Evidence of the processes of glacial erosion is not hard to find. Numerous striated surfaces attest to the power of abrasion, where blocks embedded in the glacier sole have gouged the glacier bed. Many tills contain large plates of sandstone which have been plucked from the fractured sandstone. Where these remain sharp-edged the distance of travel has been small but some boulders are facetted and bullet-shaped due to having been pressed face down on to the bedrock by moving glacier ice.
The hills of Hoy show a different assemblage of forms that suggest long phases of local glaciation. Shaded hollows have been occupied by occupied by corrie glaciers during periods when conditions were just cold enough to support small glaciers. The two major valleys of north Hoy relate in part to valley glaciation but both are glacial breaches, cut through the main watershed, and so it is likely that ice has moved through them beneath ice sheets.