Geological History: Arran over the past 600 Ma years

Arran has seen the closing of ancient oceans and the extreme volcanism of continental rifting. In between there have been periods where the climate was tropical, temperate and even arid conditions prevailed at some time. The rocks deposited throughout Arran’s history help to tell the story of evolving and changing environmental conditions and settings.

The oldest rocks on Arran are Dalradian. These metamorphic schists, quarztites and slates were originally deposited as deep marine muds and turbidites off the margin of Laurentia -the forerunner to the North American continent. The basement rocks which make up the southern part of Arran was, at this time, very far away indeed.

From Ordovician to late Silurian times, Arran was involved in the closing of an ocean and the building of mountain chains (the Caledonides) which were as high as the Himalayas are today. The Midland Valley Terrane, a volcanic island arc, was accreted to the passive margin of Laurentia, causing Precambrain and Cambrian marine deposits to become the regionally metamorphosed Dalradian rocks. During the Silurian the remaining fragments of the British Isles became sutured together by continental collision. This was a time of rapidly rising mountain chains and equally rapid erosion.

The unroofing of the Caledonides provided much sediment. During the Devonian, Arran was drifting towards the equator and the moist, tropical climate enhanced the weathering of the large mountains. These deposits are succeeded by the deltaic, fluvial and coal swamp deposits of the Carboniferous. The Carboniferous saw Arran at it’s warmest and most fertile, however the development of coal on the island is not significant due to its being on the edge of a large basin.

Drifting into latitudes equivalent to the sub-Sahara today, Arran became a desert during the Permian. The assembly of the supercontinent Pangea made it difficult for moist air to reach the continental interior where Arran was located, and large dune sands covered the landscape. The subsequent initiation of the breakup of Pangea allowed for the climate to become much more temperate at the start of the Triassic. Lacustrine (i.e. lake) and shallow marine environments prevailed. Fine sand, shale and muds were deposited during this time.

The rifting of Pangea caused sea level to increase dramatically during the Jurassic and Cretaceous. Arran would have been nearly inundated by an expansive shallow sea. The deposition of organic rich mudstones and chalk characterized this time. Only foundered blocks of these Mesozoic rocks remain on Arran. They can be found in the Central Ring Complex. The existing cover has been completely removed by Quaternary glaciation.

The Tertiary saw the opening of the North Atlantic Ocean and the final separation of North America from Eurasia. This event was coupled with intense magmatic activity, and the creation of a chain of volcanoes along the west coast of Scotland. Arran was roughly located at the centre of this volcanic chain. The igneous rocks intruded and extruded during this rime are collectively known as the British Tertiary Province.

Two major volcanoes existed on Arran. The Northern Granite and the Central Ring Complex are the exposed roots of these volcanoes. The plutons associated with the roots of the volcano have been punched up through the sediments causing widespread deformation and the creation of the structural pattern for the rocks we see surrounding the Granite today.

The last 2.5 Ma of Earth history has seen the growth and retreat of glaciers. These massive ice sheets have sculpted out the u-shaped valleys and created the corries and peaks that dominate the scenery of Arran today. Isostatic recovery of the land from the weight of the ice was slow, and the result of this is the raised beaches and terraced rivers that can be seen all over the island.