The origin of tors has a long and chequered history. The debate got under way in the mid to late 1700’s when it was speculated that among others, Brimham Rocks was an artificial feature created by druids as temples. The geological story got under way in 1814 (Maculloch). At last the religious folk-lore gave way to science and tors were deemed to be entirely natural phenomena.
Early origins of tors were still strikingly dramatic e.g. they were relict coastal features and so represented a period of time when sea level was high with boulder clay providing the evidence for marine sedimentation! Subsequently, tor formation evolved into a tw0-horse race:
a) Wind erosion in late glacial times (Periglacial)
b) Deep Tertiary chemical weathering
David Linton (1955)
Having rejected the idea of atmospheric weathering, Linton postulated that sub-surface rotting of rock by acid groundwaters with tor remnants (core-stones) therefore being the undecayed product (as seen today at Brimham). Furthermore, Linton accepted that the destructive nature of of glacial periods i.e. solifluction in periglacial times helped exhume the tors.
The clear lack of evidence for deep chemical weathering of tors in the Pennines prompted the work of Palmer, Neilson and Radley (1961, 1962) who supported the idea of post glacial tor formation.
Periglacial Weathering (Palmer et al 1961)
Palmers model of tor formation is based upon the periglacial processes i.e. the activity of solifluction to remove the loose overburden thus allowing freeze-thaw activity along the joints and natural partings to split the rocks.
Palmer and Radley (1961) on Pennine Tors considered the relationship of tors to a slowly retreating free-face or edge. 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.
It has been suggested that tors probably form by a combination of chemical and physical weathering (a view held by Gerrard (1974). Based on geological structures, tors are best developed where folding has placed the dip of bedding and cleavage planes perpendicular to the main direction of mass wasting on hillslopes or where tors have evolved from inselbergs during the pediplanation of uplifted periclinal domes. To summarise: tors can result from a variety of weathering processes, chemical or physical, so long as the general geological structure is conducive to their preservation.