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.

Pebbles at Albecq

Colourful and patterned pebbles of igneous rocks

The colours and patterns of the pebbles on the beach at Albecq on the north coast of Guernsey in the Channel Islands reflect the varied geology of the north of the island. The northern part of Guernsey is made up of igneous rocks such as granite, diorite and gabbro and formed about 400 – 500 million years ago (de Pomerai & Robinson 1994 pp 5 & 7). Most of the pebbles in these photographs would have originated in nearby bedrock that would include Cobo Granite, Icart Gneiss, Perelle Gneiss Group, and Bordeaux Diorite Complex. These rocks are constituents of what is known as the Northern Igneous Complex. The patterns in the rocks are due to the coarse mineral crystal composition. Potassium-rich orthoclase feldspar is responsible for the recurring pink-orange colour, with light grey plagioclase feldspar; quartz grains are transparent; and biotite mica is seen as small black shiny crystals (de Pomerai & Robinson 1994 p 49).

REFERENCE

de Pomerai, M. and Robinson, A (1994) The Rocks and Scenery of Guernsey. La Societe Guernesiaise, St Peter Port, Guernsey. ISBN 0 9518075 2 8.

 

Beach Stones with Iron at Rhossili

A beach stone made of iron on the Gower Peninsula in South Wales

When you visit a place often enough, you think you have seen all there is to see. For all the photographs of rocks and pebbles that I have taken over the years, I don’t think I had ever noticed so many stones made of iron as I did on my last trip to Rhossili on the Gower Peninsula, South Wales. Mostly they were lying in the narrow strip of pebbles, cobbles, and boulders that is exposed on the upper reaches of the sandy shore, right at the base of Rhossili Down.

Some of these iron stones appear to be made entirely from iron (maybe iron carbonate) while others were sedimentary rock with a solid iron centre, or with groups or bands of small iron nodules. One stone was stratified with black and rust layers. All of them were very heavy and many exhibited concretionary layers. I know very little about these stones. Their forms of iron are distinctly different from the haematite form that I have previously shown in other blog posts about Gower geology, and from the iron pyrites nodules I have featured from Dorset’s Jurassic rocks. Those stones with groups or layers of smaller nodules look like the siderite form of iron that I photographed at Joggins in Nova Scotia. Iron is recorded in Lower Carboniferous rock strata and also in the Coal Measures higher up in the geological sequence, so iron stones on this Gower beach make general sense but they may have been transported there from another location. Clearly more investigation is required.