Glacial lake

A simple model of an ice lobe within a through valley such as Strath Spey. The glacier leaves small moraines across each tributary valley floor. Drainage from the ice-free upland is blocked and pocket lakes develop in the valley heads. The lakes may spill over cols into adjoining valleys and cut channels.

Glacial lakes

Definition: ephemeral lakes created where glacier ice blocks a valley or embayment and ponds the drainage. The lakes may drain through the ice barrier or via spillways through cols. Drainage may be rapid and the floods of meltwater (jokulhaups) may form sculpted bedrock surfaces and leave behind poorly-sorted boulder gravels.

Where an ice lobe flows along the flanks of an upland massif then there is considerable potential for the formation of temporary lakes due to damming of the valleys which drain the upland. During deglaciation, the Cairngorm plateau emerged first from the ice and the lower parts of several Cairngorm valleys then became ice-free. Major ice lobes in Strath Spey and, to a lesser extent, in the Dee valley trapped a series of marginal lakes and ponds in these valleys. A series of these pocket lakes can be traced at various elevations along both sides of Strath Spey downstream from Newtonmore to Glen Feshie (Merritt et al, 2004), Knockando (Hall et al., 2004) and Rothes (Sutherland, 1984, Hall et al, 1995).

The northern flanks of the Cairngorms carry an exceptional and complex sequence of landforms associated with the advance and retreat of the Strath Spey ice lobe including lateral and terminal moraines, meltwater channels and kame terraces. Evidence of former ice-dammed lakes is particularly well-developed between Dorback and Gleann Einich. Bremner (1932) recognised that the thinning of Strath Spey ice against the flanks of the hills of Abernethy Forest had left a staged series of spillways, fans and terraces that could be related to prominent lateral moraines. In the embayment of Faesheallach Burn the major terrace is comprised largely of undeformed lake sands and provides an excellent example of a kame terrace. In the adjacent valley of the Allt Bheadhair, the lake sediments are associated with a sequence of moraines that mark ice limits across the former valley. Sections through the moraines shows contorted and faulted lake sands, deformed by minor readvances of the ice front. In lower Strath Nethy, rock-cut channels at 670 m OD acted as spillways for meltwater flowing at the margin of ice in the Glenmore embayment and this meltwater and its sediment accumulated in a lake dammed by the lower end of the ice lobe across the mouth of the Strath. The stepped series of kame terraces mark former positions of the lake floor. The largest lakes developed in lower Glen Feshie and Gleann Einich. The latter lake had a complex history, with advances of both Cairngorm ice in the upper glen and Strath Spey ice in the lower glen (Golledge, 2002). Thick sequences of sand, with local drop stones derived from ice bergs that floated on the lake, and gravel cover the floor of the glen and postglacial river incision has isolated a fine series of kame terraces. The sands deposited in these ice-dammed lakes have clear potential for dating by luminescence techniques wherever it can demonstrated that the sands were fully exposed to sunlight during transport and deposition. First results from Gleann Einich indicate that the sands there are generally poorly bleached (Everest and Golledge, 2004).