Cullompton. Report from the Geology Section
|Author(s):||Bennett. Jenny and Mather. John||Origin:||Section Conference Reports|
The Geological Distinctiveness of Cullompton
The area surrounding Cullompton is characterised by two contrasting landscapes, governed by the underlying geology. The Cretaceous Greensand ridges of the Blackdown Hills AONB rise to the east, and to the west the Permian and Triassic rocks produce the distinctive Devon red soils. The Permian rocks overlie folded and incised Carboniferous rocks of the Culm Formation, and become finer grained towards the east, grading from sandstones, breccias and conglomerates to mudstones, siltstones and sandstones.
The town itself is underlain by the Permian Cadbury Breccia, which is overlain by river sediments (alluvium) and further sands and gravels that form the Quaternary river terrace deposits. The River Culm has exploited the softer Permian rocks forming a wide valley that has been used by the M5 and the Paddington to Penzance railway line. This wide valley means that the Culm is a more sinuous river than the River Exe, with a complex system of multiple channels that often shift across the floodplain.
To the east of Cullompton the Greensand scarp slope forms the edge of the Blackdown Hills plateau and is an obvious landscape feature. The Greensand overlies the Triassic Mercia Mudstone and is itself covered by a thin layer of Clay-with-Flints, all that remains of the Chalk that once covered the area. The base of the Greensand can be picked out by changes in vegetation — it is more wooded than the Triassic material that it overlies. Its green colour is due to the mineral glauconite, and fossils, such as molluscs and sponge spicules, are abundant. The Blackdown Greensand was well known for exceptionally well preserved fossils that were much sought after by collectors and one very large collection made by the Rev. W. Downes is now held in the Royal Albert Memorial Museum, Exeter.
Chert, a silica-rich material that is slightly coarser than flint, is common and was exploited by early man for toolmaking; chert tools have been found in the Axe valley gravels. In addition, sandstone concretions occur at about 24 metres below the surface, and these are easily shaped when fresh and harden when exposed to the air. They were dug out from long horizontal galleries and were used to sharpen agricultural implements. There was a thriving whetstone (scythestones) industry here from the late 17th century until the 1920s, particularly at Blackborough, though there is evidence of working all along the western scarp. Farm workers used large quantities of these stones to sharpen tools and until the arrival of carborundum there was a ready market for the material. It was taken to Cullompton for sale and there was also an annual ‘Scythestone Fair’ in Waterbeer Street, Exeter every May or June and which was an important date in the miners’ calendar! The worked stones were known as ‘Devonshire batts’, and there is evidence that they were taken as far as Bristol and Southampton, although it is possible that they could have been exported even further afield.
To the south of Cullompton is Killerton House where some unusual igneous rocks – lamprophyric minettes – occur. These are thought to have been emplaced during post-Variscan volcanism and have been dated as approximately 291 million years old. The rocks are overlain by Permian sandstones and have been have been interpreted as lava flows although no internal structure can be seen. The Killerton Chapel and many other local buildings are built of the lamprophyre, where it may be more easily seen than in quarry exposures.
Jenny Bennett and John Mather
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