Seatown Shattered Eype Clay

The 80 metre high cliffs on the east shore at Seatown in Dorset along the Jurassic Coast are subject to land slips and rock falls. Large lumps of shattered blue-grey clay are common on the beach. They come from cascades of Eype Clay Member material that forms the lower part of the cliff exposures.

Seatown Rock Crystals

A large rock that had rolled down from the top of the cliffs at Seatown in Dorset was lying on the pebbles of the beach. It was yellow and rusty coloured. At this point along the shore, called Ridge Cliff, the rocks belong to the Dyrham Formation of the Liassic/Jurassic period. The lower section of the cliff is the Eype Clay Member of pale, blue-grey micaceous silty mudstone and shale. Above that is the Down Cliff Sand Member mostly of silts and fine sands with thin lenticles of hard calcareous sandstone. On top of this is the Thorncombe Sand Member with yellow-weathering, heavily bioturbated sands, with several horizons of large rounded calcareously cemented concretions. This boulder obviously came from one of these upper sandstone layers but I cannot say which one. Its broken edge revealed lovely abstract patterns and beautiful crystals.

Rocks & Pebbles near Twlc Point

Broughton Bay is a wide sandy expanse on the north shore of the Gower Peninsula in South Wales, facing the Loughor Estuary or Burry Inlet. A small promontory called Twlc Point at the western end of the beach has an interesting geology with an exposure of Hunts Bay Oolite from the Carboniferous Period. I have written about these strata in earlier posts such as:

Rocks on the west side of Broughton Bay Part 1

Rocks on the west side of Broughton Bay Part 2

Rocks on the west side of Broughton Bay Part 3

Brachiopod fossils in Hunts Bay Oolite at Broughton Bay

On this particular visit I was content to appreciate the way that pebbles of many types and colours on the upper shore were clustered around outcrops and boulders of the limestone which were often pink-tinged and sometimes fossiliferous.

Flints Embedded in Studland Chalk

Flint nodule embedded in chalk

Flint nodules embedded in the low cliff between two minor faults in Studland Chalk Formation exposures at South Beach on Studland Bay, Dorset, England.  At the top of the cliff face is a layer of ironstone and iron-stained flints that has caused the rusty stain on the chalk below. Elsewhere the rocks are covered with a fine coating of green algae.

A couple of useful references for the geology of the area in which these photographs of the chalk and flints were taken:

Barton, CM, Woods, MA, Bristow, CR, Newell, AJ, Weathead, RK, Evans, DJ, Kirby, GA, Warrington, G, Riding, JB, Freshney, EC, Highley, DE, Lott, GK, Forster, A, and Gibson, A. 2011. Geology of south Dorset and south-east Devon and its World Heritage Coast. Special Memoir of the British Geological Survey. Sheets 328, 341/342, 342/343, and parts of 326/340, 327, 329 and 339 (England and Wales), 9–100.

Cope, JCW, 2012 Geology of the Dorset Coast, Geologists’ Association Guide No. 22, Guide Series Editor SB Marriott, The Geologists’ Association, 191-194. A serious guide for the more dedicated amateur and professional.

Ensom, P and Turnbull, M 2011 Geology of the Jurassic Coast, The Isle of Purbeck, Weymouth to Studland, published for the Jurassic Coast Trust by Coastal Publishing, ISBN 978-1-907701-00-9, pages 96-117. A beautifully illustrated beginner’s guide to the geology of the area – one of a series of excellent publications by the Jurassic Coast Trust.

Swanage Solid and Drift Geology (map), British Geological Survey (Natural Environment Research Council) 1:50,000 Series, England and wales Sheets 342 (East) and part of 343

Flint nodule embedded in chalk

Low chalk cliff with row of embedded flints

Flint nodule embedded in chalk

Line of flint nodules embedded in chalk

Flint nodule embedded in chalk

Line of flint nodules embedded in chalk

Flint nodule embedded in chalk

Line of flint nodules embedded in chalk

Flint nodule embedded in chalk

Flint nodule embedded in chalk

Patterns in Pyroclastic Breccia near Louisbourg

Angular rock fragments embedded in a volcanic ash matrix from a pyroclastic flow in Cape Breton Island

The entire coastline north and south of Louisbourg on Cape Breton Island in Nova Scotia, Canada, is composed of Neoproterozoic volcanic rocks dating back 575 million years. A few hundred metres north of the Louisbourg Lighthouse along the Trail to Morning Star Cove and Gun Landing Cove, lies an area of seashore that offers the chance to take a close-up look at the compositions and natural patterns in rock made of pyroclastic breccia.

Pyroclastic literally means ‘fire-broken’ and is used to describe volcanic rocks made up of fragmented pieces that are normally the result of an explosive volcanic event. Clasts are pieces of broken down rock. According to the Oxford Dictionary of Earth Sciences edited by Michael Allaby (ISBN 978-0-19-921194-4) “breccia is a coarse clastic sedimentary rock, the constituent clasts of which are angular. Breccia literally means rubble and implies a rock deposited very close to the source area. The term may also be applied to angular volcanic rocks from a volcanic vent.

Rock of a similar type of origin, although not identical, has been used by the sculptor Emily Young in the creation of the carved heads that were recently on display on Neo Bankside in London. Stillness Born of History II is described as being made of “onyx with volcanic pyroclastic brecchia”.

Stillness Born of History II at Neo Bankside

Carved stone head by Emily Young displayed at Neo Bankside in LondonBeautifully textured and patterned onyx with volcanic pyroclastic breccia has been used by the famous sculptor Emily Young to create this fabulous head called Stillness Born of History II displayed (courtesy of Bowman Sculpture) at Neo Bankside in London, England, just south of the Tate Modern Gallery. Pyroclastic breccia is composed of fine-grained volcanic ash, pumice, and rock fragments larger than 2.5 inches (63.5 mm). When the fragments are smaller than this, the rock is called tuff.

Chalk Cliff Rock Texture & Pattern

Chalk pattern and texture in cliffs at Studland Bay

Textures and patterns in chalk cliffs at Studland Bay – showing white strata and darker flint nodule layers; different degrees of weathering, fracturing and smoothing of the rock surfaces; subtle inherent variations in colouring; greyscale shading brought about by dirt and biological encrustations; and more dramatic green algal coatings on the wave-washed cliff base.

Flints & Ironstones at South Beach Studland

Beach stones derived from the chalk cliffs at South Beach, Studland, Dorset, England.

The upper part of the sandy beach at South Beach, Studland in Dorset is littered with pebbles and stones of many colours and interesting patterns and textures. They are mostly flint and ironstone that has weathered out of the chalk that forms impressive cliffs from here to Old Harry Rocks and the Foreland or Handfast Point in the distance.

Click on any image to enlarge and view in a gallery.

REFERENCES

Barton, CM, Woods, MA, Bristow, CR, Newell, AJ, Weathead, RK, Evans, DJ, Kirby, GA, Warrington, G, Riding, JB, Freshney, EC, Highley, DE, Lott, GK, Forster, A, and Gibson, A. 2011. Geology of south Dorset and south-east Devon and its World Heritage Coast. Special Memoir of the British Geological Survey. Sheets 328, 341/342, 342/343, and parts of 326/340, 327, 329 and 339 (England and Wales), 9–100.

Cope, JCW, 2012 Geology of the Dorset Coast, Geologists’ Association Guide No. 22, Guide Series Editor SB Marriott, The Geologists’ Association, 191-194.

Swanage Solid and Drift Geology (map), British Geological Survey (Natural Environment Research Council) 1:50,000 Series, England and wales Sheets 342 (East) and part of 343

Joggins Rock Textures 15

Ripples from water currents over fine sediments deposited in the Carboniferous Period preserved on the lower surface of boulders from a rock fall

Joggins Rock Textures 15a – Ripples from water movement over fine sediments deposited in the Carboniferous Period, preserved in boulders from a rock fall at Coal Mine Point (also known as Hard Scrabble Point) at Joggins Fossil Cliffs, Nova Scotia, Canada.

Ripples preserved from water currents over fine sediments in the Carboniferous Period on the lower surface of boulders from a rock fall

Joggins Rock Textures 15b – Ripples preserved in stone.

Ripples preserved from water currents over fine sediments in the Carboniferous Period on the lower surface of boulders from a rock fall

Joggins Rock Textures 15c – Ripples preserved in stone.