|Geological map of Arran|
Igneous rocks, which include granites and lavas, occupy almost half of the area of Arran. These rocks were formed 50-60 million years ago (Tertiary period), when Arran was probably joined to the mainland. At that time, the area was dominated by a large volcano which later collapsed to create a surface depression, or caldera, nearly 5km in diameter. The remnants of this caldera are between the String Road and the Ross Road, although little can be seen at the surface.
In the north, the hills of Arran are carved from the remains of a large mass of molten magma which pushed its way up into the earth's crust. When this molten granite forced its way upwards, it lifted the overlying rocks by around 3000m. As those rocks eroded away, the underlying granite was exposed, which also then began to be eroded, creating the features which are visible today.
The southern half of Arran forms the most western part of the 'Midland Valley' region. This part of the island is dominated by Devonian and Carboniferous sedimentary rocks, cut by igneous intrusions. The Devonian rocks are all river deposits on Arran, with a typical age of around 380 million years. The Carboniferous rocks here (c. 320 million years) reflect tropical conditions with shallow seas, coastal deltas and swamps. Carboniferous limestones, sandstones and coal deposits can all be found here.
The igneous intrusions into the Devonian and Carboniferous sedimentary rocks on Arran are derived from rising magmas which did not break through to the surface to form volcanic lava. Instead, the magma cooled within the rocks near the surface and has since been exposed as a result of the erosion of the overlying rock. These lava flows have two forms. One was produced when magma was injected vertically into the crustal rocks to form a narrow sheet, known as a dyke. The other type, called a sill, developed where the magma flowed as a horizontal sheet between the bedding planes of the overlying rock.
|Sills on Arran|
Dykes also make a distinctive contribution to the landscape of south Arran. Although dykes are found around most of the coast of Arran, they are particularly evident on the coast between Brown Head and Dippin Head. In this relatively small area over 500 dykes cut across the foreshore. The majority of these dykes are quite small but some, especially near Kildonan, can be up to 8m high. In this area they form natural breakwaters, and tend to trap sand so that the coast here is a sequence of small sand beaches separated by the dark lines of dyke rock.
|Hutton's Unconformity - near Lochranza|
In fact, the vertical layers of schist were deposited during the Silurian Period. Over the next 100 million years these rocks became tilted by movements within the earth's crust and were eroded away. The eroded remains of these rocks were then covered with deposits of sandstone during the Carboniferous Period.
From his observations at Lochranza and alsewhere, Hutton formulated the Uniformitarian theory of geology, which suggested that processes such as sedimentation, erosion and volcanic activity caused changes in the surface of the earth which have been acting in the same manner and at the same rate over the whole of geological history. Hutton proposed that the earth was much older than had been previously thought. It was at Lochranza where the length and complexity of the Earth's history was first fully appreciated.
Arran was also one of the sites where Hutton found evidence that led to his theory that igneous rocks were produced by the cooling of molten magma within the earth's crust, rather than being laid down by water, as was believed by the `Neptunist' geologists of the time.
As the last Ice Age ended, the glaciers and ice sheets melted and the sea-level rose rapidly - faster at first than the land was rising in its 'rebound' from the weight of the ice. As a result, the coastal areas of Arran became flooded. New coastlines were formed at this higher-than-normal sea level which included beaches and cliffs, exactly as they are being formed today. This was happening about about 10,000 years ago.
Once most of the glaciers had melted, the rise in sea-level slowed and then stopped. However, the uplift of the land continued and this raised the coastal features which had been created at the end of the Ice Age to a height where they were well above sea-level. This process occurred several times and in some locations produced a stepped staircase of old shorelines reaching back - up and away from the present seashore.
Caves which were formed by the action of the sea against the cliff thousands of years ago can still be seen in the raised shorelines around Arran. The most famous is King's Cave, to the north of Blackwaterfoot, which is set above 8m of raised beach, and was formed about 6,000 years ago. This is reputed to have been a hiding place for Robert the Bruce when he began his campaign to claim the Scottish throne.
At the same time as glacial ice occupied the highland valleys, the mountain peaks themselves were subject to ferocious ice action, which created hanging valleys, spectacular corries and knife-edged ridges. Frost also worked to change the landscape by breaking down the exposed solid rock. The shattered pieces slid downhill to produce extensive screes such as those which blanket the slopes of Beinn Bharrain and Beinn Bhreac behind Pirnmill.
As the ice moved over the surface of Arran, it scraped at the rock over which it passed, picking up small rocks and stones, mixing them together and grinding them down to form a stony clay known as 'boulder clay' or 'till'. Thick till deposits can be seen on the island at several locations.
The ice also scooped up large boulders and moved them large distances. These 'erratics' can be seen in many locations around Arran, but the most spectacular must be those standing at the side of the road between Corrie and Sannox. These huge erratics were carried down from the mountains by glaciers about 15,000 years ago - although they were probably moved to their present position by mud slides in more recent times.