The Theory of Tertiary Chemical Weathering

David Linton

Linton rejected early ideas of tors having formed as a result of atmospheric weathering suggesting that where this was occurring it was acting destructively rather than constructively. He argued that weathering acted in situ and selectively. The piled and delicately poised logans of many tors, especially those on Dartmoor, could not have been transported to their present position; they must have been produced where they now stand by the slow wastage and removal of solid rock which has been structurally guided. Linton suggested the process most likely to act in this way is the sub-surface rotting of the rock by acidulated groundwater percolating along joints and from them into the body of the rock.

Rock decomposition by acidulated groundwater is well documented from the tropics, from where Linton got much of his inspiration. Embedded in the layer of rotten rock may be boulders termed core-stones, largely undecayed. Linton believed the finer grained products of rock rotting to be removed during a period of rejuvenation of local rivers or some other agency. As this removal ensues the core-stones would gradually settle down on supporting pedestals. Some stones may have come to rest in carefully balanced positions, others would have toppled and fallen to the ground. The end product was seen to be a delicately poised tower-like structure surrounded by the fine-grained products of rock decay known locally on Dartmoor as growan.

Linton's ideas were later supported from studies of different climatic environments by Wilhelmy (1958) and Lautensach (1950). However, in its application to the British Isles more generally it has met with several problems. Foremost amongst these was its inability to rest easily alongside existing theories which took account of the destructive nature of glacial periods. Linton argued, however, that deep rock rotting occurred during the Tertiary with exhumation occurring much later as a result of solifluction in periglacial areas or rejuvenation of local rivers following glaciation. He even argued that tors which were exhumed 'prematurely' during the Pleistocene may well have represented isolated nunataks and were deeply covered by neve as opposed to ice.

Indeed there have been many theories in support of Linton's ideas, but which also attempt to explain the glacial survival of tors. For example, Sugden (1968) working in the Cairngorms suggested that areas which had been irrefutably glaciated still supported tors since they were at the centre of dispersal of an ice sheet and, therefore, experienced little glacial erosion. However, this did not explain the presence of tors on the Pennines where ice movement had occurred.

Moreover, it has been shown that there is no evidence of deep chemical weathering within the immediate vicinity of tors, neither around the intensively studied granite tors of Dartmoor or the gritstone tors of the English Pennines. It is with this evidence, or rather lack of it, that Palmer, Neilson and Radley (1961, 1962) attempted a strong argument in favour of a post-glacial theory of tor formation and one which avoids the need to consider deep chemical weathering.