Phillips et al. (2006) have recently presently cosmogenic exposure ages for the Cairngorm tors, including both unmodified and glacially modified tors. Unmodified tor summits have exposure histories spanning 295, 520 and 626 ka. These tors have survived many glacial cycles but the ages indicate that the tors have formed entirely within the Pleistocene. Glacially-modified tor surfaces have 10Be exposure ages of 19 to 92 ka, indicating erosion and exposure of different tor surfaces in at least three glacial cycles, including the latest.
The age of the Cairngorm tors
The Cairngorm tors have long been regarded as preglacial features (Linton, 1955). This may be true, in as much as they are an element within the terrain of the plateau. The tor A' Chioch, for example, is being consumed by the retreat of the headwall of Coire na Ciche on Beinn a' Bhuird. Yet the evidence of pitting, marginal weathering, frost action and glacial modification demonstrates that these are evolving features. Different surfaces must be of different age, with the summit blocks being the oldest elements where glacial activity has been negligible.
The largest tors represent rock cores that are slow to weather and erode. The rate of emergence is controlled by the lowering of the surrounding ground surface, a rate which is largely unknown but probably significant over timescales of 1-2 million years. Whether or nor the largest tors of heights up to 15 m can emerge within this time frame depends largely on the duration of periods of snow and ice cover. The presence of tors on what to appear to be slopes developed in response to Quaternary drainage incision implies that small tors can emerge over timescales of 105 yr.
One way to establish the age of tor surfaces is by cosmogenic age determination. Weathering pits are also useful relative age indicators. Pits of 10-25 cm depth suggest that tor surfaces have been existence since at least the last interglacial; the largest pits of >50 cm depth may indicate surface weathering over many hundreds of thousands of years.
Emergent tor on the slope at the head of the Caol Ghleann, associated with granite weathering covers