Geology of the Isles of Scilly

A brief guide to geological events that have shaped the islands over the past 300 million years. Rock types and geological features providing visible evidence of these events are included, with Ordnance Survey grid references.

 
 


Origin of the granite bedrock

Granite outcrops and pebbles you can see on the shorelines of Scilly are about three hundred million years old. They were formed from a large body of molten rock, or magma, known as a pluton, reaching 10 km down into the ground and extending under the sea all around Scilly. The Scilly pluton is one of a group,  joined deep underground in a single mass reaching through Cornwall and Devon as far as Dartmoor. Over three hundred million years ago, the molten granite lay under sandstone and mudstone sediments at the bottom of an ancient sea located along the southern margin of Britain. Collision of ancient continents in the region of southern England and Northern France formed an ancient mountain belt, much like the Himalayas today, but extending from the Southeastern USA through France and Spain to Eastern Europe. The buoyant molten granite rose up under these mountains, cooled and crystallized, binding the deep roots of the old mountain belt. The formation of these mountains marks a key time in the geological history of the British Isles, finally taking the region away from the active edges of a continental plate and into the quieter interior. The Scilly granite pluton was thus one of the last drops of geological glue that helped assemble the rock foundations of Britain into their current state.


Minerals in the granite

The Scillonian granite you see now was a few kilometres underground when it solidified, a process which took millions of years. The chemical  composition of the granite shows that it was formed from rocks that melted deep in the Earth’s crust. Whilst still molten, the cooling granite was stressed by Earth movements, leading to flow and alignment of early formed crystals in the granite. Similar movements after it had solidified, made cracks and joints. These cracks have since been exploited by weathering and erosion, forming the spectacular tors seen around the islands. In some places, minerals crystallised out from fluids injected into the cracks. Over much of Cornwall these fluids contained commercially valuable metals, such as tin and copper. The Scilly pluton is unusual as it seems to lack these metal ores, but it does have veins of white quartz and black fibrous tourmaline


Fate of the sandstone and mudstone

The hot granite pluton heated and altered the older sandstone and mudstone “country rocks” into which it was intruded. Over the next three hundred million years, uplift and erosion of South West England exposed the overlying rock and the deep roots of the old mountain belt, including the Scilly pluton to erosion. As a result, the country rocks remain only on the seabed offshore, but pieces of the red sandstone and grey mudstone rock are found as pebbles on some beaches in Scilly. As they eroded away, fragments of the country rocks that once overlay the granite pluton were transported  as far as Devon, Dorset and Hampshire, where they formed new sedimentary layers of rock embedding dinosaur skeletons. The erosive powers of the wind and sea also sculpted dramatic granite rock formations in Scilly, and provided the white quartz and milky feldspar  grains in the sandy beaches.


Ice age deposits

During the last Ice Age, 21,000 years ago, a large glacier flowed southwards down the Irish Sea Basin reaching as far as the northern extremities of Bryher, Tresco and St Martin’s. South of this ice limit, cold tundra conditions resulted in the accumulation of orange-brown slope and wind-blown deposits known locally as “ram”. This ram is still visible in the cliffs around the islands. In places, the ram contains organic deposits containing fossil pollen of tundra plants, radiocarbon dated to the interval between 21,000 and 30,000 years ago. The northern glacial deposits contain flint eroded from the floor of the Irish sea; this resource was used locally in stone age times to make tools. Some of the flint tools are on display in the museum, on St Mary’s. It is likely that earlier glacial events affected Scilly, but any evidence for these has been eroded by the events of the Last Ice Age. As the climate warmed and the ice melted, it would initially have been possible to walk from Scilly to Cornwall, South Wales, Brittany, and Ireland, but eventually sea levels rose, forming a large single island, and ultimately the archipelago we know today.



More on the geology of the British Isles can be found at these sites:

British geological survey

Southampton University Geology

Cornish geology website

Rock specimens can be viewed at the Isles of Scilly museum; see Isles of Scilly museum website for opening times.


Further reading

British Geological Survey (1975) Geological map of the Isles of Scilly, Sheet 357/360 1:50,000 Series.


Hunter and Easterbrooke (2004) The Geological history of the British Isles. The Open  University, ISBN 0-7492-0138


McLeish (2002) Geological Science. 2nd Edition, Nelson, ISBN 0-17-448221-3


Scourse (2006) The Isles of Scilly, Field guide. Quaternary research Association, London



Written in collaboration with

Prof. Robert E Holdsworth,

Department of Earth Sciences,  University of Durham   and

Prof. James Scourse,         

School of Ocean Sciences, Bangor University                                                             

 

Main events shaping the geology of Scilly