Rock Texture & Pattern at Main a Dieu

The wooden boardwalk from the Coastal Discovery Centre at Main á Dieu on the southeast coast of Cape Breton Island, in Nova Scotia, Canada, leads to a look-out platform that is built on top of a rocky outcrop. The rock is a basalt volcanic lava flow dating from the Neoproterozoic Period around 560 million years ago. The basalt is characterised by many interesting natural fracture patterns; veins and weathered surfaces of contrasting colours; and different textures depending on exposure to aerial or aquatic erosional elements.

REFERENCES

Atlantic Geoscience Society (2001) The Last Billion Years – A Geological History of the Maritime Provinces of Canada, Atlantic Geoscience Society Special Publication No. 15, Nimbus Publishing, ISBN 1-55109-351-0.

Barr, S.M. (1993) Geochemistry and tectonic setting of late Precambrian volcanic and plutonic rocks in southeastern Cape Breton Island, Nova Scotia. Can. J. Earth Sci. 30, pp. 1147-1154.

Donohoe, H. V. Jnr, White, C. E., Raeside, R. P. and Fisher, B. E, (2005) Geological Highway Map of Nova Scotia, Third Edition. Atlantic Geoscience Society Special Publication #1.

Hickman Hild, M. and Barr, S. M. (2015) Geology of Nova Scotia, A Field Guide, Touring through time at 48 scenic sites, Boulder Publications, Portugal Cove-St. Philip’s, Newfoundland and Labrador. ISBN 978-1-927099-43-8, pp. 66-69.

Keppie, J.D., Dostal, J. and Murphy, J.B. (1979) Petrology of the late Precambrian Fourchu Group in the Louisbourg Area, Cape Breton Island. Paper 79-1, Nova Scotia Department of Mines and Energy.

St Ann’s Provincial Park

St Ann’s Provincial Park along the Cabot Trail in Cape Breton Island, Nova Scotia, Canada, was just springing into life after a cold winter when we visited earlier this year. It was a brief stop for a picnic lunch on our way from the Cape Breton Highlands National Park to the Louisbourg area on the east coast. The park  lies on the northern shore of the stretch of water known as North Gut, and has a short trail leading to a look-off where there are views over the saltmarsh and St Ann’s Bay. We did not have time to venture very far down the trail but, even by the car park, there was plenty to enjoy.

Bright green ferns of various types were uncurling their fronds. The compacted fern buds are called fiddleheads. Particular varieties in some localities are a feature on menus at this time of the year (we tried some and they were delicious). Golden mosses covered the ground, while bladed marsh plants were breaking through the winter’s debris on the water margin. Delicate white blossoms quivered on trees of the woodland edge. The greatest delight was catching sight of a snake making its way through the leaf litter. I am not certain what sort but it might possibly be a Maritime Garter Snake.

A Visit to Crystal Cliffs Beach

Beach stone with range tinted gypsum crystal in limestone at Crystal Cliffs Beach

Crystal Cliffs Beach lies a few miles from Antigonish on the north coast of Nova Scotia, Canada. It overlooks St George’s Bay close to the Northumberland Strait. It consists of a sand and pebble spit that dams back the water of Ogden’s Brook to form a large shallow lake known as Ogden’s Pond. The waters are tidal as there is a narrow inlet/outlet to the sea. In winter, the lake is more extensive as evidenced by the quantity of dead vegetation visible in marginal marshy areas. The ripples of the slowly moving water in the Pond reflected intricate patterns of blue sky and white clouds.

Boulders and pebbles dominate the upper levels of the spit, along with blanched driftwood, and sparse vegetation such as marram grass. The lower levels are mostly coarse sand. Occasional mammal bones rest on the tide line, perhaps from a seal. Cobble-size and larger beach stones of limestone, sandstone, and conglomerate are strewn across the shore – but the most noticeable and are the ones with orange and white crystals of gypsum that have come from the nearby cliffs that give the beach its name. The cliffs are composed of Early Carboniferous Limestone belonging to the Windsor Group with substantial gleaming surfaces of white gypsum. Viewed from the sea by kayak, the cliffs are said to be a marvellous sight. The only part visible from the beach at high tide, at this particular time, showed a relatively recent and massive rock fall defacing that outcrop.

The sea water lapping against the sand, on this crisp and sunny spring day, was crystal clear, revealing through a distorting lens of saline the multitudes of coloured pebbles on the seabed. The wave-textured surface made abstract patterns of sunlit reflections. It was a beautiful place to experience.

Shell Sand on Herm

Image showing size of small seashells in shell sand from Herm in the Channel Islands

The sand on the Island of Herm, which is one of the Channel Islands, is mostly made of shells and shell fragments. A good place to examine the sand is Belvoir Bay where waves and currents wash shells ashore and break them up. The small cove lies at the foot of modest cliffs of Herm Granodiorite with xenoliths; and eroding rocky outcrops strew the shore at the base of the cliffs. Hollows and crevices in these rocks are filled with coarse shell sand containing many intact little shells of both bivalve and gastropod molluscs. Even minute sea urchin tests survive. I took a handful of the sand home to photograph against a scale, and compare them with some mature-size shells from the same beach and nearby Shell Beach. I have fond memories of visiting the island and collecting shells there forty years ago.

Herm Granodiorite with Xenoliths

Macro photograph of crystals in Herm Granodiorite with a xenolith

The delightful small island of Herm lies just a short boat ride away from Guernsey in the Channel islands. The entire island is made of the Herm Granodiorite (de Pommerai and Robinson 1994). This is an intrusive igneous rock that formed below the surface of the earth, probably during the later part of the Cadomian age which lasted from about 550 to about 700 million years ago. The southern part of the island is higher than the northern part. In the south, the rock comprises a plateau with a height around 60 metres. The rock has been extensively quarried and exported in the past. The stone is particularly hard and ideal for structures like kerbstones; examples of these still feature on the Thames Embankment in London.

To the north, the area is covered by wind blown sand that hides an old wave-cut platform, glimpses of which can be seen as jagged low-lying reefs offshore.. The sand is wholly composed of shells with not only fragments but also a high proportion of undamaged miniature molluscs and sea urchin tests. The underlying Herm Granodiorite is similar to some varieties of the Bordeaux Diorite occurring in Guernsey – typically made of feldspar and quartz with some biotite and hornblende crystals. One of the main characteristics of the Herm Granodiorite is the inclusion of many contrasting lumps of other igneous rock types known as xenoliths. There is a good exposure of this rock type on Mouisonniere Beach near a stone obelisk on the marram covered dunes. [The obelisk is a navigation marker that has been constructed on the site of an earlier Neolithic standing stone (dolmen) which was taken away by quarrymen in the 19th century].

The rocky outcrop on the sandy beach is full of xenoliths. It is of special interest to geologists because of the variety in the composition and shape of the xenoliths indicating a series of different processes were involved. Some are dark, angular and made of diorite. Others are paler and more rounded; often they have a rim of darker material. The combination of crystal types and sizes varies in each type of xenolith compared with the rock in which it is embedded. The causes and possible circumstances that led to the formation of these different sorts of xenoliths are the subject of much discussion among the experts.

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 Guernesiaise, ISBN 0 9518075 2 8, pp 56 – 62.

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 1 – The St Peter Port Gabbro, 76. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pp 4 -5.