The Ordovician Period is a division of geological time spanning the interval between about 490 and 445 million years ago. In some parts of Britain the Ordovician is represented by vast thicknesses of volcanic ash and lava; in others we find black shale that formed as mud in deep seas.
Sheets of molten material
In Warwickshire, Ordovician rocks are best developed as the hard igneous rocks that are still quarried as a source of aggregate, near Mancetter. These rocks originally formed as sheets of molten material, injected under pressure into cracks within older rocks, at depth within the Earth’s crust. The sheets range from about a metre to over fifty metres in thickness, and consist of a number of different igneous rock varieties including ‘spessartite’ and ‘diorite’. This phase of igneous activity is thought to equate to the final closure of the so-called Iapetus Ocean, which existed over northern Britain, nearly half a billion years ago.
In the Griff No.4 Quarry, near Bedworth, the quarried rocks in the lower part of the quarry are Ordovician igneous rocks, intruded into older sedimentary rocks of Cambrian age. The slopes above are formed of Carboniferous Coal Measures, including thin coal seams.