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Just
how tors are formed is not fully understood.
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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. |
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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. |
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| 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. |
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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. |
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