A Walk at Rocquaine Bay

Follow in my footsteps with a virtual walk along beautiful Rocquaine Bay on the west coast of the Channel Island of Guernsey. It is protected by a long sea defence wall which has employed different construction techniques along its length; mostly using local stone but also with along stretch of reinforced concrete (probably originating from German occupation World War II fortifications). The beach is both rocky and sandy with some pebble patches. Seaweeds of every colour abound. Huge limpets with white shells cluster on the bright orange-spattered L’Eree granite bedrock while outcrops of monochrome microgranodiorite occur on the upper shore near Fort Grey. Marine worm casts cover the softer muddy sands. Streams flow across the shore, their clear shallow water reflecting sunlight from the ripple crests and creating shadow patterns. A small stone jetty looks marooned among the rocks and a multi-coloured carpet of weed. Small boats bobbing in the turquoise water, rusty buoys and chains half-buried in seaweed, and algae-encrusted mooring ropes add to the evidence for fishing and leisure boating activities.

Click on the first picture to view the images in the gallery in the sequence that they were taken during the walk.

A Vogesite Dyke at L’Eree

Detail of a vogesite dyke at L'Eree in the Channel Island of Guernsey

Dykes formed when molten lava flowed into deep cracks and fissures in pre-existing rocks millennia ago. The lava set in a sheet form within the other rock but frequently dykes are seen on the surface today, after many years of erosion and earth movements, as lines of contrasting rock type. Dykes are composed of many different mineral combinations. When I visited Guernsey in the Channel Islands last year I discovered three types of dyke cutting through the predominantly igneous and metamorphosed rocks. I have already shown some pictures of dolerite, albite dolerite, and lamprophyre dykes, all of which seem to be quite common on the island. However, on an expedition to L’Eree on the north-west coast I spotted a dyke with a very different texture cutting east-west across the other  north-south dykes. This proved to be a vogesite dyke.

Vogesite has a very characteristic texture made up of rounded mineral inclusions – ‘large euhedral amphibole phenocrysts’  – set in a fine grained groundmass of plagioclase feldspar, alkali feldspar and quartz (Roach et al. 1991).

REFERENCES

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernsaise, ISBN 0 9518075 2 8, 30-34.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 9 – Jerbourg Peninsula, 76. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, p 22.

Fault Zone Rocks at Clarke Head Part 3

Broken rocks on the beach at Clarke Head, Nova Scotia, Canada.

More pictures of the beach at Clarke Head showing the multiplicity of rock colours, patterns, and textures of the boulders, broken rocks, and small shards derived from the jagged cliffs of the fault zone. Looking at these photographs now, I am transported right back to the great time I had exploring this location – one of many that I visited on my rock hounding tour of Nova Scotia earlier this year.

 

Fault Zone Rocks at Clarke Head Part 2

More pictures of rock textures and patterns seen on the shore at Clarke head, near Parrsboro, Nova Scotia. At this site the fault zone sedimentary rocks include Blomidon Formation Triassic red sandstone and siltstone with strata laid down in repeating cycles, and preserved water ripple marks much in evidence. Igneous North Mountain Formation basalt from Jurassic period rift volcanism is present high in the cliff and not shown here for lack of accessibility. Light grey sedimentary Windsor Group Carboniferous limestone strata is also present. Large blocks of Precambrian metamorphic rock have been brought up from deep down by the faulting. These blocks, sometimes huge, are found in the breccia and include garnet-grade schist. Gypsum is common in the breccia, as boulders and as matrix. The boulders from the mega-breccia weather out from the cliff deposits and lie together with numerous smaller boulders, shards, and fragments littering the beach, varying in colour and composition as you walk along the waterline.

REFERENCES

Nova Scotia Field Guide, Arthur D. Storke Memorial Expedition, Department of Earth and Environmental Sciences, Columbia University in the State of New York, August 23 to September 2, 2012.

Fault Zone Rocks at Clarke Head Part 1

A mélange of rock textures from the fault zone at Clarke Head, near Parrsboro in Nova Scotia, Canada. The geology here is extremely complex and I have only just begun to unravel what is going on. Key research papers with precise details are not easily accessible. Others are a bit too generalised to enable me to identify exactly each rock type that I photographed….for the moment. I will update when I can be sure I have accurate identifications. The variety was wide and included igneous, sedimentary and metamorphic rocks. It is the same place that I photographed the satin spar gypsum. The colours, textures, and patterns are amazing.

Satin Spar Gypsum at Clarke Head

Crystal veins of satin spar gypsum in fault zone rocks

Clarke Head near Parrsboro in Nova Scotia, Canada, is famous for its melange of fault zone rocks. It lies on the Cobequid Fault Line that runs approximately parallel to the north shore of the Minas Basin, part of the Bay of Fundy. Clarke Head is the most southerly exposure of the fault and the area is characterised by smaller fault strands coming off the main fault, dividing different ages of rocks into adjacent blocks and giving rise to much breccia. Breccia is the deposit that is often generated at fault zones and is comprised mostly of angular broken rock fragments. Some of the breccia has massive stones measuring up to 1.5 meters in diameter and is known as megabreccia. Three main rock types are represented at Clarke Head. These are the Blomidon Formation of Triassic red sandstone and siltstone; the North Mountain Formation basalts of Jurassic age; and light gray Windsor Group limestone of Carboniferous age. However, extensive areas are so affected by faulting action that the constituent rocks are all jumbled up.

The fibrous satin spar gypsum in the images shown here permeated the brecciated areas where it crystallised in deep cracks extending through the deposits in twisted sheet-like form.

Water-worn Limestone 3

Natural sculpturing of limestone on the Worms Head Causeway in Gower, South Wales.

You would think that all the limestone strata on the Worms Head Causeway in Gower would be worn down equally to a smooth, flat, even surface – but not so. Upstanding at various points on what I suppose is really a wave-cut platform (albeit eroded by acid rain and seashore creatures as well), isolated areas remain standing. They look like giant teeth embedded in the worn surface strata. I do not know why these areas are more resistant, however, I have read that some parts of the limestone become harder by dolomitisation, a process in which the calcium carbonate is converted to magnesium carbonate by the intrusion of seawater (I think before the original sediments harden and compact). Maybe that is the explanation.