Loch of Boardhouse, one of many scenic walks around Birsay
The hamars of Fitty Hill, Westray
In the gently-dipping sandstone terrain of Orkney, it is often difficult to pick out classic landforms of glacial erosion. Low-lying areas often show a pronounced SW-NE grain to the topography, parallel to the main direction of ice sheet flow. This orientation stems largely from the deepening of valleys, with excavation of basins where ice flow was constricted between hills.
Good examples occur in North Mainland and are now filled by the Lochs of Boardhouse and Swannay. The hill flanks which now form the valley side are steepened and the up-ice (stoss) side of the hills is smoothed but there are few signs of the formation of cliffs on the lee side and so these hills are not true roches moutonnées. The ice deformed around these hills to leave egg-shaped domes with ribbed sides. On the higher hills of Rousay and Westray these ribs are exposed as cliffs (hamars) on glacially over-steepened slopes. Deep erosion has been focussed in the intervening depressions. It is the drowning of these branching valleys by post glacial sea level rise that has isolated the northern isles one from another. The smaller islands that rise as low domes above sea level can be seen as till-capped rock drumlins.
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.