Igneous rocks including granites and lavas, occupy half the island. Within the mountain masses there are two granites. The older coarse-grained granite gives rugged, dramatic scenery. All the major mountains of Arran consist of this rock. Into this granite younger fine-grained granite has intruded.

In the south, rocks from lava flows dominate the landscape. Magma injected vertically into the earths crust form narrow sheets known as dykes while sills developed where the magma flowed in a sheet between the planes of the surface rock.

Where Magna is harder than surrounding rocks it forms sills caused by erosion wearing away weaker rocks. These form steep steps in the hilly landscape above Whiting Bay. Rivers tumble over the junction between the hard and softer rocks in a cascade sometimes leading to a staircase of waterfalls. The massive bulk of Holy Isle sheltering Lamlash Bay formed from a cone-sheet sill.

Dykes appear around most of Arran - some up to eight metres high. In the north dykes cut the granite but here the granite is the tougher, and the worn away dykes form small gorges, pools and waterfalls.

The oldest of the other rock types are the schists found in north-west Arran. These underlie moors and coastal slopes flanking the granite peaks behind Lochranza, Gatacol and Pirnmill and appear in coastal cliffs. Alongside the North Sannox Burn where it passes beneath the A841 it forms first-rate sunbathing and picnicking platforms.

A geological fault separates the Dalradian rocks and the northern granite from the rest of the island. The fault curves round the granite from the coast north of Lochranza emerging again at the coast at Dougarie. To the south of the fault, the main rocks are red and white sandstones and conglomerates, with some limestone and beds of coal. These sedimentary rocks weathered to give fertile soils.

The ice ages produced boulder clay. Thick deposits appear where streams have cut down into them and in roadside cuttings along the Brodick-Lamlash Road. The ice sheet deposited blocks of the northern granite all over the south of the island.

Local glaciers polished and shaped the glens and corries to give polished walls of granite in Glen Rosa and Glen Sannox. As the glaciers melted, debris carried dropped to form ridges and mounds called moraines.

As the ice fields retreated, vast volumes of water entered the sea. Sea level rose and coastal lowlands became flooded. As the climatic warming ended, the rise in sea level slowed, while the land freed of the weight of ice rose, raising sea beds clear of the sea.

The result of all these forces over millions of years has been to produce an island with a physical geography complex and fascinating.