The trough head above Loch Avon. Undated moraines conventionally attributed to the Loch Lomond Stadial.     Image by Neil Glasser

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Schematic diagram of the thinning glaciers in the area around the Northern Corries from the LGM to final ice melt

 

Deglaciation history

The timing of deglaciation of the Cairngorms is uncertain. Until recently, the only evidence of the date of deglaciation was that provided by radiocarbon dates from organic deposits formed after ice retreat. These dates are obviously minimal and may be distorted by hard water errors. New cosmogenic exposure ages on erratic boulders may be more useful but are dependent on erosion rates. Exposure ages from the eastern Cairngorms indicate deglaciation of much of Ben Avon by or slightly after 14.9 ka; deglaciation ages from Glen Geusachan and Gleann Einich in the western Cairngorms are around 14.0 ka for realistic erosion rates (Everest and Golledge, 2004).

The pattern of deglaciation is more clear. The Ben Avon massif was probably first to emerge (Brazier, et al., 1996) and then ice progressively disappeared from the summits further west. The sequence of retreat in Strathspey can be traced southwestwards from Dorback to Kingussie by the striking series of retreat moraines and marginal lakes (see Bremner's fine series of sketches in Strath Nethy). The retreat of ice up the Dee and Gairn valleys has yet to be mapped in detail but seems to have involved a gradual retreat from a still stand position in the Howe of Cromar. The northern slope of Lochnagar shows particularly fine sequences of terminal and lateral moraines where the ice surface has been lowered against the flanks of the hill from 800-400 m OD (Brown, 1993). The final position of lobes of Cairngorm ice in Gleann Einich, Gleann Feshie and the Slochd Mhór are marked by fine series of end moraines and kame terraces associated with ice marginal lakes.

Deglaciation N CairngormsWe can make a general reconstruction of the sequence of events during ice retreat in the area around the Northern Corries. At the Last Glacial Maximum at ~22 ka, the whole of Scotland was covered by an ice sheet. In the Cairngorms, this ice sheet was probably several hundred metres thick even above the highest summits. The Cairngorm ice cap was sufficiently thick to exclude the Strath Spey ice stream from the mountains, as shown by the general of schist erratics above 800 m. By 20 ka, the ice was thinning, and separation of Cairngorm and Strath Spey ice probably occurred by ~18 ka. Thereafter, there was a series of minor readvances along the eastern margin of the Strathspey ice lobe, each producing moraine belts and damming meltwater in side valleys. At this time, the ice in the Northern Corries was quite restricted in extent but its limits can be seen on corrie sidewalls. Final retreat of ice from this part of Strath Spey was probably completed by 15 ka. Small glaciers reformed in the corries at 12 ka, now marked by boulder moraines in the inner bowls, and melted away around 1 ka later.