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

COPYRIGHT JESSICA WINDER 2013

All Rights Reserved