Carsaig, located on the southern coast of the Isle of Mull, is renowned for its stunning coastal scenery shaped by dramatic volcanic landscapes and a rich geological history. This area offers an exceptional window into ancient Earth processes, from fiery volcanic eruptions to the presence of ancient seas, all preserved in the rocks along the shore.
Geological Background
The geology of Carsaig is dominated by the Mull Lava Group, a series of basaltic lava flows formed during the early Palaeogene period, roughly 58 to 60 million years ago. This volcanic activity was part of a major episode associated with the opening of the North Atlantic Ocean, when intense volcanic eruptions produced thick layers of basalt that now form the cliffs and sea stacks of the region.
These basalt flows at Carsaig display characteristic columnar jointing—vertical, hexagonal columns created as the lava cooled slowly and contracted. The spectacular rock formations, including the famous Carsaig Arches, owe their shape to both the cooling patterns of the lava and millions of years of coastal erosion.
Between the basalt flows, thin layers of sediment and ancient soil, known as paleosols, are preserved. These represent periods when volcanic activity paused, allowing vegetation and soil to develop, before the next eruption buried the landscape again. Such layers provide valuable insight into the environment of Mull during the early Palaeogene.
Fossil Evidence at Carsaig
Unlike other parts of Mull where fossilised tree casts have been found (notably the Ardmeanach peninsula), Carsaig itself is best known for marine fossils embedded in the sedimentary rocks and shore platforms exposed at low tide.
- Ammonites and Belemnites: Fossil remains of these extinct marine cephalopods are commonly found in the Jurassic and Cretaceous sedimentary rocks beneath the basalt layers or in interbeds. Their coiled and bullet-shaped fossils indicate that this area was once covered by shallow seas.
- Bivalves, such as Gryphaea (“Devil’s Toenails”): These oyster-like fossils are plentiful in the sedimentary layers along the shore. Gryphaea fossils are significant indicators of the marine conditions that existed before volcanic activity reshaped the landscape.
- Brachiopods and Gastropods: Impressions and fossilised shells of these marine invertebrates are also found within sediment layers, further confirming the ancient marine environment.
While Carsaig’s dramatic cliffs are primarily volcanic in origin, these fossils tell a story of changing environments—from prehistoric seas to lush landscapes interrupted by volcanic flows.
The Carsaig Arches and Coastal Landscape
The Carsaig Arches are natural sea arches carved out of basalt by wave erosion, showcasing the interplay of geology and ocean forces. The cliffs and stacks formed from the layered lava flows offer a vivid demonstration of volcanic geology in action.
At low tide, the rocky shore near the arches reveals fossil-bearing sedimentary beds. These fossil layers are accessible to visitors, providing a rare opportunity to observe marine fossils set against a backdrop of volcanic rock.
Visiting Carsaig
Reaching Carsaig involves a drive along a narrow single-track road leading to Carsaig Pier, a quiet and remote spot. From here, the coastal path allows visitors to explore the arches and nearby fossil sites. The walk is rugged and involves uneven terrain, so sturdy footwear is recommended.
Fossil collecting is discouraged to preserve the scientific and educational value of the site. Instead, visitors are encouraged to observe and photograph fossils in situ.
Summary
Carsaig offers a compelling glimpse into the Isle of Mull’s volcanic past combined with evidence of ancient marine life. The basalt cliffs and sea arches are iconic volcanic features, while the fossil-rich sedimentary layers speak to a time when this landscape was submerged beneath prehistoric seas.
The site stands as a natural archive, telling a story of dramatic geological change and the persistence of life through deep time.
