Hummocky and streamlined drift mounds in Harray









The interior of a drift mound

Hummocky moraine

Definition: a strongly undulating surface of ground moraine, with a relative relief of up to 10 m, and showing steep slopes, deep, enclosed depressions and meltwater channels. It results from the downwasting (i.e. thinning) of ice which may be stagnant or active. Blocks of ice may squeeze debris released from the ice into crevasses between the blocks.

Hummocky moraine has been regarded as diagnostic of the former presence of stagnating valley glaciers at the termination of the Loch Lomond Stadial (Sissons, 1979) regarded. More recent detailed mapping has shown that the hummocks in many Scottish glens are part of integrated systems of small ridges and meltwater channels that indicate active recession of valley glaciers (Bennet, 1996) and that hummocky moraine also formed during deglaciation.

On Orkney, hummocky drift occurs on parts of Hoy and Rousay but reaches its most extensive development in Harray. Here extensive areas of ground show low hummocks, mostly less than 100 m long and 5 m high, oriented between N and W. Some retain boulders on their surfaces and others may have been lost to agricultural improvement. Available exposures are sufficient to indicate that the the hummocks often have a core or base of bedrock and consist of locally-derived, rubbly till. No detailed mapping has been completed of these mound system. It is uncertain as to whether the terrain should be interpreted as hummocky moraine formed beneath stagnant ice or drumlin fields streamlined by active ice flow.