Rocks at Trabeg on the Dingle Peninsula

Close-up of the Devonian conglomerate at Trabeg on the Dingle Peninsula

The sand looks black from a distance as you descend to the shore at Trá Chathail near An Trá Bheag (Short Strand) – otherwise known as Trabeg. The path cuts down deep through the stratified red rocks to get to the beach which is strewn with pebbles, mostly shades of red, maroon, green, grey, and white.

Trabeg is on the south coast of the Dingle Peninsula in Ireland, and is the “type section” of the Trabeg Conglomerate Formation which is exposed in the cliffs on the beach. This is place where that particular rock type was first described. The rock layers constitute part of the Dingle Group and were formed in the Devonian period between 345 and 395 million years ago. The conglomerates are composed of fairly well rounded pebbles of red sandstones and mudstones, with white vein quartz and chert. A few pebbles of volcanic rock and of grey limestone are also present.

The way in which the conglomerate rock has formed from the mass movement and subsequent accumulation of debris from terrestrial locations during, for example, river flood events, means that the pebbles are derived from a wide area covering many different geological types. The pebble beds or conglomerates are inter-bedded with layers of red sandstones and mudstones, the finer sediments of which were deposited normally by rivers during non-storm/flood times. The alternating layers are now tilted from the original horizontal orientation in which they were first deposited, and are clear to see dipping south at about 70 degrees.

As the cliffs at Trá Chathail are worn away by the action of waves and weathering, the pebbles contained in the conglomerate matrix are freed up and remain the shore below – an instant pebble beach. Added to these are pieces of other rock or matrix that became rounded into pebbles after they arrived on the beach. Some pebbles and rocks may have been transported by wave action from further along the coast were the geology is quite different: from the Eask Formation, West Cork Sandstone, Bulls Head Formation, and the earlier Silurian rocks of the Dunquin Group.

REFERENCE

Horne, Ralph R. (1976) Geological Guide to the Dingle Peninsula, Geological Survey of Ireland Guide Series No. 1, reprinted 1999.

Beach Stones at Corney Brook

There were no other visitors on the dull day that we turned off the Cabot Trail to look at the Corney Brook shore in the western Cape Breton Highlands. There was very low cloud cover, and it began to rain after a while, but there were treasures to be found – at least if you are like me and are fascinated by beach stones. Three main rock types are found at Corney Brook. The oldest are Neoproterozoic-Ordovician granitic pluton rocks of the Bras D’Or Terrane. Ordovician-Silurian metasedimentary rocks of the Aspy Terrane are slightly younger. And red sandstones and conglomerates belonging to the Horton Group come from the Devonian to Carboniferous Period.

The stones on the beach include all three types and probably a lot more due to the glaciation of the area. I wish I could identify and tell you the exact composition of each photographed pebble, stone or boulder – but that is tough for an amateur to determine. There is a great variety of colour, pattern, and texture to the stones which look dull when dry but amazing when wet. They include igneous and metamorphosed rocks like granite, gneiss, schist as well as sedimentary rocks like sandstone. It is possible to see just how difficult it is to not only understand the texts but also to convert into straight forward language for the non-specialist reader from the following detailed description that I discovered about the Corney Brook schist by Jamieson et al. (1987).

Comey Brook schist (unit 3d)
Medium- to high-grade pelitic and semi-pelitic schists, with minor marble and psammite, occurring on the Cheticamp River, Corney Brook, the northeastern end of Jumping Brook, and the central highlands near Calumruadh and Coinneach brooks, are referred to here as the Corney Brook schist. This unit is equivalent to the “medium grade belt” of Craw (1984). Pelitic and semi-pelitic members of the unit characteristically contain coarse staurolite, biotite, and garnet porphyroblasts, with kyanite at the highest grade, in a phyllitic to schistose matrix. Medium- to high-grade marbles, quartzites, albite schists and hornblendite recognized in the Corney Brook area (Plint et al., 1986) have not yet been identified south of the Cheticamp River. Centimetre- to metre-scale compositional layering, folded by tight to isoclinal folds, is interpreted as transposed bedding. Based on bulk compositions and rare relict primary textures, the Corney Brook schist is interpreted to have formed as a suite of clastic sediments interlayerd with felsic tuffs and minor basic flows – that is, it appears to represent the higher grade equivalents of units 3a-3c.

The softer sandstone cliffs are being eroded back by the sea. This has implications for the ground higher up and the roadway further back from the shore. For this reason a sea defence structure has been emplaced to protect the base of the cliffs. This is a gabion made of wire cages full of large beach stones and boulders that are stacked up into a wall, positioned at the most vulnerable part of the shore.

Rock Texture & Pattern at Black Brook Cove

Patterns of dykes in granite in the cliffs at Black Brook Cove

Black Brook Cove along the Cabot Trail in Cape Breton Island, Nova Scotia, gets its name from the dark colour of the river water which flows into it. On the southern edge of the cove, the upper banks of the estuary are piled high with large bleached driftwood lying on a bed of boulders and pebbles. Curving banks of pebbles on the main body of the beach give way to smooth waterworn rock outcrops; and spectacular jagged cliffs surmounted by pines form the northern arm of the cove.

The rocks at Black Brook Cove are part of the Devonian Black Brook Granitic Suite formed about 375 million years ago. They are igneous plutonic rocks. The magma from which they formed was created by the melting and recrystallization of meta-sedimentary rocks that were sub-ducted during the collision of the ancient land masses called Ganderia and Avalonia.

The remarkable feature of the rocky outcrops at Black Brook Cove, and at Green Cove just a little further south, is the number of criss-crossing dykes or veins of contrasting colour that create abstract angular patterns on the rock surfaces. These patterns and colours are accentuated when the rock is wet. The whole beachscape is captivating on a bright sunny afternoon but the area must look its best after a heavy downpour of rain.

The main rock is a grey granite with small black flakes of biotite. Earth movements and increased pressures on numerous occasions subsequent to its emplacement have cracked the rock and opened up fissures into which certain minerals that were squeezed out of the mother rock have entered and recrystallized. Mostly the veins formed in this way are composed of aplite or pegmatite. Both are pink-orange in colour Aplite is made of quartz and feldspar and is fine-grained with a smooth sugary texture. Pegmatite is darker and coarser with large visible individual crystals of quartz, feldspar and mica in both the black biotite and clear muscovite forms.

REFERENCES

Anoiyothin, W.Y. and Barr, S.M. (1991) Petrology of the Black Brook Granitic Suite, Cape Breton Island, Nova Scotia. Canadian Minerologist, Vol. 29, pp. 499-515.

Barr, S.M. and Pride, C.R. (1986) Petrogenesis of two contrasting Devonian Granitic Plutons, Cape Breton Island, Nova Scotia. Canadian Minerologist, Vol.. 24, pp. 137-146.

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. 94-97.

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.

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.

Pebbles at Pleasant Bay

Wet pebbles at the water's edge in Pleasant Bay, Cape Breton Island, NS.

We visited Pleasant Bay on a misty May day. It lies on the Cabot Trail in Cape Breton Island, Nova Scotia, Canada. Pleasant Bay is a small village first settled by Scottish immigrants and is nestled around a picturesque fishing harbour at the foot of steep hills. The Grande Anse River meets the sea at this point and in the background are the headlands and mountains of the Blair River Inlier composed of some of the oldest rocks in the world. The village itself lies on Carboniferous sedimentary rocks but these are less well represented in the pebbles on the beach than the more ancient igneous and metamorphic rocks like granites, gneisses and schists that have been transported downstream from the surrounding highlands. You can compare these smooth rounded wave-worn beach stones with the angular rock fragments lying on the river bed at MacIntosh Brook and the Grand Anse River near Lone Shieling not too far away.

Irish Beach Stones

Aside

Irish Beach Stones

If you are as fascinated by beach stones as I am, you will definitely enjoy looking at the new web site by Noel Tweedie at The 365 Beach Stone Exhibition where he has amassed a great collection of photographs and artwork showing amazing beach stones from the Inishowen area in the north of Ireland. His images reflect the incredible geology of the area.

Stones with holes made by Wrinkled Rock Borers (& other seashore creatures)

Beach stone with holes made by seashore creatures at Charmouth, Dorset, England.

Beach stones with holes in them excite the curiosity of most people. How did the holes get into the rock? There is no single answer but in many cases the holes in pebbles and beach stones have been made by various seashore creatures including several types of bivalve molluscs, marine worms, and sponges. The same creatures can also make holes in thick old seashells. There are  several earlier posts on Jessica’s Nature Blog describing how the holes are made, by piddocks, for example Pholas dactylus, sponges such as Cliona celata, and polychaete worms like Polydora ciliata and Polydora hoplura. Frequently, there is evidence for more than one type of organism occupying the same stone.

One of the bivalve molluscs that creates holes in stones and thick oyster shells is the Wrinkled Rock Borer Hiatella arctica (Linnaeus). Like the piddock, this species can actively excavate a burrow in soft stone for shelter and protection although unlike the piddock it can attach itself by byssus threads to the outside of solid objects or in cracks and crevices. However, once embedded in the stone it can no longer exit the burrow but obtains all it needs for sustaining life via the tunnel connecting it to the outside world. Wrinkled Rock Borers are smaller than piddocks, measuring no more than 3.8 cms in length when mature. The valves of the shell are thick and robust with distinct furrows, and the leading edges exposed to view in the burrow are rough and straight edged (truncate). Tebble (1966) says that it is not possible to distinguish between the different species of Hiatella in British waters but the descriptions apply to all the species ever recorded here. Hiatella arctica is common around the British Isles from the lower regions of the shore to considerable depths…… It has a wide geographical distribution in the northern hemisphere from the Arctic south through the Atlantic, Mediterranean and Pacific, but the particular limits of its occurrence are not known. It is almost impossible to remove empty Hiatella shells from the excavated holes without breaking them.

Hiatella holes in rock can be secondarily occupied by a similar but smaller bivalved mollusc called Irus irus (Linnaeus). This grows to about 2.5 cms in length. The protruding frill-like concentric ridges on the shell can be very distorted in shape if the shell is occupying a burrow that is too small to allow normal growth. I am not able to discount the possibility that it is Irus shells occupying Hiatella burrows in some of the beach stones illustrated here. Irus (also known as Notirus irus) occurs from low in the littoral zone to a few fathoms.

REFERENCES

Tebble, N (1966) British Bivalve Shells: A Handbook for Identification, published for the Royal Scottish Museum by HMSO, Second Edition 1976, [Hiatella p172-173 & Plate 7h; Notirus p124-125 & Plate 7g].

Hunter, W. R. (1949), The Structure and Behaviour of ‘Hiatella gallicana@ (lamarrck) and ‘H. arctica’ (L.), with special reference to the Boring Habit. Proc. Roy. Soc. Edin. B, 63 III (19): 271-289, 12 figs.