Creag an Leth-choin (Lurcher's Crag)
A steep granite slope descending from glacially-modified tor stumps on the skyline. Solifluction lobes are well-developed on this block slope. Strong structural control is also evident from the parallel rock steps that cross the slope
Dating periglacial features
A wide variety of periglacial features has been recognised in the Cairngorms, both relict and active. Often the age of these features is not clear - are they active today, are they inactive today but were active earlier in the Holocene or did the features form before the Holocene? There are several ways to establish the age of periglacial features in Scotland (Ballantyne 1984).
1. By reference to the limits of Loch Lomond Stadial glaciers
During the Loch Lomond Stadial (13.5-11 ka) glaciers developed in many corries in the Cairngorms. Today in these corries we find scree slopes, solifluction lobes and terraces which can only have formed since the ice melted. These features must have formed over the last 10 ka. Conversely, there are features in the corries which only occur outside these glacial limits, such as severe frost-shattering and the associated thick talus accumulations. These features must have formed during or before the Loch Lomond Stadial. As the climate was mild during the preceding Windermere Interstadial, it is likely that the frost weathering occurred under the severe cold of the Loch Lomond Stadial.
2. Radiocarbon dating of buried organic materials
Solifluction lobes on Cairn Gorm rest on buried vegetation dated to 5-6 kyr and so must have been active in the recent past and may be active today. Hence buried organic materials can provide maximum ages for the overlying material, Unfortunately, this approach is not useful in dating periglacial features from before the Loch Lomond Stadial as environments in the Late Devensian were too severe for plants to grow and peat to form.
3. Measuring rates of current activity
It is possible to measure how quickly small solifluction lobes and ploughing blocks move by repeated detailed surveys. Similarly, rockfall can be directly observed and rates of rock accumulation measured (for example, on snow patches). In Coire Raibert, small-scale patterned ground used to reform within three years but the process appears to be disrupted nowadays by the trampling of many walkers.
Relict periglacial features are covered with peat, soil or vegetation.
5. Relations to glacial erosion
In the Cairngorms, blockfields have been stripped from the lower parts of the plateau by glacial erosion. The blockfields must predate the last ice sheet.
6. Weathering pits
Some blocks within blockfields or plateau regolith carry weathering pits >10 cm deep which must have formed during the last interglacial or earlier. The blockfields predate the last interglacial.
7. Cosmogenic exposure ages
new technique measures the build-up over time of tiny
amounts of isotopes in rock minerals derived from cosmogenic radiation. Ages for
the surfaces of tors in the Cairngorms indicates that the oldest were first
exposed around 500,000 years ago.