Volcanoes: Calton Hill
During the early Carboniferous period around 330 million years ago, a series of underwater volcanic eruptions occurred in the shallow tropical seas covering what is now Derbyshire. The basalt lava and volcanic ash that resulted from those eruptions are now interbedded with the limestones which were being deposited around the same time. The volcanic vents were generally small, although repeated eruptions resulted in accumulations of basalt lava over a hundred metres thick in some places. Some volcanoes also deposited ash or 'tuff' which is now bedded within the limestone sequence. Sometimes (as in the Bee Low Limestones), these widely-spaced, thin bands of ash have altered to clay, known locally as 'wayboards'.
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Where the molten basalt 'magma' failed to reach the surface, it became emplaced within the limestone strata where it solidified as an intrusive mass known as a 'sill'. The Calton Hill sill is a large intrusion of olivine dolerite, a coarse-grained variety of basalt. The sill has been quarried extensively for roadstone, but the site is now restored and is a site of geological interest. The crushed dolerite produces strong, durable aggregate materials. There were once many small quarries working igneous rocks in the Peak District, but all are now closed and aggregate production is now concentrated on large-scale extraction from the thick and consistent beds of Carboniferous Limestone.
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