Glacial modification of granite tors in the Cairngorms, Scotland

• Published in: Journal of Quaternary Science
• Main Authors: Hall, A. M., Phillips, W. M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2006
• Subjects: Cold-based glacier
• Online Access: http://dx.doi.org/10.1002/jqs.1003; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fjqs.1003; https://onlinelibrary.wiley.com/doi/pdf/10.1002/jqs.1003

Abstract A range of evidence indicates that many granite tors in the Cairngorms have been modified by the flow of glacier ice during the Pleistocene. Comparisons with SW England and the use of a space–time transformation across 38 tor groups in the Cairngorms allow a model to be developed for progressive glacial modification. Tors with deeply etched surfaces and no, or limited, block removal imply an absence of significant glacial modification. The removal of superstructure and blocks, locally forming boulder trains, and the progressive reduction of tors to stumps and basal slabs represent the more advanced stages of modification. Recognition of some slabs as tor stumps from which glacial erosion has removed all superstructure allows the original distribution of tors to be reconstructed for large areas of the Cairngorms. Unmodified tors require covers of non‐erosive, cold‐based ice during all of the cold stages of the Middle and Late Pleistocene. Deformation beneath cold‐based glacier ice is capable of the removal of blocks but advanced glacial modification requires former wet‐based glacier ice. The depth of glacial erosion at former tor sites remains limited largely to the partial or total elimination of the upstanding tor form. Cosmogenic nuclide exposure ages (Phillips et al. , 2006) together with data from weathering pit depths (Hall and Phillips, 2006), from the surfaces of tors and large erratic blocks require that the glacial entrainment of blocks from tors occurred in Marine Isotope Stages (MIS) 4–2, 6 and, probably, at least one earlier phase. The occurrence of glacially modified tors on or close to, the main summits of the Cairngorms requires full ice cover over the mountains during these Stages. Evidence from the Cairngorms indicates that tor morphology can be regarded as an important indicator of former ice cover in many formerly glaciated areas, particularly where other evidence of ice cover is sparse. Recognition of the glacial modification of tors is important for debates about the former ...