of Rhossili Down, on Rhossili Beach, Gower, South Wales, UK (7)

Rhossili rocks: A boulder of quartz conglomerate, from the Old Red Devonian Sandstone strata of Rhossili Down, on Rhossili Beach, Gower, South Wales, UK (8)

Rhossili rock texture: Detail of the composition and texture a boulder of quartz conglomerate, from the Old Red Devonian Sandstone strata of Rhossili Down, on Rhossili Beach, Gower, South Wales, UK (9)

Rhossili rocks: A boulder of quartz conglomerate, from the Old Red Devonian Sandstone strata of Rhossili Down, on Rhossili Beach, Gower, South Wales, UK (10)

Detail of the composition and texture a boulder of quartz conglomerate, from the Old Red Devonian Sandstone strata of Rhossili Down, on Rhossili Beach, Gower, South Wales, UK (11)

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Curious sea-wall rocks at Lyme Regis

Rock texture: Close up detail of natural pattern in rock with holes in a sea wall at Lyme Regis, Dorset, UK, on the Jurassic Coast (1)

Here are some really unusual rocks that are incorporated in sea walls around Lyme Regis in Dorset, UK. I don’t know what they are. Perhaps you do. I’d love to know what they are. I’m uncertain whether they are local in origin – maybe picked up from the beach – or whether they have been imported from elsewhere. Some of the rocks used in the construction of The Cobb sea defence and harbour were quarried on the Isle of Portland further east along the Dorset coast. Other Cobb rocks were shipped in from much further afield – Norway, I think.  

I’m struck by the honey-comb appearance of these sea wall rocks, the networks of cavities and intricate partitions in the stone – almost bubbly and volcanic. I am wondering whether the holes in the rock are an integral part of the rock’s formation or whether they represent the effects of years of driving rain and pounding seas dissolving softer areas of the stone and leaving harder parts relatively unaffected. It’s a bit of a puzzle.

Rock with holes: Close up detail of natural pattern in rock with holes in a sea wall at Lyme Regis, Dorset, UK, on the Jurassic Coast (2)

Rock texture: Close up detail of natural pattern and texture in rock with holes in a sea wall at Lyme Regis, Dorset, UK, on the Jurassic Coast (3)

Rock pattern and texture: Close up detail of natural pattern and texture in rock with holes in a sea wall at Lyme Regis, Dorset, UK, on the Jurassic Coast (4)

Rock texture: Detail of natural pattern and texture in rock with holes in a sea wall at Lyme Regis, Dorset, UK, on the Jurassic Coast (5)

Rock with holes: Close up detail of natural pattern and texture in rock with holes in a sea wall at Lyme Regis, Dorset, UK, on the Jurassic Coast (6)

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