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Geology

The geology of Harbertonford and the surrounding areas.

 

By Rory Scammell BSC.

 

Regional Overview.

The southwest of England is one of the most interesting regions of England to observe the geology for there are many types and ages of rocks, all visible within a small area. From the granites of Dartmoor to the highly metamorphosed rocks of the Lizard peninsula in Cornwall, there are world class outcrops

 

Harbertonford sits upon rocks of middle and upper Devonian age, 360-380 million years old. The Devonian is a period in geological time which bears the name of the county due to the wonderful outcrops visible of that age, indeed this was the place where these rocks were first studied. The Devonian period was when fish first evolved legs and started to walk on land. Large coral reefs populated by weird and wonderful animals such as armoured sharks filled the shallow seas. Large forests were growing all over the continents due to the evolution of seed bearing plants. The actual continents were in a different formation to today with the land mass Gondwana towards the south of the globe, with another landmass, Euramerica, on the equator. The different position of the continents is due to continental drift, where the continents sit on mobile slabs of rock that slide upon the earths molten interior. The South of England would have sat between these two major continents, and due to globally high sea levels, would have been submerged at this time, allowing sediments to be deposited.

 

During the late Carboniferous, 325-300 million years ago, the South West underwent an Orogeny, an event where colliding continents cause the crust to become folded up and raised often forming a mountain chain. A similar thing today is happening with the Himalayas where the Indian plate is colliding with the European plate resulting in a raised ridge of mountains. The sediments formed within the South West were uplifted, folded and heated, causing melting of some of the sediments creating large voids full of molten material which then cooled to form granite. Since the Orogeny, the regions crust has been stretched and eroded, allowing these once buried rocks to become uncovered, as well as accumulations of more recent sediments such as the new red sandstones to the east.

 

Local Geology.

Harbertonford sits upon sedimentary and igneous rocks, rocks that formed in the sea due to the settling of sediments, lavas and volcanic particles. Within the surrounding areas there are outcrops of limestone. The sedimentary rocks are found in the form of slate, shales, limestones, volcanic Tuffs. The sedimentary slates and shales would have originally started out as muds and silts, which have been baked and hardened by the pressure and temperature of being buried at depth. The layering visible within the slates and shales is not due to them being lain down in layers, but is caused by crystals growing perpendicular to the pressures squeezing the rock when the rock is buried. Limestones are formed from the bodies of animals and corals which are buried and squeezed together, resulting in a rock rich in the mineral calcite. The volcanic Tuffs are formed by underwater eruptions which create the beds of lavas (such as the local Diabase) and broken particles of volcanic glass which mixed with the sediments. These then are compressed to form the Tuffs and Diabase seen in the area.

The presence of limestone and the ever present need for cements caused many lime kilns to be built within the area. Today unused, these kilns were once used for roasting lime, to make quicklime. Lime kilns can be found on the road near the Watermans Arms, Tuckenhay.

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