Discover France and Explore the Fossils and Geodiversity of the Crozon Peninsula in Brittany
- Wayne Munday
- 21 minutes ago
- 7 min read
Sip back and discover France and explore the fossils and geodiversity of the Crozon Peninsula located on the Atlantic coastline of Brittany. Forming the dramatic western tip of the Armorican Massif the peninsula hosts the Armorique UNESCO Global Geopark designated in 2024. Covering 1,600 Km² of the Massif and encompassing the Crozon Peninsula, Arrée Mountains and Brest Bay this region reveals over 500 million years of Earth’s history and a succession of Ordovician fossils. The Crozon Peninsula, with its iconic Armorican Sandstone cliffs at Pen Hir and Cap de Chèvre, displays the erosional sculpting of Ordovician sandstones, while the Huelgoat Forest exposes vast granite boulder fields shaped by weathering and tectonics. The peninsula’s Ordovician succession includes the Armorique Formation (Grès Armoricain), Postolonnec Formation, Kermeur Formation and Tufs et Calcaires de Rosan Formation indicating environmental change from shallow high-energy tidal flats to deeper low-energy marine settings to volcanically influenced shallow seas. Fossil assemblages encompass trilobites like Colpocoryphe grandis and Morgatia hupei, echinoderms, brachiopods and diverse trace fossils of a sea floor ecosystem over time. The terminal Ordovician Hirnantian glaciation left a lasting imprint on marine biodiversity coinciding with one of Earth’s earliest mass extinctions.
At the heart of this region is the Armorique UNESCO Global Geopark designated by UNESCO as a Global Geopark in 2024 covering an area of 1,600 Km2 the geopark encompasses the western side of the Armorican Massif, the Crozon Peninsula, Arrée Mountains and Brest Bay. This is an area of rugged coastal cliffs, wind-swept mountain peaks, granite boulder forests and expansive bays that covers 47 municipalities accessible via the gateway towns of Brest, Morzine and Huelgoat. The Crozon Peninsula has an enduring legacy as a region built upon a remarkable historical, strategic and cultural significance shaped by its dramatic geography. Forming the western tip of the Armorican Massif the peninsula juts into the Atlantic and shelters the Rade de Brest one of Europe’s largest deep natural harbours. Its strategic location has made Crozon a key military and maritime hub with commanding views of the Atlantic allowing control over seaborne access to the important naval port of Brest. From the Middle Ages through to the Napoleonic era fortifications have been constructed to protect the coastline including the Tour Vauban at Camaret-sur-Mer. The peninsula’s defenses were later enhanced during World War II as part of the German Atlantic Wall reflecting its enduring military relevance.
Beyond its strategic role the Crozon Peninsula preserves a rich cultural and archaeological heritage. Evidence of continuous human settlement dates back to the Neolithic and Bronze Age with monumental sites such as the Alignments of Lagatjar and burial cairns. The peninsula was later inhabited by Gallic communities and saw Roman influence followed by early Christian developments at the Abbey of Landevennec.
The Armorique UNESCO Global Geopark stretches from the vertical cliffs of the Crozon Peninsula to the windswept summits of the Arrée Mountains. This ancient terrain has been forged by sedimentation, magmatism, tectonic upheaval and sea level change starting from the Upper Neoproterozoic during the Ediacaran Period some 635 million years ago through to the Paleozoic Era.
During the Upper Neoproterozoic and early Paleozoic sediments accumulated in a shallow marine environment forming layers that would later undergo profound deformation. Episodes of volcanic activity, metamorphism and the intrusion of granitic plutons and magmatic dykes transformed these deposits.
The pivotal moment in the Armorican Massif's history was the influence of the Variscan or Hercynian Orogeny that began during the Devonian Period and continued through into the early Permian caused by the collision of the continents of Laurussia and Gondwana, which led to the formation of the supercontinent Pangaea. This mountain-building event folded and faulted the region’s sedimentary rocks, uplifted vast granitic bodies and left behind remnants of what were once mountains rivaling the modern Alps in height. The Arrée Mountains are some of the eroded peaks of this orogeny and Brest Bay is the lowland area shaped by both tectonic subsidence.
The Crozon Peninsula, with its dramatic white Armorican Sandstone cliffs at Pen Hir and Cap de Chevre, showcases the erosional sculpting of ancient sedimentary layers. Granite exposures in the Huelgoat Forest, often referred to as the “Chaos of Rocks” reveal large boulders fractured and piled by centuries of weathering and tectonic activity. Volcanic rocks and dykes also punctuate the landscape.
Among the interesting features of the Armorique Geopark is the presence of Kersantite, a distinctive igneous vein rock formed from the rapid crystallisation of mantle-derived magma within dykes the narrow conduits that channel molten material toward the Earth's surface. These Kersantite dykes, some stretching for hundreds of meters and exceeding ten meters in width, originated from the partial melting of metasomatized peridotite that has undergone a change in the chemical composition in the Earth’s upper mantle. Rich in olivine and pyroxene, the peridotite was altered by fluids from subducted oceanic crust, generating volatile, alkali bearing magmas that ascended and solidified into dark, crystalline lamprophyric rock marked by large phenocrysts or crystals. Kersantite veins, including the notable Rostiviec sill at Loperhet was formed during the final magmatic stages of the Variscan Orogeny. Kersantite is highly resistant to erosion yet easily workable, Kersantite became a building stone in Brittany used in historic churches and sculptures such as those in Landerneau’s Church of Saint-Houardon.
The Armorique Geopark preserves an fossil record spanning the Ordovician Periods despite not being a classic fossil location. As part of France’s Armorican Massif the succession of rocks reveals a marine ecosystem influenced by profound environmental change across the Early to Late Ordovician.
The Armorique Formation, also known as the Grès Armoricain, forms the foundation of the Ordovician succession of fossiliferous rock on the Crozon Peninsula. Deposited over 450 million years ago during the Tremadocian – Floian Stage the first and second stages of he Ordovician Period. This formation represents one of the earliest marine transgressions over the Cambrian basement of the Armorican Massif. Its thick, quartzitic sandstones were laid down in high-energy, shallow tidal environment.
The beds of the Armorique Formation capture a wealth of trace fossils including the vertical burrows of Skolithos, along with Daedalus, Diplocraterion, Tigillites, Monocraterion, Cruziana and Arthrophycus the evidence of ancient worms and arthropods actively burrowing across tidal flats. Although body fossils are rarer, fine-grained layers preserve delicate remains such as the trilobite Colpocoryphe grandis.
In the coastal outcrops of the Armorique Formation seen at Pen Hir and Cap de Chèvre. Pointe de Pen Hir is famed for its offshore rock stacks in the Iroise Sea known as the Tas de Pois (“Pile of Peas”), a cluster of six granite islets sculpted by relentless marine erosion. The area is now a protected nature reserve and supports colonies of seabirds and attracts visitors to see its dramatic cliffs, walk its panoramic coastal trails and explore the nearby Cross of Pen-Hir a memorial, inaugurated by General Charles de Gaulle in 1960, honouring the Free French Bretons who founded the WWII resistance group Sao Breiz. In contrast, Cap de Chèvre presents a wilder, more rugged coastline defined by steep sea cliffs and ancient folded sedimentary rocks.
The Postolonnec Formation, overlies the Armorique Formation, represents a Middle Ordovician sequence of shales and sandstones deposited in a quieter, deeper marine shelf environments during the Dapingian – Darriwilian stages of the Middle Ordovician. This formation preserves a diverse marine ecosystem adapted to lower-energy conditions, with a rich assemblage of fossils that provide insights into Ordovician benthic or sea bed life. Trilobites such as Morgatia hupei and Crozonaspis struvei are particularly characteristic, while brachiopods and ostracodes further illustrate biodiversity. More rarely found in the formation are specialised organisms such as echinoderms including the ophiuroid (Brittle Star) Taeniaster armoricanus and the mitrate Mitrocystella incipiens.
The Kermeur Formation succeeds the Postolonnec Formation and consists predominantly of lithified mud or argillaceous shales, silty mudstones and fine-grained tidal sandstones with a high content of clay minerals. Deposited in deeper low-energy marine settings during the Late Ordovician during the Sandbian–Katian stages. The Kermeur Formation reflects a continuance in the subsidence of the Crozon Basin and is rich in trace fossils of burrows from Bifungites, Arenicolites and Teichichnus.
Capping the sequence is the Tufs et Calcaires de Rosan Formation dating to the Katian–Hirnantian stage of the Late Ordovician. A combination of volcanic ash deposition and intermittent carbonate sedimentation laid down just before the start of the Hirnantian glaciation which occurred approximately between 445.2 - 443.8 million years ago and represents one of the most significant climatic events of the early Paleozoic. Lasting around 1.4 million years, this glaciation was marked by the expansion of extensive ice sheets, a substantial drop in global sea levels, and widespread cooling that reshaped Earth’s climate. Notably, it coincided with the Late Ordovician mass extinction the only major glacial episode directly associated with such a dramatic loss of marine biodiversity including trilobites, brachiopods and other benthic organisms.
The Tufs et Calcaires de Rosan Formation is exposed along the Crozon Peninsula, records a volcanically influenced shallow marine environment with tuffs, limestones and pillow lavas. Its benthic fauna is dominated by articulated brachiopods of Strophomenata and Rhynchonellata including Hedstroemina, Leptaena and Dalmanella.
The Crozon Peninsula and the Armorique UNESCO Global Geopark is a remarkable place to visit. From ancient seas that deposited sedimentary layers to Hercynian mountains now reduced to eroded peaks, the region preserves over half a billion years of Earth’s history. Its granitic plutons, Kersantite veins, Armorican Sandstone cliffs and fossil record of marine invertebrates gives a glimpse into a pre-Mesozoic ecosystem.
