Contorted Strata;
The mesmerising rocks of Mouthmill.
Blackchurch Rocks
Blackchurch Rock is a huge sea stack which stands over 22 metres (72 feet) tall and is made of sandstone and mudstone with quartz veins. It has eroded to form a huge pyramid shaped outcrop with two gravity defying arches.
Surrounding Black Church rock is a fascinating and complex section of cliffs and coast which features many interesting geological phenomena.
The cliffs and shore of Mouthmill are characterised by their dramatic twisting and buckling rocks, known as the Contorted Strata. This notable geological feature of the area provides a spectacular view into the age and power of the Earth, culminating in the natural beauty of Blackchurch Rock itself, so named after its giant archways appearing as sacred doors.
Formation
Mouthmill Beach showcases the edge of the "Crackington Formation", a sequence of folded rocks dating to the Carboniferous period.
These rocks began as sediments at the bottom of an ancient sea around 320 million years ago. Over vast eons these sediments piled up and gently formed layers of strata along the seabed. The image to the left shows this sea floor frozen in stone, on the inside of the northern strata sheet of Blackchurch Rock.
The sandstone, mudstone and slatestone deposits still contain evidence of primitive marine life such as spiral shelled Goniatite fossils. The striking folding and uplifting of these rocks occurred during the formation of Pangaea over 300 million years ago in an event known as the Variscan orogeny.
Such impressive buckling strata is a phenomenon which is sought by geology enthusiasts from all over the world. A few miles up the coast at Hartland a famous section of cliff with striking chevron folds is a widely known text-book example. Chevron folds can also be spotted at Mouthmill Beach.
Wandering Stones - The Pebbles
The tide line at Mouthmill is dominated by uncountable grey pebbles on top of the ancient strata and shifting sands. These cobbles typically originate as substantial rocks dislodged from cliff collapses. The rocks undergo a process of fragmentation and rounding due to the continuous tumbling and impact within the waves. Over the decades the number of pebbles on the shore varies greatly, and storms can suddenly reduce or increase their quantity overnight. Our efforts to stabilise cliffs have a long-term consequence of diminishing the supply of pebbles to the coast.
The prevailing Westerly wind means that smaller stones are slowly washed along the coast towards the East in a process called long shore drift. This means the pebbles at Mouthmill will eventually join the famous Pebble Ridge at Westward Ho! beach - another notable local natural feature. If you visit Blackchurch Rock you will quickly learn that stepping on wobbly pebbles and wet rocks is very hazardous. Well practiced locals appear to skip across these stones with ease, but it can require lots of coordination and concentration to pick the right stones to step on.
Coastal Erosion
In a geological time context, cliffs are always receding, constantly nibbled away by the waves over thousands of years. Although the vertical leaning strata gives Mouthmill a natural strength, recent history proves it is still vulnerable to erosion. Ten years ago a large collapse caused a part of the foot path to fall into the sea (now re-routed). In the distant past, what is now Blackchurch Rock was most likely hidden inside the rocks of the cliff, yet to be exposed by the crumbling of the coast.
The wave cut platform at the shore is craggy and sawtoothed, this is caused by layers of shale being easier to erode than the harder stone in the strata. The flat view of the strata highlights the chevron folds and buckling and the long views make spotting syncline and anticline axes easier.
It is not clear how this particular piece of coastline has changed since our Bronze age ancestors walked here. We have precise mapping and written records going back to the 1800s which accurately describe the coast as it is now. It is possible that 4000 years ago when Gallantry Bower was built that Blackchurch rock was still connected to the cliff, perhaps connected by another arch, or hidden completely, but we may never know.
Cliff collapses aren't necessarily bad, they are just a natural part of coastal geology. It is natural erosion which masterfully carved the sculpture that is Blackchurch rock, after all.
When visiting Blackchurch Rock see if you can spot the following:
- Chevron Folds
- Flexural Slips
- Oblique Joints, Cracks and Fractures
- Synclines and Anticlines
- Cliff Slips
- Horizontal and Vertical Strata
- Quartz Seams
- Goniatite Fossils
Interesting Tid-bit: This area does not contain dinosaur fossils because the section of bedrock pre-dates the Jurassic period by many millions of years!
Mouthmill is part of the Marsland to Clovelly Coast Site of Special Scientific Interest (SSSI)
Further Reading
Geological Society - Plate Tectonic Stories Hartland Quay, Devon
Depositsmag.com 2020 Article - Fabulous folds: Variscan tectonics in southwest England
Thank you for reading this page about the geology of Mouthmill valley and Blackchurch Rock. It is my pleasure to present my findings on this page and was very enjoyable to research and write. It sent me down a path of education and appreciation which only deepened my love of the coast. If you enjoyed it, please consider reading my page on the human history and landscape of the valley. I am merely an enthusiast but I tried my best to stick to facts and to keep it as simple and informative as possible. I am open to comment, corrections and critique - please send me an email and let me know. I also make and sell sculptures and photographs of Blackchurch Rock which I would love for you to check out here, you can pick up a piece of art and help support the running costs of this website.
Buy a BCR sculpture by a local artist.