Rock Textures at Little Tor, Gower

Little Tor cliff at the east end of Oxwich Bay in Gower, South Wales, is made of Carboniferous Limestone of the Hunts Bay Oolite Sub Group. In common with beach outcrops of the same type of rock at Broughton on the north Gower coast, and Tenby that lies further west in Pembrokeshire, the surface is marked on a small scale with scalloped depressions and branching runnels that are the result of acid erosion and sand abrasion, giving rise to interesting textures and patterns.

The small sinuous etchings are called microrills (Ford and Williams 2007). They are typically 1 mm wide, round bottomed dissolution channels that are found close together. The pattern is reminiscent of rain running down a window pane. On gentle rock slopes they have curving paths and divide and rejoin in a network-like pattern. On steeper gradients the channels are straighter. Some microrills are made by slightly acidic water flowing down the rock surface but in other instances they are caused by the “water moving upwards, drawn by capillary tension exerted at an evaporating front. Capillary flow is believed to explain much of their characteristic sinuosity”.

REFERENCE

Ford, D. and Williams, P. (2007) Karst Hydrogeology and Geomorphology. John Wiley & Sons, Chichester, England. Revised Edition, p324.  ISBN 978-0-470-84997-2.

Great Tor, Gower

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

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.

North Mountain Basalt at Cap d’Or

 

The brooding basalt cliffs at Cap d'Or in Nova Scotia

Cap d’Or or Cape of Gold on the northern shore of the Minas Basin in Nova Scotia is in fact a misnomer. The golden glints in the cliffs which gave the place its name when Samuel de Champlain saw them in 1604 were really veins of copper. The glistening copper has now been mostly mined out but the cliffs remain absolutely spectacular. On the cold and misty day that I visited, the dark massive faces of rock looked their most brooding and atmospheric, almost like a fantasy backdrop from a Tolkien inspired scene.

The Jurassic period North Mountain Basalt outcropping at Cap d’Or. Basalt was originally molten lava that was extruded onto the surface about 206 million years ago. This happened in a number of episodes, each event giving rise to rock that had a different appearance. This is most easily seen in images 2 and 4 where the colour and texture of the rock at the top of the cliff is different from that at the base of the cliff. Details of the columnar fracturing that occurred as the lava slowly cooled, can be seen most clearly along the shoreline where the waves have washed the rock.

REFERENCES

http://www.davidkjoyceminerals.com/pagefiles/articles_capedor.asp

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 Coce-St. Philip’s, Newfoundland and Labrador. ISBN 978-1-927099-43-8.

A Walk Around Partridge Island

You would never guess the historical and commercial importance of the site as you walk along the arcing pebble ridge of this quiet beach. The beach leads to Partridge Island on the north shore of the Minas Basin in Nova Scotia – that’s part of the Bay of Fundy in Canada. Its location and geological treasures have marked it out as a very special place and once made it a hub of activity. Partridge Island is made of Jurassic North Mountain Basalt overlying Triassic red sandstone of the Blomidon Formation. The basalt rock formation extends 50 kilometres along the shore from Cape d’Or in the west to Economy Mountain in the east. Continental drift 200 million years ago caused cracks or faults in the Earth’s crust through which molten lava surged and solidified as basalt as it spread out over the sedimentary sandstone around it.

It was probably the Mi’kmaq people who first discovered the site as they travelled by canoe through lakes and rivers from Amherst, pausing at Partridge Island before continuing across the Minas Basin to Blomidon on the further shore and beyond. The island was a traditional collecting place for red ochre that was used in burial ceremonies. The lava stones were used in the sweat lodges. They found rocks, semi-precious stones, and minerals on the beach (such as agate, jasper, quartz, and basalt) from which they crafted axes, chisels, knives, arrowheads, spears, stone wedges, and drills for stone and leather work. In the Mi’kmaq legends, Glooscap presented an amethyst necklace to his Grandmother. Amethysts are still found here along with calcite, chabezite and stillbite. Gas bubbles and openings that had formed in the basalt while it was still molten later provided ideal spaces for mineral crystallisation. The powerful tides rushing through the narrow Minas Channel into the Minas Basin break off pieces of rock each year to reveal new sources of minerals.

From the early 17th to the mid 18th centuries, Acadian boatmen ferried goods from a station at Partridge Island across the Basin to Blomidon. Following the British expulsion of the Acadians, this became a permanent ferry service linking north to south Nova Scotia and connecting with an overland route as far as Halifax. Partridge Island was captured by Rebels from the American Revolution in 1776 and later retaken by the British who then built a fort there. This saw the setting up of a commercial outlet for essential provisions (tools, broadcloth, rum, and molasses) and the development of an important community with its own ferry link, militia outpost, postal and legal services.

Sadly, by the 1850’s the community began to decline in importance as the nearby Mill Village took advantage of the sheltered harbour and grew into the town now called Parrsboro. Very little remains to remind people of the former importance of Partridge Island but visitors still flock there to collect semi-precious stones on the beach. Geologists can admire the basalt columns stretching up from the shore on the east side of the island, and observe how the basalt sits on the red sandstones on the west of the island. The cliff strata on the mainland opposite the island, at the eastern end of West Bay, are older and highly fossiliferous Carboniferous limestones belong to the Windsor Group, Mabou Group, and Parrsboro Formation – out of chronological sequence due to major faults running from east to west.

Pebbles, salt marsh, and areas prone to flooding separate the island from the mainland. This piece of land was known as Wasoqun by the Mi’kmaq, meaning heaven or “the place where the spirits of the people and the animals live in harmony”.

N.B. The notice boards located on the beach at Partridge Island have been the only source of the historical information used in this post. The “Geological Highway Map of Nova Scotia” also proved invaluable additional facts for understanding the geology.

Bluestone Formation Rock Textures at Point Pleasant Park 1

On the southwest shore of Point Pleasant Park in Halifax, Nova Scotia, Bluestone Formation bedrock with an early Ordovician age outcrops on the beach. It comprises thinly bedded metasiltstone and slate with occasional calcareous concretions. The rock originated as deep sea muds and fine-grained sands which were compacted and lithified into shale 450 million years ago (MacBourne et al., 1986). The rock was later metamorphosed into slate by heat and pressure that built up during massive movements of the earth’s crust. It was further “baked” by the heat generated by an intrusion of molten magma (a batholith of granite was created) at which time new crystals of anadalusite and cordierite were formed in the Bluestone Formation strata. The closer the Bluestone slates and metasediments had been positioned relative to the injection of granite, the greater the heat, and the greater the number of the crystals formed. The small crystals are now weathering out from the surface of the rock by the action of the elements. This is leaving many characteristic small pits and embuing the exposed rock s with a strange texture.

Some of the layers in the rock cross-sections in the illustrated outcrops near Chain Rock Beach (above) show undulating bedding planes and cross-stratification. I think that these are the equivalent of the stone sand ripples on the surface of exposed bedding planes in similar rock on Black Rock Beach at the other side of the park (see the previous post – Rippled Rock at Point Pleasant Park).

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.

Fraser, J. A. (2010) Trends and Architecture of the Bluestone Formation Turbidites in Point Pleasant Park, Halifax, Nova Scotia. Submitted in Partial Fulfillment of the Requirements for the degree of bachelor of Science, Honours, Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia.

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 Coce-St. Philip’s, Newfoundland and Labrador. ISBN 978-1-927099-43-8, pp 128 – 131.

MacBourne, B., Donohoe, H. and Devanney M. (1986) A Walking Tour of Rocks, Minerals and Landforms of Point Pleasant Park, Nova Scotia Department of Mines and Energy, Information Circular 7.

White, C. E., Bell, J. A., McLeish, D. F., MacDonald, M. A., Goodwin, T. A. and MacNeil, J. D. (2008) Geology of the Halifax Regional Municipality, Central Nova Scotia. In Mineral resources Branch, Report of Activities 2007, Nova Scotia Department of Natural resources, Report ME 2008-1, p. 125-139.

Rippled Rock at Point Pleasant Park

Point Pleasant Park occupies the southern extremity of the promontory on which the city of Halifax is built in Nova Scotia. It juts out into the harbour and was a militarily strategic position in the past, as evidenced by old structural remains and modern memorials (MacBourne et al. 1986). The bedrock underlying the park is Late Cambrian to Ordovician Period slate and minor metasiltstone which formed on the Meguma terrane lying on the continental margin of the much  larger Gondwana between 499 and 470 million years ago.

A rocky outcrop of siltstone on Black Rock Beach, seen shortly after you enter the park from the east entrance, is remarkable for the ripple marks on its sloping surface. These were formed by a turbidity current and subsequently preserved under a layer of mud. Sediments were derived from the erosion of Gondwana and transported by large rivers across the land mass before being deposited. The rocks here are called turbidites and are part of a succession of similar layers 4 kilometres deep (Hickman Hild and Barr, 2015). The rocks are the part of the Halifax Group which used to be called the Halifax Formation, which has recently been re-classified into three – the Bluestone Formation, Cunard Formation, and Beaverbank Formation (White et al. 2008). The Black Rock Beach strata belong to the Bluestone Formation.

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.

Fraser, J. A. (2010) Trends and Architecture of the Bluestone Formation Turbidites in Point Pleasant park, halifax, Nova Scotia. Submitted in Partial Fulfillment of the Requirements for the degree of bachelor of Science, Honours, Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia.

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 Coce-St. Philip’s, Newfoundland and Labrador. ISBN 978-1-927099-43-8, pp 128 – 131.

MacBourne, B., Donohoe, H. and Devanney M. (1986) A Walking Tour of Rocks, Minerals and Landforms of Point Pleasant Park, Nova Scotia Department of Mines and Energy, Information Circular 7.

White, C. E., Bell, J. A., McLeish, D. F., MacDonald, M. A., Goodwin, T. A. and MacNeil, J. D. (2008) Geology of the Halifax Regional Municipality, Central Nova Scotia. In Mineral resources Branch, Report of Activities 2007, Nova Scotia Department of Natural resources, Report ME 2008-1, p. 125-139.