Patterns and textures in cliffs of Upper Carboniferous rocks, also called the Lower Coal Measures, showing stratification, cross-bedding, and rust deposits. The rocks can be seen on the beach at Saunderfoot, Pembrokeshire, Wales, near Coppet Hall.
This gallery displays a selection of the most colourful and interesting rocks that have been featured in posts here at Jessica’s Nature Blog over the past couple of years. While I am out walking on beaches, I am always drawn to the colours of the rocks, sometimes bright and other times more subtle, and the many different patterns and textures. Initially it is the way that the rocks look that is so appealing. So much of what I see seems like amazing natural abstract art. I try to frame the composition so that it stands alone as an attractive image in its own right. But then I get curious and lots of questions come into my mind. I always want to know what kind of rock is it? What is it called? How old is it? What is it made of? How did it get to look like that? What happened while the rock was buried? What are the elements doing to it now that it is exposed?
As an amateur with a keen interest in geology, I start by looking at maps. I try to pinpoint the exact location where I photographed the rock. Then I try to get hold of the correct geology map. Geology maps have a lot of information about the age of the rock, the type, the period in which it was laid down or developed, as well as the distribution of the different rock types in the locality. Often there are references to special papers, memoirs and so forth that discuss the geology of the area. Sometimes these publications are available on-line. I do a lot of Googling. Sometimes a visit to the library is needed. Libraries and the internet don’t always have the information I am seeking so I buy books too. Sometimes books about a specific place, and sometimes more general textbooks. I need those too because it is quite difficult to understand everything. Geology is a complex subject with a great deal of specialist terminology.
Once I am fairly certain what the rocks are, I try to write a bit about them in a straightforward way so that anyone else who is truly interested will be able to understand. It is fascinating. Slowly I learn more about the rocks and can fit the pieces together into the bigger picture. Walking along shorelines becomes a whole new experience when you are able to visualise the former environments in which the bedrock originated, or the drift geology was created, when you begin to understand what has happened to the strata over the millions of years since they came into being, and when you first begin to grasp what processes are affecting them once they are exposed to air. I love it when I can recognise strata belonging to the same geological period in different parts of the world, and see their differences and similarities, whether in situ or in buildings, walls and other structures. I begin to feel an enormous sense of wonder and awe, as well as an enormous feeling of humility, at this hugely significant part of the natural environment, a part on which everything else in nature depends or by which it is affected.
This is the third in a series of photographs of Silurian rocks from Clogher Bay. A brief examination of the literature indicates that the rocks in these pictures belong to the Drom Point Formation which has accumulated to a depth of 300 metres and is part of the Dunquin Group of Silurian Period strata in Ireland. The Drom Point and Croagh-marhin Formations consist of shallow-marine, fossiliferous siltstones and very fine to fine grained sandstones.
You don’t have to be a rockhound to be impressed by the spectacular scenery at The Hopewell Rocks. Tall cliffs of sloping red strata rise high above the Bay of Fundy shore, with an abundance of naturally worked shapes, caves, arches, and free-standing pillars of rock called sea stacks. At high tide, people can kayak around the stacks, also known locally as “Flower Pots” because of the groups of full-grown trees that grow on top of them – as they also do right to the cliff edges, with their root systems often clearly visible. At low tide, it is possible to descend a staircase to the ocean floor itself and explore these geological phenomena close up. Viewing time on the seashore is limited by the enormous and potentially dangerous rise and fall of the tides in this narrower northern neck of the Bay, where in some places, and at certain times, the sea can rise by as much as 56 feet.
At one time, about 600 million years ago, this part of Canada’s New Brunswick Province started its life near the Equator. Here it was subjected to uplifting earth movements that incorporated it into the Appalachian Oregon, an ancient mountain chain that now stretches from New Foundland to Florida. By 360 million years ago, the Appalachian building activities had ended and were followed by predominantly erosional processes.
The rocks exposed at Hopewell originated specifically in that part of the Appalachians called the Caledonian Mountains. Erosion by water and wind about 350 million years ago, in the Lower Carboniferous Period, steadily wore down the mountains, creating massive volumes of boulders, stones, gravel, sand and mud. Near the highland areas, flash floods tore through the valleys and canyons, washing away loads of eroded sediment and depositing it as stony and gravelly debris. Further from the highlands, sediment formed alluvial plains with sorted layers of sand and mud. The region covered by these terrestrial deposits in present day Atlantic Canada is called the Maritime Basin.
Over time, the coarser material in the erosion deposits on the flood plain became consolidated and cemented together with finer sand and silt. Because the land lay near the equator, the climate was hot and dry. Iron-bearing minerals became oxidised, and the rocks turned into redbeds. The series of red rock layers is now known as the Hopewell Cape Formation; this is the rock exposed in the cliffs and sea stacks at Hopewell today – eventually brought to its current position by Continental Drift, the tectonic movement of continental crustal plates.
In the first instance, the variably-textured sedimentary strata were deposited in horizontal layers. However, earth movements tilted them to angles between 30 and 45 degrees. The tilting of the rocks caused horizontal cracks to form parallel to the bedding planes, and also vertically at right angles to the strata. These lines of weakness in the rocks have become the points of entry for weathering agents – glaciers, tides, snow, ice, and winds. Erosion by these forces widens the cracks and steadily works away at the softer horizontal strata. The expansion of water as it changes to ice is a significant factor in the enlargement of cracks and crevices, and the breaking up the rock. Sandstone is softer than the conglomerate and easy for waves to wear away. The overall result is that broad columns of rock are carved into the cliff face. Undercutting at the cliff base creates caves and arches. Eventually, some columns are completely separated from the cliff face and become sea-stacks or “flower pots”.
The erosion activities are on-going. Extreme weather events and storms of recent years may accelerate the processes. The cliff face is gradually receding. Sea stacks eventually collapse and new ones are formed. A sea stack can last as little as 100 years or as long as a thousand. However, there is no need to panic about seeing the sights at Hopewell as soon as possible for fear that they will all disappear into the sea – geologists have calculated that there is enough conglomerate in the Hopewell Cape Formation to make “flower pots” for the next 100,000 years.
Photographs of the amazing Dunmore Head on the Dingle Peninsula on the West Coast of Ireland where the swell of azure blue waves crashes in white surf against the steeply sloping beds of Devonian strata in the cliffs, and breaks on the pinnacle-sharp rocks below. A small sandy cove, incredibly accessible even by car, is where visitors are privileged to picnic, build sand castles, brave the surf on boards, play among the rocks, and marvel at the views.
Don’t forget, you can click on any photograph to enlarge the image!
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