Rocks at Presqu’ile

Phyllite rock face on the Cabot Trail in Cape Breton Island

The Cabot Trail road leading to Presqu'ile and Pillar Rock in Cape Breton Island, Nova Scotia, Canada.Presqu’ile means “almost an island” and it refers to a narrow stretch of coastline just off the Cabot Trail in Cape Breton Island, Nova Scotia in Canada. It is nearly separated from the mainland by a long narrow lake. The road passes first along the eastern lake shore before crossing to the western shore; and just on the bend is where a track cuts down to the of the shore of Presqu’ile.  Parallel fault lines run along each side of the lake and one of these extends along the beach between the sea stack Pillar Rock and the mainland, where it has been responsible for interesting changes to the rocks.

Three different rock types originating in different geological periods lie incongruously side by side where they have been brought together by major faulting. Most noticeable is the phyllite rock that forms expansive, pale, gleaming surfaces beneath the highway and extending seawards. This is a metamorphic rock that started life as muddy sediment accumulating late in the Ediacaran or early in the Cambrian period (about 550 to 509 million years ago) on the margin of the ancient micro-continent of Ganderia. It was subsequently converted to shale and, when Ganderia collided with Laurentia in the Silurian period (443 to 418 million years ago), was buried by earth movements at a depth of about 8 kilometres and baked by temperatures as high as 300 degrees centigrade. This resulted in its deformation into phyllite by a realignment of the crystals. It was deformed again when Avalonia collided with Ganderia in the Devonian period (418 to 360 million years ago). Veins of quartz and calcite are common in the phyllite.

The black basalt of the sea stack Pillar Rock, lying just off shore from the phyllite cliffs and separated from them by a fault line, was extruded by volcanic activity in the Devonian period. Looking north-east along the shore, the cliffs are composed of sandstones from the Carboniferous period (360 to 300 mya). This odd juxtaposition of rocks from different periods is (I think) due to thrust faulting.

Mechanical digger moving granite boulders for coastal rip-rap sea defence at Presqu'ileThe weakened area of the fault line is reinforced against erosion by wave action by massive rip-rap boulders of granite obtained from Neil’s Harbour further along the Cabot Trail. There were road maintenance works going on during May, and the activities of heavy plant being used to arrange the boulders on the beach prevented access to the site on my first attempt. The digger had gone when I revisited a few days later and the light proved much more favourable for taking photographs.

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 84-89.

Atlantic Geoscience Society (2001) The Last Billion Years – A Geological History of the Maritime Provinces of Canada, Atlantic Geoscience Society Special Publication No. 15, Nimbus Publishing, ISBN 1-55109-351-0.

Bee Burrows in Redend Point Rocks

Soft red and yellow sandstone with holes made by bees

These curious rocks occur at Redend Point between Middle and South Beach at Studland Bay in Dorset, England. I have featured them several times in the blog before. The bright red and yellow patterned sandstones are Eocene Creekmoor Sand (also known as Redend Sandstone) and are part of the Poole Formation in the Bracklesham Group. The sandstone is very soft and easily eroded. It is also easily carved and provides a surface for much graffiti. However, the busiest carvers are small bees which excavate burrows in which to lay eggs. Now the sandstone has weathered away you can see the empty pupal cases from last year.

Rock textures at Saundersfoot

Natural rock textures and patterns

The natural textures and patterns in the cliff rock strata near Coppet Hall at Saundersfoot, South Pembrokeshire, Wales, really caught my eye on a first visit to the location. The stratigraphy is intriguing and complicated – and I have yet to work out exactly what I am looking at. I need a detailed geological map of the area and access to published papers for that. However, I think it fairly safe to say that they belong to the Upper Carboniferous Period, probably the Namurian, also known as the Lower Coal Measures, comprised of sandstone and mudstone layers, with coal seams and layers of iron nodules. I’ll check it all out when I can. High quality anthracite coal was open-cast mined not far away, and there used to be a local iron smelting industry.

My key guide to the geology of Gower and South Wales (George 2008) only describes in detail the geology of the stretch of beach immediately north of this place, and immediately south of it, leaving me a bit stumped as to an explanation for its peculiarities. I am definitely going back to this coastline to spend some quality time exploring the intricacies of its geological history, and photographing some of its marvellous natural abstract compositions.

REFERENCE

George, G. T. 2008, The Geology of South Wales: A Field Guide, gareth@geoserv.co.uk, 978-0-9559371-0-1.

Rock Textures at Eype 2

 

View of the beach at Eype in DorsetThe shore at Eype is littered with large boulders, several tons in weight, that have broken from the strata high on the cliff and then slip-slided down the lower mudstones and clays to the beach. They are all rocks belonging to the Jurassic Dyrham Formation. that includes a fascinating assortment of mudstones, sandstones, and limestones, some with ironstone nodules or carbonate concretions, and lots with fossils. I cannot with confidence identify the specific rock types illustrated in all the close-up photographs I took. It is quite a complicated geology at this coastal location. However, a general picture of the represented rock types follows. Fossils are found in more or less all the strata, ammonites are said to be common, but the ones I saw were mostly fragmentary shells and bullet-shaped belemnites

An accurate and up-to-date source of information about the geology of this locality is the British Geological Survey’s Geology of south Dorset and south-east Devon and its World Heritage Coast, published in 2011 by the Natural Environment Research Council. All the information that follows has been obtained from this book.

The Dyrham Formation is comprised of three members. At the base of the cliff is the Eype Clay Member which is a pale, blue-grey micaceous silty mudstone and shale. The base of the Eype Clay Member is marked by The Three Tiers  about a metre thick with three prominent sandstone beds separated by shales and mudstones. Higher up is a band of calcareous nodules, the Eype Nodule Bed. At the top of the band is Day’s Shell Bed with a rich fauna of juvenile bivalves and gastropods.

Above the Eype Clay member is the Down Cliff Sand Member made up of silts and fine sands with thin lenticles of hard calcareous sandstone. At its base is a fossil-rich layer known as the Starfish Bed, with abundant brittle-stars. At its top is the Margaritatus Stone which is hard, grey, iron-shot limestone.

At the top of the Dyrham formation is the Thornecombe Sand Member, sitting on the Down Cliff Sand Member. The bottom-most layer is the blue-grey Margaritatus Clay, above which are yellow-weathering, heavily bioturbated sands, with several horizons of large rounded calcareously cemented concretions. There is an impersistent band of limestone running through the middle of this, and a shelly Thornecombiensis Bed sealed by sandy mudstone atop it.

So you can see that there are many different rock layers and types in the stratified cliff, often obscured by land slips, and it is quite difficult for an amateur like myself to correctly identify pieces of these strata when they are lying on the shore.

However, one noticeable feature in the beach boulders was the occurrence of bioturbation: this is defined as a disruption of sediment by organisms, seen either as a complete churning of the sediment that has destroyed depositional sedimentary structures, or in the form of discrete and clearly recognisable burrows, trails, and traces (trace fossils). The most easily recognisable trace fossils are the largish burrows of Crustacean Thalassinoides – which you can see in images 4 and 9.

View of the beach at Eype in DorsetAnother phenomenon that is responsible for some of the more unusual colouration and patterning of the rocks, is the transformation of blue-grey rock to yellow by the weathering process on exposure to air, which oxidises iron minerals in the stone. Iron staining, iron nodules (often in association with fossil fragments), and veins of iron, also contribute to rich colour patterns both within and on the surface of the boulders. Sometimes the colours are exhibited as a thin outer layer that is exfoliating into abstract patterns of contrasting hues on the rock.

COPYRIGHT JESSICA WINDER 2014

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The Cliffs of Moher Strata

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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Raised Beach Deposits on the Oregon Coast

Sea cliff exposing raised beach deposits at Neptune State Park in Oregon, USA

Sixty miles due west of Eugene the Suislaw River hits the Pacific. From here, where the town of Florence straddles the mouth of the river, the U.S. Highway 101 follows the shoreline south to San Francisco, and northwards all the way to Washington State. We were headed north as far as Yachats.

The views along the coast road were stunning. At first, sand dunes on both sides. Thereafter, accompanied all the way by the seashore and pounding ocean to our left, and forested slopes of the Cascade Range to our right. The shore was unlike anything I had previously encountered at home in Great Britain, because the Oregon shore had been greatly influenced by relatively recent volcanic activity. However, one element was familiar: an ancient wave-cut platform and accompanying raised beach or terrace deposits.

The terrace deposits were once extensive along the Oregon Coast but are now present only as segments in more protected areas of the shore. They were formed during the Late Pleistocene period when the sea level was higher than it is today. The sea at that time cut away or levelled the basalt bedrocks on the beach and created a layer of sediment including rounded pebbles and sand which remained in an elevated position, marking the site of the Late Pleistocene beach, when the sea level subsequently lowered.

The raised beach or “terrace sediments range in thickness from a few feet to tens of feet and have been weakly consolidated into sandstone and conglomerate capable of maintaining a vertical sea cliff on the seaward side” (Lund 1971). The pictures below illustrate the appearance of segments of terrace and sea cliff at Neptune State Park on the central Oregon Coast.

Earlier posts in Jessica’s Nature Blog have described aspects of the rocky shores in the region of Yachats, just a few miles north of Neptune State Park. Most of the town of Yachats (population 650) is built on a segment of Late Pleistocene raised beach or terrace. Raised beach deposits occurring on the Gower Peninsula in South Wales have also been described in a previous article in this blog. Raised beach deposits containing abundant shells and pebbles with holes made by sea creatures will be discussed in a following post.

REFERENCE

Coastal landforms between Florence and Yachats, Oregon by Ernest H. Lund, February 1971, The ORE BIN, Volume 33, No. 2, pp 21-44.

COPYRIGHT JESSICA WINDER 2014

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Cliffs at West Bay

East Cliff at West Bay near Bridport (1) - Towering golden yellow cliffs of the Bridport Sand Formation on the Jurassic Coast in Dorset, UK.

The spectacular yellow cliffs at West Bay near Bridport in Dorset are soon to form the back-drop for the new television series called Broadchurch. They really are a remarkable geological phenomenon – like so much of the landscape on the World Heritage Jurassic Coast.

The East Cliff (1) is largely composed of the Bridport Sand Formation with 43 metres depth of it exposed out of an overall depth of approximately 49 metres. It has been calculated that each metre of rock would have taken about 20,000 years to accumulate on the seabed where it originated. So it would have taken about 860,000 years for the depth of the rock exposed in the cliff to accumulate. This all happened in the Upper Lias phase of the Early Jurassic Period (about 176 – 205 million years ago).

The sediments of which the cliff is formed consist of alternate beds of soft fine-grained crumbly sandstone, and harder sandy limestone (2). The striped nature of the cliff is one of its most notable characteristics. The depth of each bed of soft sandstone gets progressively smaller with an increase in height above the beach. The sand beds vary in thickness from 30 centimetres to 3 metres. The carbonated-cemented sandy limestone layers are consistently narrower and range only up to 75 centimetres and have irregular upper and lower surfaces.

The harder calcareous sandstone layers stick out as horizontal parallel jagged ledges (3 – 5). They are more resistant to erosion by wind-blown sand than the softer sandstone. They are also typically bio-turbated, meaning that the original soft sediments on the sea bed were churned up and burrowed into by marine invertebrate animals. The tunnels or burrows were subsequently filled in with a calcium-rich sediment – forming trace or ichno-fossils of the burrows and tunnels. These are the hardest part of the rock, and when weathered and sculpted by the elements, they remain like a network of inter-connected cords. From a distance, they have a general honeycomb appearance and this type of erosional feature is known as tafoni (6).

Vertical cracks or joints, known as gulls, extend at intervals from top to bottom of the cliff (7). These were mostly created a very long time ago as part of a cambering process which affected the whole hill of which the cliff forms the eroded seaward edge. As far as I can understand it (and I may have got the wrong end of the stick here), this cambering was due to softer substrates lying beneath the Bridport Sand Formation rocks in river valleys to the east and the west of this BSF structure. Lack of support for the rock in these locations led to a kind of subsidence that opened up vertical cracks and led to the development of a sort of fanning of the strata and a curving of the top of the hill (21 – 22). This is one of only a few places in Britain where cambering rock strata can be observed.

The gulls are usually in-filled with sand and debris. However, these joints are particularly susceptible to erosion and are worked at by the elements so that they gradually recede landwards, creating angular recesses between protruding columnar buttresses right along the length of the cliff.  Vegetation can colonise the cliff in these places because the roots can take hold among the infill debris (8 – 9). Caves can form where wind or waves clean out and extend the cracks (10 – 11). This can happen high on the cliff but is more common at the base.

The friable or crumbly texture of the sandstone means that rock frequently falls from the cliff and ends up as boulders on the beach (12). Wind and rain work on higher strata while the sea undercuts the lower layers. At the base of the cliff where waves frequently work (14, 18), and in newly exposed rock after falls (13), the true colour of the Bridport Sand Formation is revealed as blue-grey. The Bridport Sand Formation comprises fine-grained sandstone, a quartz arenite, regularly alternating with hard calcite-cemented strata. Within this matrix is pyrite. When pyrite is exposed to air, it starts to oxidise, forming a rust-like thin layer on the rock (15). It is this process that converts the natural blue-grey to the wonderful yellow colour of the rocks in the cliff.

At the base of the cliff for some distance along its length, it is possible to see an anomaly in the rock layers (19 -20). Most of the alternating bands of rock are virtually level, horizontal, and parallel to each other. However, there is an exception to this. There is a bed of hard rock which undulates up and down in a discontinuous or broken set of curves. This is thought to be a scour structure where an extreme storm event disturbed the shallow water marine shoal sands on the seabed early on in the development of the Bridport Sand Formation, just after the layer have been deposited and consolidated.

REFERENCES

Bridport Sand Formation at East Cliff , West Bay by Ian West

Brunsden D and Goudie A, (1981,1997) Classic Landforms of the West Dorset Coast,  Series Editors R Castledean & C Green, The Geographical Association, ISBN 1 899085 19 x.

Melville R V and Freshney E C (1982) The Hampshire Basin and adjoining areas, British Regional Geology, Institute of Geological Sciences, NERC, HMSO, ISBN 0 11 884203 x.

THE PICTURE GALLERY

If you click on any picture in the gallery below, the image will be enlarged and show the descriptive caption. From there it is possible to either view the photograph in full-screen size or to the use the arrows to scan through all the pictures in the gallery in sequence.

COPYRIGHT JESSICA WINDER 2013

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