Silurian Rocks at Arisaig, NS

Silurian rocks from Arisaig, Nova Scotia, Canada.

The Northumberland Strait shoreline of Arisaig Provincial Park in Nova Scotia, Canada, is described as one of the best sections of Silurian rock in the world. The strata are shales, sandstones, and siltstones from the Arisaig Group which was deposited in the early Silurian Period dating from about 443 to 424 million years ago.

I was fascinated by the way that some of the rocks were made up hundreds of extremely fine layers that were breaking up very easily. As far as I understand it, these darker shale layers were the result of deposits created in the coastal waters of the time by storm events rather than by tides or currents; and they are known as tempestites.

Hickman Hild and Barr (2015) say that the uninterrupted accumulation of fine-grained sediment during the Silurian Period, exposed here along a continuous 5 kilometre stretch, suggests that the area was tectonically quiet for at least 20 million years.

REFERENCES

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 50-53

More rock textures & patterns at Saundersfoot

Textures and patterns in rock strata at Saundersfoot

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.

Rock Formations at Langland Bay

Carboniferous limestone rock formation at Langland Bay

This post provides a context for the earlier post of mostly close-up images in Rock Textures at Langland Bay 1. Langland Bay is a popular beach near Swansea in South Wales. It is located on the south coast of the Gower Peninsula. It has a wide stretch of lower sandy shore, and pebbles landward in the central part. There are also wide expanses of low-lying rock platforms with hundreds of shallow pools in which to hunt for seashore creatures. To each side of the bay low cliffs of Carboniferous period sedimentary rock are overlain with much more recent glacio-fluvial and later poorly consolidated deposits.

The distinct layers of Hunts Bay Oolite, High Tor Limestone, and Penmaen Burrows Limestone form the southwest limb of the Mumbles anticline that extends from east to west. Here at Langland the sea has breached the rocks to create the embayment. The strata are riven by numerous joints and minor faults that have allowed the sea to penetrate, eroding away the rock to form small coves, undercuts, caves, tunnels, and passages to explore. The photographs shown here probably do not do justice to the site, as it was a very dull and overcast day when I visited, but I hope they serve to illustrate that the geology of Langland Bay is interesting from many points of view.

REFERENCE

Bridges, E. M. (1997) Classic Landforms of the Gower Coast, Series Editors R. Castleden and C. Green, The Geographical Association and The British Geomorphological Research Group, page 17. ISBN1-899085-50-5.

All sorts of colourful & interesting rocks

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.

Rocks Near Cape Enrage

West of the promontory bearing the famous Cape Enrage lighthouse in New Brunswick, across a wide wetland valley blocked by a shingle bank, is another spectacular promontory with pine topped cliffs. The rocks here belong to the Mabou Group of The Enrage Formation, part of the Cumberland Group from the Late Carboniferous Period. My visit was a brief one and I wished I had known at the time that this is a site where  fossils may well be found. Unfortunately, I didn’t know what to look for and simply marvelled at the natural rock textures and fracture patterns without discovering their hidden secrets. Further east on the shore beneath Cape Enrage itself I was much luckier and found some plant fossils – but more of that later!

Rocks at Fermoyle on the Dingle Peninsula

Red Devonian sandstone rocks at the beach with fucoid seaweeds

The place where I took these photographs is marked on the map as an island but it is actually just a tiny promontory near to the village of Fermoyle, along the Dingle Way, on the north coast of the Dingle Peninsula in Ireland. I am sure that most people visit the location for its wonderful long unsullied sandy beach. However, I was drawn to this particular part, at the extreme western end of the beach, because of its fascinating geomorphology. The rocks are sandstones and conglomerates (mostly but not exclusively red) of the Glengarriff Harbour Group from the Devonian Period. The bright olive, lime, yellow and orange colours of the seaweeds, and the black, yellow and white of encrusting lichens, clash garishly with the red rocks. The rock strata are clearly defined: sometimes on-end, sometimes as flat bedding planes, and in one place a dome of strata lies cut-away and exposed. Beach stones rather than pebbles cover a portion of this area; and there are also occasional huge boulders composed of conglomerate scattered along the shore nearest the inlet from Brandon Bay.

Rock Textures at Fall Bay (1)

Limestone rock texture on the coast

The rocks at Fall Bay are arrayed like the riffled pages of a book. Layer after layer of Carboniferous Limestone is sequentially spread out across the west side of the bay. Each layer has an observably different texture; some are bioturbated with bioclasts and fossils such as fragmentary crinoids and corals. The bedding planes of some strata have deeply sculptured surfaces from weathering and bioerosion. Lichens, barnacles and limpets colonise the rocks and take advantage of the meagre shelter offered by cracks, crevices, and solution hollows.