Tor Formation

Just how tors are formed is not fully understood.

Indeed controversy has surrounded their origin.
If we define a tor as a residual mass of rock capping hills or high ground then many theories can explain their origin. They are not just restricted to granite regions and appear to be found in more than one climatic region of the world. Theories centre on weathering and erosion.

Consider then the granite tor, so characteristic of Dartmoor, Linton in 1955 advocated deep chemical weathering as the exponent, suggesting that where joints in the rock were closer together the rock would be more deeply weathered and so easily removed by later erosion. He saw a prolonged chemical weathering under tropical conditions as the main factor in tor genesis. A second theory favored by arctic workers suggests mechanical weathering during the ice age was responsible. King believed them to be nothing more than the residual remains of sub aerial erosion surfaces.
Geomorphologists, working on surface processes, tend to advocate one of the above mechanisms, however as a geologist I favour Colin Bristows explanation. That chemical weathering under tropical conditions played a significant roll, as did mechanical weathering during the ice age, but taking much much longer, in conjunction with a process which took millions of years, from within the granite itself.

We have seen how the slow cooling of the granite, containing radioactive minerals, maintained the temperature within, producing large crystals. That a layer cooled forming a capping which trapped molten magma with volatile gases and hydrothermal liquids beneath. Gases escaped through joints and fissures to the surface, rotting the granite in areas, weakening the granite as it altered minerals to secondary forms.

This allowed surface ground water to attack the granite in these areas. More recently in the ice age, mechanical erosion has removed the material, leaving the core stones or tors surrounded by clitter. This process started in the Permian and Triassic arid landscape as saline groundwater penetrated from above and if we assume Devon remained part of an island into Jurassic times, continued when tropical forests covered the land. In the Eocene, about 40 million years ago, Devon was again subjected to hot tropical weathering which stripped the Cretaceous chalk cover almost completely away. During this time Dartmoor granite was again subjected to deep seated weathering through the overburden. This weathering was associated with erosion in the Oligocene when the Bovey Tracy region, laying on the Sticklepath fault line, received quantities of sands, gravels and clays. These clays are a weathering product of feldspar crystals from the granite. Ball clay, as it is known, is a top quality clay used in high grade white ceramic and paper making processes. Interbedded are lignite deposits, which would suggest rafts of tree material washed into the lake basin from time to time.

In the Pleistocene and recent periods of earth history Britain was subjected to repeated ice ages with warm interglacials. The southern extent of the ice sheets did not reach Dartmoor, but during the cold periods Dartmoor was subjected to periglacial processes of weathering and erosion. This was to shape the scenery we see today. Very cold sub-Arctic conditions were followed by African style hot periods when rhinoceros and hippopotamus roamed Devon. In the four cold stages, frost action would split, heave and carry away vast quantities of loosened material by the process of solifluction. Freeze thaw of the permafrost would lift and mix layers producing head deposits. Tors were exhumed from burial and masses of material washed down slope. So in this way our familiar tors were born, a process which began over 250 million years ago and one which will continue into the future.