The passage of ice sheets has given a pronounced SE-NW grain to the topography of Mainland and Rousay and excavated the sounds between the islands. The rocks embedded in the ice left scratch marks, known as striations, on the bedrock, giving the general direction of movement of the ice towards the northwest. The ice carried rocks from its source area in northern Scotland, including metamorphic erratics, and from the bed of the North Sea, including chalk and flint, but the bulk of the material is derived more locally from the Devonian sandstones. Much soft sediment was scraped up by the ice from what is now the sea bed and the resultant mix of mud, sand, broken shells and stones gives the layer of boulder clay that now covers much of Orkney in a blanket from a few centimetres to 20 metres deep. The best place to view this material is at Scara Taing, Rousay, where three distinct layers of boulder clay are superposed, each resting locally on a striated sandstone pavement. The final movement of the thinning ice sheet moulded the boulder clay of Harray and Rousay into a series of low, elongate hills or drumlins.
The timing of glaciation on Orkney is unclear. Evidence from offshore indicates that the Ice Age started about 2.4 million years ago. The major ice sheets developed at intervals after 850,000 years ago, with the last (Late Devensian) ice sheet expanding after 32, 000 years ago. This ice eventually retreated and thinned, with its final disappearance from Orkney about 15,000 years ago.
After a brief amelioration of climate, the short and suddenly cold period known as the Loch Lomond Stadial, 13,000 to 11,500 years ago, terminated the Devensian cold stage. A small glacier probably reformed in Enegars corrie on Hoy and conditions of arctic severity prevailed throughout Orkney. Permafrost gripped the ground, with frost shattering of barren rock surfaces and the churning and solifluction of boulder clay deposits. After 11,500 years ago, the climate changed abruptly to warmer conditions than the present time and sea water levels rose rapidly.