Rock Texture & Pattern at Dog’s Bay

Posted on August 10, 2017 by winderjssc

Natural pattern and texture in igneous rocks

Juxtaposition of granite and intrusive dike material

Contrasting igneous rock types at Dogs Bay

The rocks at Dog’s Bay in Connemara, Ireland, are part of the Galway Batholith. In particular they are composed of the Errisbeg Townland Granite riven by faults and many dikes containing other younger intrusive igneous rocks. The juxtaposition of the different rock types is a phenomenon marked by contrasting, colours, textures, and patterns. The whole rocky terrain has been levelled off and smoothed by ice sheets and reflects many glaciation features.

Feely, M. Leake, B.E., Baxter, S. Hunt, J. and Mohr, P. A Geological Guide to the Granites of the Galway Batholith, Connemara, western Ireland.Geological Survey of Ireland, 2006.

Volcanic Tuff near Louisbourg Lighthouse – Part 5

Posted on July 6, 2017 by winderjssc

Rock texture in solidified intrusive lava

Rock texture in compacted volcanic ash or tuff

Rock texture in compacted volcanic ash or tuff and intrusive solidified lava

Rock texture in boulders of compacted volcanic ash or tuff

Not all the rock exposed at Louisbourg Lighthouse is composed of tuff. Molten lavas intruded the tuff at later stages forming harder bands of igneous rock with a contrasting greenish colour and distinct fracture patterns. The textures of the two kinds of rock are very different.

Volcanic Tuff near Louisbourg Lighthouse – Part 4

Posted on July 5, 2017 by winderjssc

Fine layers of ash in volcanic tuff from Cape Breton in Nova Scotia

More rock textures from the compacted ash in tuff deposited from explosive volcanic eruptions during the Neoproterozoic period at Louisbourg in Cape Breton, Nova Scotia, Canada.

Volcanic Tuff near Louisbourg Lighthouse – Part 3

Posted on July 1, 2017 by winderjssc

On a whole range of scales, there are variations to the simple layering of the tuff (which is made of volcanic ash) and constitutes swathes of faintly striped and banded rock on the shoreline at Louisbourg on Cape Breton Island in Nova Scotia. Subsequent to the deposition and consolidation of the volcanic ash into tuff rock, the build-up of great pressures from earth movements at different times during geological history has caused both minor and major fractures in the rock. Small cracks sometimes filled up with dissolved minerals that crystallised to form veins of contrasting coloured material. In other places, intrusive molten lava squeezed its way into weak areas between or across the layers forming large-scale dikes. The igneous rock type of the dikes may be a greenish colour, and often cracks upon weathering in a characteristic way giving it distinct fracture patterns that are not present in the tuff.

[We stayed at the most excellent Louisbourg Harbour Inn while we explored this part of Cape Breton Island.]

Volcanic Tuff near Louisbourg Lighthouse – Part 2

Posted on June 30, 2017 by winderjssc

The volcanic ash deposits or tuff found in coastal rocks around Louisbourg Lighthouse in Cape Breton show subtle colour banding. Originally, ash from volcanic eruptions fell into lakes occupying the valleys around the volcanoes, and accumulated in horizontal layers, each representing an individual eruption event. The colours of the ash layers differed slightly according to the content and the temperature. When ash remained very hot on its journey through the air from the volcanic vent, the particles often melted together on landing, forming welded tuff. Welded tuff has a purple colour instead of the more normal shades of grey. We can see the layers as colour bands because we now see the layers of consolidated ash in cross-section. The layers were originally deposited in horizontal beds in water. Over the great period of time that has elapsed since deposition (575 million years) earth movements have brought the layers into an almost vertical orientation so that they are now viewed end on.

The textures are varied but in a quiet way with combinations of different sized fragments and changes of hue in the finer ash and small pyroclastic rock pieces. One of the images below shows an example of a volcanic bomb. This was in the first instance a glob of molten lava that was spewed from the vent along with the ash, becoming rounded in shape as it fell through the air, and then landing and forming a depression in the soft ash surface. Subsequent ash falls buried it.

[We stayed at the most excellent Louisbourg Harbour Inn while we explored this part of Cape Breton Island.]

Volcanic Tuff near Louisbourg Lighthouse – Part 1

Posted on June 29, 2017 by winderjssc

The rocks around Louisbourg Lighthouse on Cape Breton Island in Nova Scotia belong to the Lighthouse Point Member of the Main-a-Dieu sequence (formerly assigned to the Fouchu Group). They were deposited 575 million years ago, following a series of explosive volcanic eruptions in a subduction zone at the edge of a continental plate. They are the youngest rocks from the Coastal Belt and represent the end of an era of violent volcanic eruptions in this part of Avalonia. The pyroclastic deposits around Louisbourg Lighthouse are made of compacted pumiceous ash in ignimbritic units, and the resulting rock is called tuff. Layers within the tuff, distinguished by varying shades of grey, represent a series of separate eruption events. The grey layers transition into a purple layer of welded tuff where the ash and debris remained hot enough to melt the individual particles together as they landed.

[We stayed at the most excellent Louisbourg Harbour Inn while we explored this part of Cape Breton Island.]

Rocks composed of volcanic ash (tuff) near Louisbourg Lighthouse in cape Breton, Nova Scotia.

Seatown Beach Boulders

Posted on May 26, 2017 by winderjssc

Strange shaped boulder splashed by waves on the pebble beach at Seatown in Dorset

Boulder splashed by waves at Seatown in Dorset

Boulder on the pebble beach at Seatown in Dorset

Boulder on the beach at Seatown on the Jurassic Coast

View looking east at Ridge Cliff and East Ebb on Seatown beach in Dorset

Boulders on the beach at Seatown on the Jurassic Coast

Boulders on the water's edge at Seatown in Dorset

Boulders from many rock falls at East Ebb beneath Ridge Cliff at Seatown in Dorset

Boulders on a pebble beach washed by waves

Boulders in the waves of the ebbing tide at Seatown in Dorset

Boulders of various Jurassic rock types at Seatown in Dorset

Strangely shaped boulder on the beach at Seatown in Dorset

Boulders at East Ebb beneath Ridge Cliff at Seaton in Dorset

As you walk east along the shore at Seatown in Dorset, you reach Ridge Cliff from which numerous boulders have fallen over the years, and accumulated across the beach and into the water. What is most interesting is the great variety of shapes, colours, textures, and compositions. They represent all the different strata that make up the 80 metre high cliffs.

Seatown Shattered Eype Clay

Posted on May 25, 2017 by winderjssc

Pile of Eype Clay Member debris from a minor land clip on cliffs at Seatown

Fractured piece of Eype Clay Member material from a minor rock fall on cliffs at Seatown

Lump of dry shattered Eype Clay on a pebble beach

Strange rusty marks in a fallen piece Eype Clay Menber at Seaton, Dorset

Precarious cliffs at Seatown, Dorset, are subject to land slips and rock falls

Odd rusty formations in a piece of broken Eype Clay at Seaton in Dorset

Shattered pieces of Eype Clay from a minor cliff fall

Heap of Eype Clay Member debris from a minor rock fall on cliffs at Seatown

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.

Rocks & Pebbles near Twlc Point

Posted on May 15, 2017 by winderjssc

Rocks and pebbles near Twlc Point at Broughton Bay on the Gower Peninsula in South Wales

Pebbles near Twlc Point at Broughton Bay on the Gower Peninsula in South Wales

Beach boulder and pebbles at Twlc Point in Gower

Fossilierous beach stone at Twlc Point in Broughton Bay, Gower, South Wales

View from Twlc Point in Broughton Bay, Gower.

Detail of the pattern in a beach boulder at 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

Posted on April 30, 2017 by winderjssc

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