Discover a coastline where ancient Carboniferous sandstones and shales have been compressed into spectacular folds, creating some of the most striking geological exposures in south-west England.
From the cliffs around Bude to the dramatic chevron folds of Millook Haven and Crackington Haven, the Culm Coast reveals a remarkable story of deep-water sedimentation, mountain-building and millions of years of erosion.
The coastline around Bude, Millook Haven and Crackington Haven provides an exceptional opportunity to examine the structure of the rocks beneath north Cornwall. Layers of sandstone, mudstone and shale were deposited during the Carboniferous Period, roughly 359 to 299 million years ago – the rocks at Bude itself are around 315 million years old. Geologists have long called these rocks the Culm, after the thin seams of soft, sooty coal found in them, and the stretch of coast from Bude north towards Hartland is known as the Culm Coast.
These sediments accumulated in ancient basins under changing marine and delta-influenced conditions. Some layers were laid down by sediment-laden underwater currents, while other parts of the succession record shallower-water, deltaic and lake environments. The precise environment varies between formations and through the sequence.
Later, powerful tectonic forces compressed the rocks during the Variscan mountain-building episode. Instead of remaining flat, the original sedimentary layers were bent, folded, fractured and, in places, overturned. Subsequent erosion by the sea has exposed these structures in the cliffs and foreshore, allowing visitors to see the results at close range.
The area is especially important for understanding how sedimentary rocks behave when subjected to intense compression. The sharply angular folds at Millook Haven are a particularly memorable example.
The coastline provides a natural cross-section through folded Carboniferous rocks, from the sandstone cliffs of Bude to the zig-zag folds of Millook Haven.
Coastal exposures reveal the alternating sandstones and mudstones of the Carboniferous succession.
Tight, angular folds make Millook Haven one of Cornwall's best-known examples of folded strata.
Sandstone and shale layers provide further evidence of the region's complex sedimentary and structural history.
The rocks along this coast record several stages in the development of an ancient sedimentary basin and its later deformation.
During the Carboniferous, the region formed part of a sedimentary basin in which mud, silt and sand accumulated. The deposits became layers of mudstone, shale, siltstone and sandstone as they were buried and compacted.
Some sandstone beds record sediment transported by underwater density currents. Other beds and formations record changing water depths and deltaic or lake-related conditions. These changes are preserved in the rock sequence.
As additional sediment accumulated, the lower layers were buried beneath increasing thicknesses of material. Compaction squeezed water from the sediment, while minerals helped bind grains together to form sedimentary rock.
The resulting alternation of relatively resistant sandstone and generally finer-grained mudstone or shale created layered rocks that would later respond differently to tectonic stress.
Later in the Carboniferous and into the Permian, continental collision and associated tectonic movements compressed rocks across south-west Britain. This mountain-building episode is known as the Variscan Orogeny.
The Culm Basin succession was folded and faulted. The orientation, tightness and shape of individual folds vary across the region, reflecting the complexity of the deformation.
Over millions of years, uplift, weathering and erosion gradually exposed the folded rock succession. Along the north Cornwall coast, waves and cliff erosion reveal structures that would otherwise remain hidden underground.
The modern cliffs are therefore a natural geological record, although exposure changes over time and some sections may be covered by beach material or fallen rock.
The term "Culm" is historically associated with the Carboniferous rocks of Devon and north Cornwall. Modern geological mapping distinguishes a number of named groups and formations within this wider succession.
The older of the two local formations, the Crackington Formation is made mainly of rhythmically bedded dark mudstones with grey turbidite sandstones and siltstones. Its partial type section is about 10 km of sea cliffs between Rusey Cliff and Wanson Mouth, named after Crackington Haven.
The formation is particularly relevant to the folded Carboniferous rocks seen along sections of the north Cornwall and north Devon coast.
The Bude Formation is characterised by thick sandstone beds alternating with mudstones and shale intervals. Some beds are massive, while others preserve sedimentary structures that help geologists interpret their origins.
The formation is named after Bude; its type section is the coast north of the town between Duckpool and Gull Rock, exposing about 1,290 m of strata. The British Geological Survey describes it as laid down in a large freshwater lake, with occasional marine incursions recorded by black shale bands.
Across the border in north Devon, around Westward Ho! and Bideford, the Bideford Formation sits between the Crackington and Bude Formations. It includes mudstones, siltstones and thick cross-bedded sandstones arranged in coarsening-upward cycles.
It is not exposed on the Cornish coast, but helps explain how the basin filled up between Crackington and Bude times.
Formation boundaries and the rocks exposed at any particular location should be checked against British Geological Survey mapping. Similar-looking beds are not necessarily part of the same formation.
One of the most remarkable features of the Culm Coast is the sharp zigzag pattern formed by folded sedimentary layers. These are known as chevron folds, named for their angular, V-shaped or zigzag appearance.
Chevron folds commonly develop where alternating layers of contrasting mechanical properties are compressed. Strongly layered rocks can buckle into relatively straight limbs joined by sharp hinges, producing a distinctive angular geometry. The folds at Millook Haven are particularly striking and have been highlighted by the Geological Society of London as a major geological site.
Not every fold on the Culm Coast has the same shape. Some are upright, while others are inclined, tight or recumbent. Their geometry records the complex deformation of the Carboniferous rocks and should be interpreted from the actual exposure rather than from a single photograph.
Several coastal locations around Bude provide opportunities to observe sedimentary layers and deformation. Exposures vary with tide, weather and beach conditions, so plan visits carefully.
Bude gives its name to the Bude Formation, an important part of the Upper Carboniferous succession of north Cornwall and north Devon. Coastal exposures in the wider Bude area reveal thick sandstone beds, mudstone intervals and folded strata.
Look for: changes in bed thickness, sandstone and mudstone alternations, sedimentary structures and folded rock layers.
Beach guides: Summerleaze · Crooklets · Northcott Mouth · Sandymouth · Duckpool
Millook Haven is famous for its tightly folded sedimentary rocks and dramatic chevron patterns. The cliff section provides an exceptional illustration of how layered rocks can buckle under compression.
Look for: angular fold hinges, straight fold limbs and repeated bands of sandstone and finer-grained rock.
Millook beach guide · south of Widemouth Bay
Crackington Haven and the surrounding coastline provide further exposures of the Carboniferous succession. The rocks illustrate the relationships between sedimentary layering and later deformation.
Look for: sandstone beds, finer-grained intervals, dipping strata and evidence of folding. Check local geological mapping for the formation at a specific exposure.
The wider coastline around Bude includes further sedimentary exposures. The appearance of the rock platform and cliffs changes with tides, weathering and the movement of beach material.
Look for: bedding, joints, changes in rock type and the contrast between more resistant sandstone and finer-grained beds.
The north Cornwall coast is exposed to Atlantic weather and powerful waves. Geological observation is best undertaken from safe public viewpoints and established paths, with care around cliff edges and the foreshore.
Some rock exposures are easier to see at lower water. Check local tide times and allow plenty of time to return. Never enter a cove or foreshore route if the tide could cut off your exit.
Keep away from cliff edges and the base of unstable cliffs. Rockfalls can occur without warning, and wet rock platforms may be slippery. Follow local signs and coast path guidance.
Use photographs and field notes to record folds and bedding. Avoid hammering, breaking or removing rock samples where this is prohibited or could damage important geological exposures.
Consult the British Geological Survey's mapping before assigning a formation name to a particular outcrop. Geological boundaries may be complicated by folding and faulting.
The Culm Coast is one part of Cornwall's geological story. Compare its strongly folded Carboniferous rocks with the Devonian sedimentary formations and granite landscapes found elsewhere in the county.
The cliffs north and south of Bude are made of the Bude Formation – thick beds of grey sandstone with mudstones and a few distinctive black shale bands. They are Upper Carboniferous in age, roughly 315 million years old. The type section of the formation is the coast north of Bude, between Duckpool and Gull Rock.
‘Culm’ is an old name for the soft, sooty coal found as thin seams in the Carboniferous rocks of Devon and north Cornwall. Geologists borrowed it for the whole succession, which is now formally called the Culm Supergroup. The stretch of coast between Bude and Hartland is often called the Culm Coast.
The best-known folds are at Millook Haven, south of Widemouth Bay, where the cliff face shows a stack of sharp zig-zag folds. Crackington Haven and the cliffs north of Bude, around Northcott Mouth and Sandymouth, also show folded beds. The folds are best seen from the beach at low tide.
The sandstones and mudstones were laid down as flat layers. Towards the end of the Carboniferous, the Variscan mountain-building episode squeezed the region from the south. Because the rocks were made of alternating strong sandstone and weaker mudstone layers, they buckled into folds with straight limbs and sharp hinges – the classic chevron shape.
Not entirely. The British Geological Survey describes the Bude Formation as deposited in a large freshwater lake on a broad shelf, with sand carried in by river-fed underwater currents. Thin black shale bands with marine fossils record times when the sea flooded in. The older Crackington Formation, seen at Crackington Haven, is made of deeper-water turbidite sandstones and mudstones.
Fossils are not common, but some black shale bands contain nodules with goniatites (coiled shelled animals related to squid) and fish remains, and trace fossils can be found on bedding surfaces. Please photograph rather than hammer the cliffs.
Millook Haven is one of the Geological Society of London’s 100 Great Geosites in the UK and Ireland, chosen for its spectacular folds. Treat it, and the rest of the coast, as an outdoor museum: look, photograph and leave the rocks in place.
The north Cornwall coast faces the Atlantic and the cliffs are unstable. Check tide times before you go, keep away from the base of the cliffs, take care on slippery rock platforms, and make sure the incoming tide cannot cut off your way back.
The following sources provide additional information about the folded Carboniferous rocks, geological formations and regional history of the Culm Coast.
Geological terminology and formation boundaries follow published geological sources. Individual exposures should be checked against current geological mapping, especially where strata are tightly folded or faulted.