Barns of Bynack interior
Post-glacial differential weathering of a quartz vein on glaciated granite slab
Periglacial modification of tors
Much attention has been given to the origin of tors. The modification and eventual destruction of tors surfaces has received less attention. Leaving aside the widespread evidence of glacial modification of tors, it is clear that many processes are operating to lower tor surfaces and to render tors unstable.
Clach Bun Rudhtair, lower tower, showing weathering pits up to 1 m deep
Weathering pits and lapies are common on tors that have experienced little glacial modification. The pitting reflects the importance of moisture in weathering, with granular disintegration concentrated wherever water can gather. These pits can reach considerable depths and may coalesce to form larger depressions. On Clach Bhàn (The Stone of the Women), for example, there is a large internal joint-controlled cavity with a shaded pool.
The tor surfaces, like all granite surfaces in the Cairngorms, are also weathering slowly by granular disintegration. Evidence of this process is seen on Barns of Bynack, where granular grit accumulations occur within gullies and cavities inside the tor. Upstanding quartz veins indicate that this process operates at rates of 16 mm/ka. Frost weathering operating within and between minerals is probably the main cause of this surface crumbling.
Individual tor blocks may be rendered unstable by a combination of steepening of the adjacent slopes, removal of lateral support by weathering and the removal of neighbouring blocks and undercutting by the growth of basal weathering notches and the removal of weathering.
Frost wedging, with the growth of ice lenses in joints, may also have contributed to block movement. This process has been of limited effectiveness today and in the Holocene, as ice-moulded tors lack aprons of large blocks. The heaving, however, of large granite blocks within blockfields and large scale patterned ground suggests that significant tor modification may have occurred under permafrost conditions during the Pleistocene.
Creag Mhór summit - ice-moulded tor with few signs of