Beach Boulders at Charmouth (East) 2

Natural fracture patterns in beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Here are some more pictures of the boulders at the eastern end of Charmouth Beach in Dorset, England, all exhibiting natural fracture patterns in sedimentary rock belonging to the Jurassic Charmouth Mudstone Formation. I’m not sure which particular layer they come from but it could be the Black Ven Marl Member. Perhaps someone can help me out with the identification? These images show the boulders at the foot of the cliff adjacent to the landslip or mud slide. In contrast to the dark boulders at the water’s edge shown in the previous post, these are dry and therefore lighter in colour.

I wonder if these boulders could have been the inspiration for an artwork in the sculpture park in Tout Quarry on the Isle of Portland featured in an earlier post.

Portland Stone sculpture at Tout Quarry, Isle of Portland, Dorset, UK on the Jurassic Coast - polyhedron (11)

Beach Boulders at Charmouth (East) 1

View looking east at Charmouth Beach, Dorset, England.

The shoreline at Charmouth looked particularly dramatic on this April visit as storm clouds periodically burst and blue skies were only intermittent. Charmouth Beach lies on the World Heritage Jurassic Coast in Dorset, England. The rocks are mainly Jurassic Period Charmouth Mudstone Formation. The character of the cliffs changes as you walk from west to east because the sedimentary rock layers gently slope and disappear beneath the beach surface level while new rock strata are freshly revealed at eye level. The predominance of softer rocks has led to a great deal of cliff slippage, and this means that the chronological sequence of the layers is frequently obscured by fallen debris; it makes it difficult to distinguish which rocks are which. The numerous rockfalls regularly contribute to the boulders on the beach and in this post I feature some boulders that exhibit some interesting fracture patterns. Of course these are not the only rock type on the beach, and I will post some more photographs of other patterns and textures in boulders and in the cliff face on the eastern half of Charmouth Beach in due course.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach Boulders at Charmouth Beach (East) 3

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Detail of pattern and texture in a beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Close-up of pattern and texture in a beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

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.

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.