Geology

The Isle of Arran possesses such a variety of rocks types and formations within a small area that it's a wonderful destination for anyone interested in the earth sciences, whether they're professionals, students or enthusiastic amateurs. We've put together some information about the Geology of Arran, the reason why Huttons' Unconformity is so famous, and how the Ice Ages have shaped the Isle of Arran.

Geology of Arran

Geological map of Arran
The geology of Arran
The rocks of north Arran, are divided geologically from the rest of the island by part of the Highland Boundary Fault - a great crack in the earth's crust which separates Highland Scotland from the Lowlands of the Midland Valley. In Arran, the fault cannot easily be seen at the surface, but it curves round the granite from the coast north of Lochranza to emerge again at the coast at Dougarie.

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
Sills on Arran
The sills and dykes of south Arran are clearly evident within the landscape here. The sills are harder than the rocks into which they have intruded, and as erosion has worn away those surrounding weaker rocks the sills have been left standing 'proud'. These sills form the steep 'steps' in the landscape that give the hills above Whiting Bay their character. Sills form the dramatic cliffs at Bennan Head, Dippin Head and Drumadoon Point, and have also shaped the waterfalls for which the area is well known. Because the rivers cannot erode the hard sills as rapidly as the surrounding rock, cascades or waterfalls have been created where water flows over junctions between the two rocks.

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.

Huttons' Unconformity
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Those who are familiar with geology will immediately associate Arran with 'Hutton's Unconformity'. This is one of the sites at which James Hutton (1726 - 1797), first identified this type of geological structure. He was then able to put forward a theory about the geological history of the earth that was to have as profound an effect upon society as did Charles Darwin's 'Origin of the Species', as he was the first to propose that the earths' surface had evolved over an immense period of time.

Hutton's Unconformity - near Lochranza
Hutton's Unconformity - near Lochranza
Visiting Lochranza in 1787, Hutton explored the coast to the north of the village. Here, he noticed that one rock strata which was inclined nearly vertically was overlain by another rock strata which was almost horizontal. On seeing this, he concluded that the layers of the lower strata (schist) had been tilted and eroded over a long period of time before the upper strata (sandstone) had been deposited on top of it.
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.

The Ice Ages
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Although the underlying geological structure of Arran was formed many millions of years ago, the character of the landscape we see today largely results from the action of ice during the Ice Ages, the most recent of which ended about 10,000 years ago. The description and interpretation of landforms, such as those created by the Ice Ages, is actually referred to as Geomorphology, rather than Geology, but let's not quibble here . . .

Raised Shorelines
Over the last 2 million years, Arran was repeatedly covered by ice sheets that spread out from the Scottish Highlands. The weight of this ice depressed the surface of the land, and Arran was pushed downwards into the earths' crust. Once the ice began to melt, Arran began to rise up again. This uplift - or 'rebound' - after the melting of the last ice cover has taken place gradually, over a period of many thousands of years - and it is still continuing today.

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.

Kings Cave
Kings Cave
Relatively minor fluctuations in sea-level since the end of the Ice Age has meant that there has been several different levels of shoreline since then. The features such as raised beaches and dry cliffs associated with these different sea-levels are found around the whole island. The highest ones are 30m above the present sea-level. Another series of raised shorelines can be seen in some places at 20m a.s.l., but the most extensive raised beach is that seen at 10m above the present sea-level. This raised shoreline is well developed along the coast between Brodick and Corrie, where the road runs for miles along the surface of the old beach, and the old cliff face can be seen 30 or 40m inland.

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.

Glaciated landforms
The Ice Age sculpted the landscape of north Arran in a very dramatic fashion. Even at times of less intense cold, Arran's mountains contained a local ice-cap. This ice was centred in the northern hills and from there the glacial ice flowed downhill to carve deeply into the rocks. The action of the ice created classic 'U-shaped' valleys, such as Glen Rosa, Glen Sannox and Glen Catacol. Glacial striations and polished rock surfaces can be easily seen in these valleys today.
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.

Glacial erratics
Glacial erratics
At the end of the last Ice Age, the glaciers of Arran began to melt at their snouts, and the debris that they were carrying was dropped to form moraine ridges, which mark the position of maximum extent of the glaciers. Moraine ridges can be seen at a number of locations on Arran, but the most easily accessible are in Glen Rosa, where they form a series of mounds on the valley floor. These are 'recessional moraines' and were formed as the Glen Rosa glacier paused in its retreat. Each pause was marked by a new terminal moraine. Glen Cloy is also a good place to see moraine, as the pre-historic fort known as 'Bruce's Castle' is sited on a major moraine ridge.

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.
 

Isle of Arran