Worm Casts at Pembroke Bay

Lug worm casts and blow holes on a sandy beach

Lug worm casts and blow holes were widespread over the low-tide sand at Pembroke Bay on the Channel Island of Guernsey when I visited in early October. They had a more scattered distribution and the casts were not so fine as those I have seen on other parts of the island at Rocquaine Bay and Cobo Bay. Two species of Arenicolidae have been recorded for Guernsey and I wonder if I have been looking at the burrows and traces of the two different types. Here on the beach at Pembroke Bay I think they could well be Arenicola marina (Linnaeus) whereas those I had photographed else where could be Arenicola ecaudata Johnston which prefers the rich mud between stones or in rock crevices at low water. Both types of cast are shown in the gallery below. Click to enlarge the images and see the descriptions.

Ebb Traces in the Sand at Whiteford

Ripples patterns in wet sand at the beach

Ripple patterns in wet sand on the seashoreThe shape of natural abstract sand sculptures, like these ripples on the seashore, result from complex interactions of water and substrate which are the subject of much research in the field of fluid mechanics. They are described as “small-scale three-dimensional bedforms due to interactions of an erodible bed with a sea wave that obliquely approaches the coast, being partially reflected at the beach” (Roos & Blondeaux 2001). Different combinations of three main perturbation agencies create different ripple designs.

REFERENCE

Roos, P.C. and Blondeaux, P. (2001) Sand ripples under sea waves. Part 4. Tile ripple formation, J. Fluid Mech.  vol. 447, pp. 227-246.

Ripples patterns in wet sand at the beach

Ripples patterns in wet sand at the beach

Ripples patterns in wet sand at the beach

Ripples patterns in wet sand at the beach

Ripples patterns in wet sand at the beach

Gneiss Rocks & Metasediments at Havelet

Metamorphic rock textures and patterns

Each beach you visit as you travel around the coast of the Channel Island of Guernsey seems to reveal a different combination of rock types. On the southern part of the island these are predominantly metamorphic rocks. In Havelet Bay close to St Peter Port, just south of the main rock bathing pool, and before the aquarium, metasedimentary rocks and gneisses outcrop on the pebble beach where the waves have polished the rock surfaces to reveal the patterns and textures. The actual contact between the Perelle Gneiss and the metasediments cuts through the centre of the swimming pool. The metasediments are described as interbanded semi-pelitic schists with migmatites (Roach et al. 1991).

Wave action warning sign at Havelet bay in GuernseyI particularly remember this site because I nearly got knocked off my feet by a freak wave. I did not realise until later when I came across this sign that the ferries and other large ships going into and out of the harbour at St. Peter Port can create a wash with great impact especially around high tide.

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 91-94.

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.