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Ages 13+
The Geology of Combe Martin
An accessible introduction for geology enthusiasts, local history buffs, and curious visitors.
Published on March 18, 2025 | Last modified on March 25, 2025
Authors: The Combe Martin History Project
Content is licensed under CC BY-NC-ND 4.0.
Introduction
Combe Martin’s geology, seascape, and heritage are intrinsically linked. The jagged cliffs, coves, and rocky shores reflect millions of years of geological activity. Besides the town's horticultural heritage, the coastline has shaped Combe Martin's identity through mining, quarrying, and maritime traditions.
This article, aimed at readers aged 13 and up, provides an accessible introduction to Combe Martin's geology, with a brief glossary and a bibliography at the end. A local map is included on our page footer.
Ongoing natural processes continue to influence the landscape, linking past and present. Combe Martin sits on Devonian-era sedimentary rocks. One of the longest main streets in Britain is lined by buildings showcasing local stone, cob earth constructions, and the local architecture.
Beneath the surface, mining tunnels reveal Combe Martin’s rich silver and lead mining legacy. Iron mining, limestone quarrying and numerous lime kilns also played a key role in the local economy.
In all, Combe Martin's polymetallic mining and geological heritage shaped the village’s development, influencing its landscape, economy, and buildings. Remnants and monuments to its industrial past can be visited today.
The Combe Martin Valley Fault, a major geological feature in the area, and the towering Hangman cliffs, further highlight the region’s dramatic geology.
Visit Combe Martin Museum and Information Point on Cross Street, and explore artifacts from the area's mining and quarrying past. Examine and magnify local rocks in the museum's activity pod.
Combe Martin Bay, Hele Bay, and Samson's Bay hold SSSI (Site of Special Scientific Interest) status due to their significant geological features. The diverse geology, marked by various rock types, folds, and faults, contributes to the site's national importance.
The Geology of Combe Martin
The area's geology primarily consists of sedimentary rocks deposited during the Devonian period, otherwise known as the Age of Fishes which lasted from approximately 419 to 359 million years ago. These formations include shales, siltstones, sandstones, and localised limestone beds.
Combe Martin's oceanic climate significantly influences its geology, through historical sedimentary processes and ongoing weathering and erosion.
Combe Martin's Geological Faults and Fractures
Complex geological structures created a double-edged sword for Combe Martin silver mining. The folded and faulted terrain concentrated argentiferous galena in rich veins, making some areas highly productive. However, these same features led to irregular ore distribution.
- Folds are geological structures that form when rocks undergo gradual deformation. This process typically occurs deep beneath the Earth's surface, where rocks are subjected to intense pressure and elevated temperatures.
- Faults are geological discontinuities that occur when tectonic forces cause rock masses to fracture and shift relative to one another. These breaks in the Earth's crust are often associated with seismic activity, as the sudden release of built-up stress can trigger earthquakes.
- Faults and fractures acted as pathways for groundwater, leading to significant flooding in mine shafts and tunnels in Combe Martin. This persistent flooding issue was a major obstacle for silver mining operations in the area.
- Some mineral-rich deposits or veins were thick and rich in argentiferous galena, while others were thin, lens-shaped, or pinched out entirely.
- Irregular ore veins made mining operations inefficient and risky. Miners could spend significant time and resources excavating barren rock, or chasing veins that abruptly ended.
The Significant Combe Martin Valley Fault
The Combe Martin Valley Fault is an important geological feature that has shaped the landscape of the area. According to Douglas J. Shearman of Imperial College London (1967), this fault is a large crack in the Earth's crust that has moved the rocks on either side in two ways:
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It has shifted the rocks sideways by about one and a half miles.
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It has also moved one side up (or the other side down) by about 500 feet.
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In contrast, the San Andreas Fault in the United States is a major continental fault that extends approximately 1,300 kilometres long, and about 25 kilometres deep, through California.
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The Combe Martin Valley Fault happened millions of years ago. It has played a crucial role in creating the valley and influencing the area's unique geology, including its silver and lead deposits.
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Now inactive, the fault runs through Southwest England and is part of a larger system of cracks; its effects can be seen in the cliffs and shoreline around Combe Martin.
In summary, the Combe Martin Valley Fault has shaped the cliffs and valleys we see today, and thus plays a crucial role in the area's geology, mining history, and landscape formation. It is therefore an important feature for both scientific study and local heritage.
The Ilfracombe Formation
The Combe Martin area's rock formations belong to the Ilfracombe Formation, which is divided into several members (listed in descending order):
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Kentisbury Slates Member
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Combe Martin Slates Member, which includes three distinctive limestone beds: Jenny Start Limestone, Combe Martin Beach Limestone, and David's Stone Limestone
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Lester Slates-and-Sandstones Member
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Wild Pear Slates Member
These sedimentary rocks were deposited in a shallow marine environment during the Middle Devonian period (approximately 388–383 million years ago). Fossils found in the limestone beds, including corals (tiny sea animals that build hard skeletons) and brachiopods (marine animals with hard shells that resemble clams), help confirm this ancient environment (paleoenvironment).
The Variscan Orogeny
The region's geology was significantly shaped by the Variscan Orogeny, a major mountain-building event that occurred during the late Carboniferous to early Permian periods (approximately 300–290 million years ago).
According to the UK Geological Society, the Variscan Orogeny resulted from continental collisions that formed the supercontinent Pangea by the late Palaeozoic, around 250 million years ago. This tectonic event caused extensive folding and faulting in the area, creating complex geological structures.
The assembly of Pangea is widely accepted to have occurred during the late Palaeozoic, specifically in the Carboniferous to Early Permian periods (approximately 359–250 million years ago).
One notable feature is a large fault running through Combe Martin Bay, with an estimated displacement of about 2 km. This fault, however, is of Tertiary age (approximately 66–2.6 million years ago). The Tertiary period is now divided into the Paleogene and Neogene periods.
Combe Martin's Limestone Beds
The limestone beds in the region contain fossil corals, and brachiopods (a class of crustaceans) indicating a shallow sea floor paleoenvironment (preserved in the rock record at some time in the past).
These fossils, particularly in the Jenny Start Limestone and David's Stone Limestone, suggest a Givetian age (Middle Devonian).
This rich fossil record provides valuable insights into the ancient marine ecosystems that once thrived in the Combe Martin area, during the Devonian period.
Combe Martin's High Water Table
Combe Martin’s geology, along with its location near the River Umber and water draining from nearby hills, results in a high water table. The local clay makes drainage difficult, increasing the risk of flooding. This has historically posed challenges for mining, especially in winter.
The high water table also affects local infrastructure, requiring careful management of drainage and sewer systems. It also shapes the area's ecosystem, influencing vegetation patterns.
While the raised water levels have to be managed, the high water table creates favourable conditions for wetland vegetation and supports species adapted to saturated soils.
Coastal Influence on Soil
Combe Martin's coastal location affects its soil through salt-laden air, diverse geology, and coastal erosion processes. The steep topography influences drainage, while the microclimate impacts soil temperature and moisture. Coastal vegetation and marine inputs also contribute to the soil's organic content.
While shale —fine-grained, clastic sedimentary rock— is present in the area, Combe Martin's soil is a complex mixture influenced by various geological and biological factors.
Geology of the Country Around Ilfracombe and Barnstaple (British Geological Survey, 1985)
Key Points:
- This memoir provides a detailed geological overview of North Devon, covering formations, structures, and economic geology.
- Geological Formations: Includes the Ilfracombe Slates Formation (marine-origin slates, sandstones, and limestones), Baggy Sandstones Formation (sandstones and siltstones from marine and freshwater settings), and Pilton Mudstone Formation (shales and fossil-rich sandstones).
- The Ilfracombe Slates are layers of ancient mud and sand, once part of a seabed teeming with life. Today, you can still find fossils of those sea creatures in the rocks.
- Structural Geology: Examines folds, faults, and deformations shaping the region.
- Economic Geology: Discusses historical mining and the area's mineral resources.
Fossil Crinoids in North Devon
A study by E. GÅ‚uchowski and G. Racki (2005) examined fossilised crinoid stems—marine invertebrates that resemble sea lilies—found in Devonian and Carboniferous rocks in North Devon.
The researchers identified several crinoid species, including one from Combe Martin and others from sites such as Baggy Point and Saunton Sands.
Their findings also highlight similarities with crinoid fossils from Europe, North America, and Asia, suggesting these marine animals were widespread in ancient oceans (GÅ‚uchowski & Racki, 2005. Proceedings of the Yorkshire Geological Society, 55(3), 161–170).
Why is This Crinoid Fossil Study Interesting?
The study is interesting because it reveals North Devon’s ancient marine life, connecting local marine invertebrate crinoid fossils to those found worldwide. It helps scientists understand past ecosystems, species evolution, and geological changes while also being valuable for education and fossil enthusiasts.
Conclusion
Combe Martin's geology is more than just an academic subject; it's an integral part of the village's identity and heritage. The rocks themselves provided the raw materials for local buildings and industries, most notably silver mining, while shaping the landscape.
The British Geological Survey memoir, Geology of the Country Around Ilfracombe and Barnstaple (1985), is a detailed and valuable resource for understanding North Devon’s geology.
The Combe Martin Valley Fault plays a crucial role in the area's geology, mining history, and landscape formation. It is therefore an important feature for both scientific study and local heritage.
In 2005, a study by E. GÅ‚uchowski and G. Racki examined fossilised crinoid stems—marine invertebrates that resemble sea lilies—found in Devonian and Carboniferous rocks in North Devon.
Research identified several crinoid species, including one from Combe Martin and others from sites such as Baggy Point and Saunton Sands.
Combe Martin's seaside location shapes its soil composition through marine air, varied geology, and erosion. The steep landscape and local clay affect drainage, while the oceanic climate influences soil conditions. Coastal plants and sea-derived materials further enrich the soil's organic content.
However, the village's rocky landscape and seascape, its proximity to the River Umber, and the surrounding hills, also results in a high water table. This feature historically challenged mining operations, increased flood risks, and necessitates careful management of drainage systems today.
By understanding the geological history of Combe Martin—its Devonian rocks, evidence of ancient seas, and the legacy of the Variscan Orogeny—we gain a deeper appreciation of the forces that have shaped both the village and its people.
Article and images © Combe Martin History Project 2023-2025
Explore More
If you’re interested in fun geology activities, games, and quizzes, check out GeoMôn’s Kids’ Activities. The GeoMôn website provides a variety of educational materials, including a special section for children with interactive geology activities. Learners may need supervision and should follow the safety guidelines.
Learners can also explore the Earth Learning Idea (ELI) website for even more exciting hands-on geology activities and experiments, bringing Earth science concepts to life in a fun and practical way.
GeoMôn’s Kids’ Activities features games, videos, quizzes, and hands-on projects aimed at sparking curiosity and enthusiasm for earth sciences among young learners. It’s a great way to learn more about rocks, fossils, and the geological history of places like Combe Martin.
Glossary of Geological Terms
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Argentiferous galena: A mineral composed of lead ("led") sulphide (PbS) that contains silver (Ag). It's the most important ore of lead ("led") and a significant source of silver.
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Brachiopods: Marine animals with hard shells on their upper and lower surfaces. They are important index fossils.
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Carboniferous: A geological period that lasted from about 359 to 299 million years ago, characterised by vast forests and the formation of coal deposits.
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Clastic: Sedimentary rocks or particles that are composed of fragments of older rocks or minerals, such as sand, silt, or clay, which have been transported and deposited by natural forces.
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Combe Martin Fault: A significant geological fault in North Devon, formed by movements of the Earth's tectonic plates. It represents a zone of weakness in the Earth's crust and has played an important role in shaping the region’s geological history. Studying this Fault helps scientists understand past geological events and the processes that have shaped the landscape
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Coral: Marine organisms that secrete calcium carbonate to form protective exoskeletons. Over time, coral colonies grow into vast underwater structures known as coral reefs. A notable example is the Great Barrier Reef in Australia, the world's largest coral reef system.
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Devonian: A geological period of the Paleozoic era, spanning from about 419 to 359 million years ago, often known as the "Age of Fishes".
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Earthquakes: Intense shakings of the Earth's surface caused by the sudden movement of tectonic plates. This movement releases energy as seismic waves, causing the ground to shake. Earthquakes often occur along fault lines where these plates meet. They can also be triggered by volcanic activity or by human and industrial actions.
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Fault: A fracture or zone of fractures between two blocks of rock, which allows the blocks to move relative to each other. In other words, a Fault is a crack in the Earth's crust where two blocks of rock can move past each other.
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Fold: A curve or bend of a planar structure such as rock strata, bedding planes, or foliation in any type of rock. In other words, A Fold is a bend or curve in layers of rock. This happens when rocks are pushed or squeezed over time, causing them to bend instead of breaking.
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Ilfracombe Formation: A geological formation in North Devon, consisting of alternating beds of sandstone and mudstone with occasional limestone beds.
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Limestone: A sedimentary rock composed mainly of calcium carbonate (CaCO₃), often formed from the skeletal fragments of marine organisms.
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Metamorphosed: Rocks that have undergone metamorphism, a process involving heat and pressure that changes the mineral composition and texture of existing rocks.
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Neogene: The later of two geological periods of the Cenozoic Era, spanning from about 23 million years ago to the beginning of the Quaternary Period.
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Orogeny: A process in which a section of the earth's crust is folded and deformed by lateral compression to form a mountain range.
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Paleogene: The earlier of two geological periods of the Cenozoic Era, spanning from about 66 million to 23 million years ago.
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Paleozoic: An era of geological time spanning from about 541 to 252 million years ago.
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San Andreas Fault: While the Combe Martin and San Andreas (slicing California in two) faults are important geological features in their respective regions, the San Andreas Fault is a much larger and more significant structure in terms of its impact on plate tectonics and seismic hazards.
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Sedimentary rock: Rock formed by the deposition and consolidation of sediment, usually in layered deposits.
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Slates: Fine-grained, foliated metamorphic rock derived from an original shale-type sedimentary rock.
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Tertiary: An obsolete term for the geologic period from 66 million to 2.6 million years ago. Now divided into the Paleogene and Neogene periods.
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Variscan Orogeny: A geologic mountain-building event caused by the collision of tectonic plates during the late Paleozoic era. It formed the Variscan mountain belt across Western and Central Europe, including regions in Portugal, Spain, Ireland, and areas of Britain including Cornwall and Devon.
