Ice carves out valleys and corries
During the Ice Age a succession of ice sheets and mountain glaciers covered Scotland. The ice was not stationary, but moved leaving scratches called striae on the rocks, as stones and boulders in the base of the ice acted like a gigantic sheet of sand-paper. The ice flowed south from the Highlands over the region, scouring out deep hollows in the Kyles of Bute and on either side of Arran. The ice, too, picked out the softer rocks and left the harder volcanic rocks like granite, dolerite and felsite upstanding.
Throughout this period, valley glaciers carved out typical alpine valleys such as Glen Rosa. Alternate freezing and thawing of the rocks on the higher ground, combined with the glacial action, created the typical mountain hollows known as corries after the Gaelic word for these features. Where two corries erode towards each other, the result can be a saw-toothed ridge called an aręte such as A’ Chir, the cock’s comb. These effects of glaciation are particularly noticeable in the granite hills.
Mounds of glacial debris called moraines were abandoned in the lower parts of the glaciated valleys, when the ice melted. The ground moraine (glacial till) left by the last ice-sheet covers much of the low ground of south and west Arran. Rocks and boulders were rounded and carried by the ice, sometimes long distances from their source; rock fragments from Ailsa Craig have been found as far afield as Pembrokeshire in South Wales. These are called erratics (wanderers) and large ones can form spectacular features.