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.

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.

Beach Stones with White Lines & Patterns

Natural pattern in a Charmouth beach stone

Just east of the outlet of the River Char on Charmouth beach, in the area close to Raffey’s Ledge, the upper shore is strewn with many large irregular stones. Amongst these, the most noticeable are those with white patterns and lines, which on closer inspection turn out to be crystalline calcite-filled cracks in the matrix of the rock. I have been looking at these strange stones over the years and wondering what they were (see the earlier post Pebbles with white lines on Charmouth beach). Now I think I have the answer. They are the worn remnants of the inner cores of Birchi Nodules. Birchi Nodules appear high in the cliff above this section of shore and have a complex structure resulting from a series of processes in the sediments that took place millions of years ago before the sediments compacted into rock. The large ovoid or discoid Birchi Nodules can be seen scattered along a line below the more continuous stratified rocky Birchi Tabular Bed at the top of the cliff. The rest of the cliff below is mostly composed of darker thinly-bedded shales.

These remnants of the inner cores of Birchi Nodules are also a kind of septarian nodule. The stones illustrated here from Charmouth are partial remains that have been worn smooth by rolling around on the beach for a long time. Further east along the coast at Ringstead I have seen complete septarian nodules  that have freshly fallen from the cliff face of a different type of rock formation  (Septarian Nodules at Ringstead).

[I found out about Birchi Nodules from the most excellent on-line resource for the geology of the Dorset Coast written by Ian West. This is a veritable cornucopia of information but requires that you continuously scroll down the page to locate the items in which you are interested. It is well worth the effort if you really want to find out the information.]

Pebbles in a Stream

The bright sunshine created wonderful effects on the rapidly moving, crystal clear water of a chalk stream as it flowed over rounded pebbles. Light reflected from the rippled surface of the water. The stones below were covered with some sort of brownish algae that disturbed the flow and made either rainbow-coloured prisms or golden patterns of reflection.

Pebbles at Havelet

There are different combinations of colours and patterns in the pebbles of different beaches in the Channel Island of Guernsey. The assortment of pebbles in each location reflects the local geology of that area. There is a higher proportion of metamorphosed rocks compared with igneous rocks on the southern coast of the island, as seen here for Havelet which is just west of St Peter Port. Many of the rocks belong to the Southern Metamorphic Region dating back as far as 2,500 million years ago. Rocks from this region include gneisses and schists. The pebbles at Havelet can be compared with accumulations of pebbles on the north coast, such as those at Albecq, where the stones are mostly derived from younger igneous rocks such as granite, diorite and gabbro dating from about 700 million years ago in what is known as the Northern Igneous Complex.

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.

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, pp 21 – 22, & 75 – 78. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0.