The age of corrie moraines and postglacial environmental change in the Cairngorm Mountains, Scotland
In this thesis the hypothesis that the corrie moraines in the Cairngorms were built during the Little Ice Age of the 16th-19th centuries is tested. Today the Cairngorm massif is the snowiest area of Britain and it is not unreasonable to suggest that it would be the first area to support a marginal glaciation when temperatures fell in the Little Ice Age. This is in contradiction to the more widely held view that the moraines relate to the Loch Lanond Advance of 10,800-10,300bp. In collecting data to test the hypothesis information has also been obtained which has allowed reconstruction of the environment of the corries since they last contained ice. Three different methods have been used to date corrie moraines. Sediments collected from the base of three corrie lochans enclosed by moraines have been dated by pollen analysis and radiocarbon assay. The pollen profiles obtained have been divided into local pollen assemblages and correlated with regional pollen assemblages for which good radiocarbon chronologies exist. Two of the corries contain sediments from the middle of the pine- birch phase putting their age at approximately 6-7,000 years whilst the third profile exhibits the birch-hazel phase and is likely to date from 9,000bp. Six radiocarbon dates on the corrie sediments confirm the magnitude of these estimations. The inference from this data is that the corries have not been glaciated for at least 6000-9000 years because if they had the sediments would have been removed. The glaciation immediately prior to both 6,000 and 9,000bp is the Loch Lanond Advance so it is likely that this is the period when the corries were last glaciated. Thirdly, lichenometric dating is used. The theory on which this thesis is based emerged from previous findings of small lichens inside moraines and the suggestion that their growth was retarded by Little Ice Age glaciers. Detailed measurements given in this thesis confirm the earlier data. Of the nine corries with moraines eight have smaller lichens inside the moraines than outside them. With the Little Ice Age hypothesis shewn to be invalid alternative explanations for this pattern are sought. It is concluded that it is most probable that lichen growth inside the moraines has been restricted by snow cover (but not glaciers) during the Little Ice Age. Inside the exceptionally high altitude moraine in Garbh Choire Mor there is evidence that lichens are being killed by semipermanent snow. A temperature drop of only 0.5 C would be needed to bring semipermanent snow down to the altitude of the majority of the other corries. Therefore, semipermanent snow in the corries during the Little Ice Age, when temperatures may have dropped as much as 2 C, is easy to conceive. Absolute dating of the period of snow lie is not possible both because of the lack of dated surfaces which are needed to construct a growth curve and because of the differences in lichen sizes which occur between corries, possibly resulting from differing geology and altitude. However, as the Little Ice Age is the only known period within the range of lichenametry it is reasonable to assume that it is the period involved. Further investigation of the corrie sediments has permitted identification of disturbances in the pollen profiles which might affect the interpretation of the chronology, whilst it also proved interesting in its own right. Reference to pollen, diatoms, particle size, organic and chemical content indicate an environmental and probably climatic optimum in the early and mid Postglacial. At 3,000bp conditions began to deteriorate to the levels of the present day. Interestingly there is no evidence of an elm decline at 5, 000bp, the date at which it normally occurs in Britain, nor is there evidence of climatic deterioration at that time. Thus suggestions that the elm decline that is found elsewhere at 5,000bp is due to climatic deterioration cannot be substantiated, especially as the northerly location and the high altitude of the Cairngorms makes the area one of the climatically most sensitive in Britain. There is no evidence of any further environmental deterioration in the Little Ice Age as might have been expected.