Tag Archives: rock layers

Silurian Rocks at Arisaig, NS

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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

Great Tor, Gower

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The limestone formation of Great Tor on the Gower Peninsula

The magnificent limestone outcrop that separates Three Cliff Bay from Oxwich Bay on the Gower Peninsula in South Wales is known as Great Tor. It is composed of steeply dipping layers of Carboniferous Period sedimentary rock called Hunts Bay Oolite Sub Group.

The limestone formation of Great Tor on the Gower Peninsula

The limestone formation of Great Tor on the Gower Peninsula

The limestone formation of Great Tor on the Gower Peninsula

The limestone formation of Great Tor on the Gower Peninsula

The limestone formation of Great Tor on the Gower Peninsula

The limestone formation of Great Tor on the Gower Peninsula

Rocks at Spencer’s Island

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There is an island called Spencer’s Island just off shore in the north Minas Basin in Nova Scotia, Canada. The same name also applies to the small community on the mainland opposite the island. It is the rocky outcrops along the beach of the mainland that feature here. While the island itself is mainly igneous basalt extruded in the Jurassic period, the steeply dipping strata along the mainland shore are composed of Carboniferous period Cumberland Group rocks (approximately the same type as are found at the famous Joggins Fossil Cliffs further north).

At the moment, I have not managed to find information to explain the details of these rocks but they certainly are interesting to look at. I know that they are composed of sandstones, siltstones, and shale (I didn’t notice any conglomerate or coal which are typical of this rock group). They are rapidly wearing away. They are severely impacted by strong tides and currents especially over the winter months. The outcrops reveal multiple layers of sedimentary deposits which occur in a cyclical way with harder more solid bands alternating with softer finely laminated ones. This reflects the depositional environment which was changing on a regular basis. All the rocks are subject to fine fracturing on a massive scale so that the slightest touch would cause the crumbling fragments to cascade to the beach. Sometimes the weathering process has led to falls of large boulders. Sometimes the outcrop is reduced to fine scree.

It was a very cold, dull, windy, wet day in late May when we visited Spencer’s Beach and we were very glad to discover a delightful beach cafe where we filled up on coffee and clam chowder. The beach was once a busy ship-building yard – famous as the place that built the Mary-Celeste.

Rock Textures at Fall Bay (1)

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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.

Caswell Bay Mudstone Formation

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These three galleries show pictures of rock texture and pattern in strata of Caswell Bay Mudstone Formation (CBMF) at Caswell Bay on the Gower Peninsula in South Wales. This is the type location for the CBMF – the place from which the rocks were first described and named. The rocks are part of the Pembroke Limestone Group of the Tournasian/Visean epoch of the Mississipian subdivision of the Carboniferous Period. The Carboniferous Period lasted from 359 to 299 million years ago (mya) but the Tournasian/Visean part only lasted from 359 to 326 mya. The CBMF were deposited around the middle of that period. The total thickness of rocks deposited during the Tournasian/Visean epoch was around 750 metres but the CBMF is just a narrow band – with estimates of its thickness varying from 0-14 metres (George, 2008) to 3 – 7.5 metres thick (Barclay, 2011). The CBMF is sandwiched between the Gully Oolite Formation limestone below and the High Tor Limestone Formation above.

Barclay (2011) says that: the Caswell Bay Mudstone Formation is composed of thinly bedded calcitic and dolomitic mudstones and micritic limestones (George, 1978; Ramsay, 1987). The basal bed is a calcrete (Heatherslade Geosol of Wright, 1987b), with beds of algal laminate and oncoid limestone. The rocks are pale grey to greenish grey, buff, brown and yellow, locally with some red staining.

Barclay says there are some but not many fossils. Also that: the rocks formed in shallow water environments referred to as “lagoon phase” by Dixon and Vaughan (1912). They are interpreted as shallow-water, peritidal deposits formed in a tidal flat lagoon complex behind a beach barrier in a humid climate, with abundant evidence of sub-aerial exposure in the form of dessication cracks (Riding and Wright, 1981; Wright, 1986; Ramsay, 1987).

The Caswell Bay Mudstone Formation lies unconformably on top of of the sub-aerially weathered palaeo-karst surface of the Gully Oolite Formation limestone. It means that there was a time lag between the deposition of the limestone and the next phase of deposition of the mudstones. The palaeo-karst surface is full of dissolved pot-holes. These pot-holes are a common karstic feature on Gower south coast beaches – with Mewslade Bay and the Worms Head Causeway exhibiting some good examples.

The Gully Oolite Formation limestone was deposited in warm tropical seas at a time when sea-level was standing still or slowly falling. The extended period of sub-aerial weathering that created the palaeo-karst surface occurred during a significant relative fall in sea-level (George, 2008, 85). The Caswell Bay Mudstone Formation was formed during a subsequent phase of slow sea-level rise.

References

Barclay, W. J. (2011) Geology of the Swansea District: a brief explanation of the geological map Sheet 247 Swansea,  British Geological Survey, Natural Environment Research Council,  ISBN 978-085272581-8, pp 4-6.

Dixon, E. E. L., and Vaughan, A. (1912) The Carboniferous succession in Gower (Glamorgan) with notes on its fauna and conditions of deposition. Quarterly Journal of the Geological Society of London, Vol. 67, pp 477-571.

Geological Society Field Guide to Caswell Bay.

George, G.T. (2008) The Geology of South Wales: A Field Guide, published by gareth@geoserve.co.uk, ISBN 978-0-9559371-0-1,  pp 82- 86.

George, T. N. ((1978) Mid Dinantian (Chadian) limestones in Gower. Philosophical Transactions of the Royal Society, Vol. B282, pp 411-462.

Ramsay, A. T. S. (1987) Depositional environments of Dinantian limestones in Gower, 265-308 in European Dinantian environments, Miller, J., Adams, A. E. and Wright, V. P. (editors). (Chichester: John Wiley & Sons Ltd.)

Riding, R. and Wright, V. P. (1981) Palaeosols and tidal flat/lagoon sequences on a Carboniferous carbonate shelf: sedimentary associations of triple disconformities. Journal of Sedimentary Petrology, Vol. 51, pp 275-293.

Wright, V. P. (1986) Facies sequences on a carbonate ramp: the carboniferous Limestone of south Wales. Sedimentology, Vol. 33, pp 221-241.

Wright, V. P. (1987b) The ecology of two early Carboniferous palaeosols, 345-358 in European Dinantian environments. Miller, J., Adams, A. E. and Wright, V. P. (editors). (Chichester: John Wiley & Son.)

A walk along the shore beneath Rhossili Cliffs

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Blue tidal pool water and limestone rock face

From the southern sandy shore of Rhossili Beach in Gower, the cliffs tower overhead, bearing the village itself. Sheep with bright red and purple markings nonchalantly graze the craggy upper slopes. Visitors to the Worms Head Causeway are minute figures among the hummocks of a former castle, peering recklessly over the edge to the beach below.

The path down from the village to the beach has been disrupted by last winter’s land slip, and heavy machinery continues to make a new, easier way to the shore. The red earth scars of the recent and many previous movements are visible along the face of the fault-line valley that separates the Carboniferous Limestone Rhossili headland from the greater height of the Old Red Sandstone in Rhossili Down. Boulders litter the beach at this point. Some loose rocks are red sandstones and conglomerates from the Down. Many of the larger boulders are composed of angular limestone fragments (something to do with glaciation I think – maybe till) held together by a crystalline matrix that formed from calcium-rich groundwater percolating  between the stones. Some boulders are huge chunks of Black Rock Limestone or similar from the headland and must weigh many tons.

Standing far out on the shore allows a panoramic view of the cliffs, from the soft red soil and erratic turf of the land slip area, along the bare rock exposed strata of the basal third of the cliffs, to the tidal island of Worms Head beyond. The cliff face is scalloped in and out by early quarrying activities. The distinct diagonal arrangement of the dipping rock layers contrasts with the horizontal colour banding caused by the colonisation of the rock surface between tide levels by organisms with different tolerances to exposure.

In places, tidal pools of strangely blue water skirt the pale, barnacle and mussel encrusted rock. Sand ripples like the lans and grooves of massive fingerprints decorate the beach, and create intricate arrangements around isolated boulders, reminding me of Japanese Zen gardens. Rounded smooth limestone pebbles in caves and alcoves bear fossil Sea Lily stems. And everywhere, sharp-edged fragments on the beach are evidence for the continuous weathering of the cliff face where each rock fall is signified by the fresh exposure of frequently orange-coloured stone

The Cliffs of Moher Strata

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Rock strata at the Cliffs of Moher

The Cliffs of Moher rise to 214 metres above the sea and stretch for 8 kilometres along the west coast of Ireland in County Clare. They are spectacular. This impressive site has formed the backdrop for box-office hits like the ‘Princess Bride’ and ‘Harry Potter and the Half-Blood Prince’. The first landfall west of the cliffs is the Atlantic coast of Canada – 3000 kilometres away. You can get an idea of the vast scale of the cliffs by noting the minute figures of the people walking along the cliff-top footpath and rocks in the photographs below.

The Cliffs of Moher take their name from a ruined promontory fort known as “Mothar” that once stood at the southern Hag’s Head end of the cliffs but which was pulled down in Napoleonic times to make way for a signal tower.

The Cliffs are part of the famous Burren landscape. However, they are composed of relatively more recent Upper Carboniferous Namurian Period rocks – the Central Clare Group and Gull Island Formation, that overlie the Clare Shale Formation. These sandstones, siltstones, mudstones, and shales were all deposited on top of the more familiar Carboniferous Visean limestones of the perhaps better known eroded scenery of the Burren. The sediments of which the cliff rocks are made, were originally brought down by a large river to a huge delta system that was subsequently consolidated into rock about 300 million years ago. The Burren and the Cliffs of Moher are a recognised UNESCO Global Geopark.

Over long periods of time, some places at the base of the cliffs have been worn away by wave action to form sea caves. In fact, Ireland’s largest surfing wave (called Aileen’s) is at the base of these cliffs. Continued eating-back of the rock where these caves have been scooped out of the cliff, can lead to the formation of sea arches through which the tides can freely flow. When the arches themselves eventually collapse, isolated pillars of rock named as sea stacks are created. The final stage of erosion by the sea causes the stacks to be worn down to ever-decreasing stumps of rock. These caves, sea stacks and stumps can be seen from the cliff-top pathways.

The strata in the cliffs are almost horizontally bedded and the ledges which jut out from the cliff face, where the alternating hard and soft rock layers have been differentially weathered, form ideal nesting sites for thousands of puffins, guillemots, razorbills, fulmars, kittiwakes, peregrine falcons, and choughs. Wild flowers abound on the cliffs in the summer months. The great height of the cliffs provides an ideal viewpoint to look for dolphins and seals in the water below – and maybe even the occasional basking shark, humpback or Minke whale.

The best vantage point of all is the top of O’Brien’s Tower perched near the cliff edge. It was purpose built in the 1835 for tourists who, even then, were flocking to marvel at the cliffs. Up to a million visitors a year now come to the Cliffs of Moher and, even on a cold March day early this year when I was there, people were arriving by the coach-load to enjoy the wonder of the cliffs and to learn about their significance in the fascinating and extremely well-presented Cliffs Exhibition. Although most visitors walk no further than O’Brien’s Tower, this is enough to give them a real sense of awe at this incredible geological site.

O'Brien's Tower at the Cliffs of Moher

REFERENCES

Cliffs of Moher Visitor Experience, Co. Clare – Visitor guide leaflet.

Sleeman, A. G., Scanlon, R. P., Pracht, M. & Caloca, S. (2008) Landscape and Rocks of the Burren: A special Sheet in the Bedrock Geology 1:50,000 Map Series, published by Geological Survey Ireland, ISBN 189970257-1.

Hennessey, R., McNamara, M., and Hoctor, Z. (Compilers) (2010) Stone, Water and Ice – A geology trip through the Burren, The Burren Connect Project, ISBN 0-9567204-2-9.

COPYRIGHT JESSICA WINDER 2014

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Red Point Rocks on Grand Manan

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Off the coast of Canadian New Brunswick, in the Bay of Fundy, lies an island that is split geologically in two. A fault runs from north to south in Grand Manan Island and it can be clearly seen only at Red Point in the Anchorage Provincial Park. It is visible as a diagonal line in the cliff. There are distinctly different rocks on each side of the fault.

To the west of the Red Point Fault are volcanic rocks known as the Southwest Head Member of the Dark Harbour Basalt – part the Late Triassic Flood Basalt dating from about 201 million years ago. Photographs of this type of rock showing textures and fracture patterns are shown above. The Red Point Fault itself is shown below. It is visible as a diagonal line in the low cliff structure with a stream flowing out of the fault across the shore. This is “a rare exposure of a major displacement intra-basin fault” (McHone).

The geological fault between basalt on the left and meta-siltstone on the right at Red Point on Grand Manan Island.

To the east of the fault, the rocks are metamorphic and not volcanic. They are laminated meta-siltstones of the Long Pond Bay Formation of the Cambrian-Ediacaran Formation dating from 539 million years ago. Photographs of the colours, textures, layers, and fracture patterns in these rocks are shown below.

The Mesozoic basalts of the western two-thirds of the island conceal the ancient Ediacaran (Neoproterozoic) and Cambrian (Palaeozoic) metamorphic rocks lying below them. However, in the eastern third of the island, the rocks have all moved upward along the Red Point Fault, with the more recent strata of the Mesozoic basalts gradually being removed by erosion so that the ancient metamorphic layers are now exposed at the surface. “At least 300 meters (and possibly much more) of vertical offset is required for this juxtaposition” (McHone) … of the Dark Harbour Basalt with the Long Pond Bay Formation meta-siltstones.

Click here for a geological map of Grand Manan. The map is one of many invaluable downloads about the geology of Grand Manan made available on the website earth2geologists run by J. Gregory McHone who is a professional geologist personally responsible for much of  the mapping and research on the island, and who runs the Grand Manan Museum.

McHone, J. Gregory Mesozoic Geology of Grand Manan, Graduate Liberal Studies Program, Wesleyan University, Middletown, CT.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Storm damage at Ringstead Bay

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Wooden steps, smashed by storms, leading down to the east half of Ringstead Bay

Ringstead Bay is well known for its slumping low cliffs. Bad weather gradually erodes them and more fossils regularly appear on the shore after heavy rain and storms. However, the storms over the last two months (January & February 2014) have had a significant impact on the shoreline in this location – as you would perhaps expect given the media coverage of the dramatic changes to be seen on Chesil Beach and the Isle of Portland only a few miles away, which are actually visible from Ringstead.

The first thing I noticed on my visit of 1st March was that  I couldn’t go to the eastern part of Ringstead Bay where I usually prefer to walk. This is the part that leads to the White Nothe chalk cliff. The lower half of the wooden access steps had been destroyed in the storms; and descent to the eastern shore from just past the caravan site was dangerous and impossible for me. I returned to the coastal path to find another point of entry to the beach – walking further eastwards to the zig-zag path that crosses the undercliff near the “Burning Cliff”. This is steep and not hazard-free at the best of times but right now is officially closed as parts of the path have slipped away altogether.

I retraced my steps to the cluster of houses near the car-park and went to the beach via the concrete slipway, turning right and westward as I reached the shore. This direction leads to Bran Point and Osmington Bay. On every visit to this particular place over the last 14 years there have been pebbles, large pebbles, and lots of them. They shift around a bit, sometimes a gradually flattened slope to the water line, and at other times piled up into steep terraces. But always millions of pebbles. Now there were none. Not a single one. Just coarse sand with maybe a bit of pea gravel as far as the eye could see.

Isolated rocks bearing fossil assemblages stood proud of the sand. The soft cliffs had been scrubbed by the storm surges to remove the surface vegetation on the lower levels revealing strata previously hidden from sight. Slides and falls of mud, rocks, and clays had left fresh surfaces exposed to view. Intriguing ‘fracture’ patterns decorated the clays as they began to dry out.

It has always been possible to see exposures of the Jurassic Period rock strata at the top of the Ringstead shore. The low cliffs at the western end of the bay are mostly made up of Later Corallian Formation Beds and Kimmerdge Formation Clays above them. The way these rock beds have slumped is an expected feature at this location.

However, the strata have to some extent always been obscured on my many previous visits  by overgrowing vegetation, superficial weathering deposits, and of course the pebbles. The big difference now, at least for the moment, is that most of the pebbles have been washed away and those that remain seem confined to one area. The earlier exposures of rock have been scoured by the storms to present fresh clean faces. The new rock falls and slides have uncovered new sections and revealed more fossils.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Calcite Veins in Black Rock Limestone

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Crystals in limestone rock cracks

On the beach below the Coast Guard Lookout, at the tip of the Rhossili Headland in Gower, jagged rocks stick up in lines parallel to the shore. This is the landward edge of the wave-cut platform that forms the causeway between the mainland and the Worms Head. The causeway is covered by the sea twice a day. Here on the beach, the rocks have not been worn down so much as the rest of the causeway, and the repeating rows of strata with their pronounced bedding planes are clear to see. The rock layers comprise Black Rock Limestone which was formed in the Carboniferous Period.

At some time in the past, cracks opened up in the rocks, breaking it at right angles to the rock layers. These fissures became filled up with calcium from solution in ground water that precipitated out as large calcite crystals on the sides of the spaces. Red sediments containing iron, haematite, also percolated down from the former covering of Triassic Period rocks, infilling the gaps between the lining of calcite crystals in the cracks. Today, the once hidden rocks are exposed to the surface in the base of the anticline that once connected the mainland to Worms Head, and the red and white stripes of calcite and haematite are visible cutting across the grey limestone rocks on the shore.

COPYRIGHT JESSICA WINDER 2013

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