The rock that is quarried at Mancetter is known as Diorite or Lamprophyre. This is an igneous rock of the granite family that was injected into a sequence of ancient shale rock over 430 million years ago during the Ordovician period of geological time.
Diorite (also known as Lamprophyre)
Diorite is an igneous stone (from the Latin for Fire) –formed from molten rock. This particular kind of igneous rock is generally a good indicator of past tectonic action where two massive pieces of the earth’s crust have collided resulting in one being forced down, or subducted into the upper mantle.
The Diorite at Mancetter varies in colour from light to medium blue grey and is formed by the crystallisation of numerous minerals, including feldspar, quartz, biotite, hornblende and pyroxene. It is the interlocking nature of the crystalline structure that makes this a very tough and durable rock. When made into rock chippings it is particularly suitable for use on busy roads because of its skid resistance, which comes from its reluctance to ‘polish’ during its life on the road
The Diorite was injected into the surrounding shale rock in the form of molten sheets that split open layers of shale as it flowed in along the beds. These sheets vary in thickness from a few centimetres to over 30 metres and are known as Sills.
Shale is a fine-grained sedimentary rock formed from compacted layers of ocean muds and silts. It is characterised by breaks that form, parallel layers or beds. The shale found at Mancetter is amongst the oldest found in England and has been dated as 530 million years old, placing it in the Cambrian period.
The shale beds at Mancetter took millions of years to create, and are hundreds of metres thick. Locally, the shale forms part of the Stockingford Shale Group. The beds found in the quarry are just a part of this overall group and are known as the Outwoods Shale Formation. These names came from the local places where they were first described by geologists.
The shale at Mancetter is black or dark grey in colour and weathers to a buff colour. It often has layers of iron pyrite or ‘Fool’s Gold’ as well as some very rare fossils.
Fossils are the preserved remains or impression of living creatures or plants. Important fossils have been found at Mancetter, including some new to science. The most interesting are trilobites – segmented, prehistoric marine ‘wood louse-like’ creatures. Palaeontologists have found a number of different species of trilobites throughout the Outwood Shales and have been able to date the rocks accurately as the trilobites evolve. Trilobites lived 530 million years ago on the sea floor and buried into ocean mud at the same time as the shale was forming. They fed on tiny fragments of dead animals or plant matter that ‘rained’ down through the water from above.
Through careful study of these fossils, Professor Charles Lapworth (1842-1920) and his students, found that the rare trilobites found at Mancetter were also found in Canada, mainland Europe and Asia. From this discovery, it was deduced that most of the British Isles, North America, Europe and Asia were once joined together as a supercontinent, now known as Gondwana. It is thought that this primitive landmass and its shallow seas were close to the present day Antarctica.
Gondwana, with arrows showing the direction it broke up to form the present day land masses.
The shales formed in quiet conditions, below cold or, at best, cool seas. The water was sufficiently shallow to sustain some forms of life, as seen by fossils found at the quarry. Olenis Austriacus is one of the rare trilobites found at the quarry.
Olenis Austriacus, found in 2003
Much of the geological interpretation of Mancetter Quarry has been undertaken by local geologists – both professional and amateurs. In recent years, the Warwickshire Geological Conservation Group has worked to improve and protect some key rock exposures. This work was jointly funded by Tarmac and the Aggregate Levy Sustainability Fund and has produced a useful educational resource.
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