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 Guernsaise, 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.

On the Dorset Coast 1

Warm weather, blue skies, and hazy sun bring out visitors to enjoy the Dorset Coast in England. The milky waters of the sea at one location take on many shades of blue from navy to torquoise. Tranquil waves, lapping gently on the black and white pebbles of the beach, create natural abstract patterns of reflection. While people in leisure craft of all descriptions from motor boats, yachts, and dinghies to kayaks, take full advantage of the lovely day, and revel in the sparkling waves.

Pebbles at Langland Bay

Pebbles from Carboniferous Period rocks at Langland Bay

The pebbles at Langland Bay are all sedimentary rock as far as I can see but they include many different rock types from shores further away. Red and green Devonian sandstones, siltstones and conglomerates; light and dark bluish-grey Lower and Upper Carboniferous Period limestones (some with fossils), and shales; Namurian sandstones, grits, shales and coal measures with black and iron-bearing deposits; and no doubt the occasional erratic brought in by the ice sheets in periods of glaciation.  Consequently there is a great variety of colours, textures and patterns. Pebbles with holes made by sea creatures such as piddocks or other boring bivalved molluscs, or by marine polychaete worms and sponges are also a frequent occurrence. The pictures show the pebbles mostly in the the positions where they were found although I may have moved the odd one or two.