Glacial modification of granite tors in the Cairngorms, Scotland

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 wetbased 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 together with data from weathering pit depths , 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 existence of nunataks and refugia.

Age and weathering status of granite tors in Arctic Finland (~68° N)

“…An important inference for this study is that the bedrock chemical weathering and the grus/soil stories may not reflect identical histories. The work described in Phillips et al (2006) is expanded upon in Hall and Phillips (2006) where a nuanced description of the impact of cold-based ice upon tor form in the Cairngorm Mountains of Scotland is presented and combined with a set of associated cosmogenic nuclide exposure dates.…”

“…The impact of cold-based ice on the landscape varies from an almost entirely protective one to one that permits varying degrees of modest modification. Hall and Phillips (2006) have shown in the Cairngorm Mountains of Scotland that the morphology of tors previously covered by coldbased ice may be combined with cosmogenic nuclide dating to construct a meaningful geomorphological history. Similar research has been conducted in the Parkajoki region of northeastern Sweden (Hätterstrand and Stroeven et al, 2002a), which at ∼ 67-40′ N, 23-15′ E, lies slightly south and ∼ 250 km to the west of our research area.…”

The first stages of erosion by ice sheets: Evidence from central Europe

“…(i) stripping of regolith from and transport of boulders across low angle surfaces (Kleman, 1994), (ii) progressive removal and transport of large blocks and superstructure from rock outcrops, including tors, in circumstances that preclude toppling, rock wall collapse and periglacial mass movement downslope (André, 2004;Hall and Phillips, 2006), (iii) preferential development of small lee-side steps on small granite protuberances , (iv) streamlining of small hills and development of lee side cliffs, with the formation of roches moutonnées (Jahns, 1943;Lindström, 1988;André, 2001), (v) development of large scale hill asymmetry, with stepped cliffs on the lee side of the hills (Sugden, et al, 1992) and (vi) streamlining of large hills parallel to former ice flow (Rudberg, 1954;Ebert and Hätterstrand, 2010).…”

“…Previous attempts to examine the inception of glacial erosion have relied on theoretical models (Harbor, 1995;MacGregor et al, 2000), on comparisons of unglaciated and glaciated terrain (Feininger, 1971;Hall, 1985;Lidmar-Bergström, 1996;Johannsson et al, 2001) and on space-time substitutions which place large glacial landforms in sequences of progressive morphological development (Sugden et al, 1992;Hall and Phillips, 2006). We adopt a new approach to this problem by examining the morphology of granite hills on the northern footslopes or Foreland of the Sudetic Mountains ( Fig.…”

Scottish Landform Examples: The Cairngorms – A Pre-glacial Upland Granite Landscape

“…The Cairngorms hold perhaps the best example of a glaciated tor field in the world (Ballantyne, 1994;Hall and Phillips, 2006a;Phillips et al, 2006). The tors occur at altitudes of 600-1240 m O.D., spanning almost the entire elevation range of the exposed Cairngorm pluton (Figure 2).…”