The Theory of Periglacial Weathering

Palmer, Neilson and Radley

Heavy criticism was laid at Linton's ideas by Palmer et al (1961, 1962) who argued that there was no evidence for deep rock rotting having occurred under a tropical climate. They also argued convincingly against the tors being residualem core-stones/em and pointed to an arrested periglacial cycle using the presence of tors in the tundra of Alaska and in areas corresponding to the margins of the Wisconsin ice sheet in North America as supporting evidence. Furthermore, the presence of tors in areas that have irrefutably been glaciated also lended support for a post-glacial origin for tors.

Palmer and Neilson (1962) worked on the granite tors of Dartmoor and developed a model of tor formation based on Peltier's (1950) periglacial cycle, i.e. a period of freeze-thaw activity followed by solifluction in a periglacial climate.

Palmer and Neilson (1962) envisaged an initial stage involving the loss by solifluction of soil from the tops of hillsides and rounded hilltops. The action of frost on the granite is the next stage and would be accentuated on the joints and natural partings. The break-up of such a bedrock by frost producesblockfields. Massive results are possible on hilltops where permafrost is absent, while permafrost at the foot of the hill limits the effects of congelifraction, or frost shattering, to a shallow active layer.

The downhill movement of blocks freed by frost action would be facilitated on steeper slopes, but tors are best developed on intermediate slopes. On gentle slopes movement is restricted and there is little scope for the differential weathering necessary for tor isolation, on steep slopes movement is easy. Palmer and Neilson believed that the relation of particular stack-like tors to the scarps behind them strongly suggested that some tors were outliers of scarps and were left behind during scarp retreat. The cycle was halted before completion in the case of those hilltops retaining tors.

The periglacial idea for the formation of tors had been discussed earlier for the English Pennines by Palmer and Radley (1961). The relationship of tors to a slowly retreating free-face or edge was more clear. The downhill movement of blocks over the predominantly shale slopes of the Pennines was thought to have isolated and undermined the capping gritstone to produce a free-face. Palmer and Radley suggested the gritstone caprock, with its water-holding properties, was subject to diurnal or seasonal freezing and thawing most effective along the joints. The 'active-layer' above the permafrost led to the extensive series of landslides of melted shale over frozen shale.

The opening of joints on the free-face may have been aided by undermining. It is probable that several forces worked together to induce free-face retreat. Differential jointing led to variable retreat which in turn produced different types of tor.