Page:VCH Warwickshire 1.djvu/40

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A HISTORY OF WARWICKSHIRE bournes, south-west of Atherstone, this rests on the edges of the Stockingford Shales, including two sheets of diorite. As these latter have not affected the Coal Measures they must have been intruded in pre-Coalmeasure times ; and Professor Watts seems disposed by general considerations to think that the intrusions are of immediately post- Cambrian age. CARBONIFEROUS Between the period of the Cambrian rocks of Nuneaton and that of the Coal Measures which overlie them there is a great gap, unfilled in our district by any known formation. We know that during this enormous interval thousands of feet of muds and volcanic ashes the Ordovician rocks were deposited over what is now Wales and the west and north of England ; but none of these is known to occur eastwards of the Malvern district, and it therefore seems probable that what is now central England was occupied by an extensive island formed of the upraised Cambrian sediments which stood up above the waters of the Ordovician Sea. This land tract however slowly sank and contracted in area, for the Silurian deposits, which immediately followed the Ordovician, extend farther eastwards over the subsiding area ; but the higher parts of the district seem still to have kept their heads above water during this and the succeeding Devonian period, for these vast accumulations of mudstones, limestones and red sandstones are unrepre- sented in our county ; and it is practically certain that parts of the old island were still in existence as such while the Carboniferous or Moun- tain Limestone and Millstone Grit of Derbyshire and Yorkshire, Wales and Ireland were accumulating. This Lower Carboniferous sea lay to the north, east, and south of our area ; we even obtain a glimpse of its coast-line at Grace Dieu in Charnwood Forest, but nearer than that it appears not to have approached. By the time that the higher ridges of Cambrian rocks at the north of the county had sunk to the water level the physical aspect of the midlands had changed. The sea had become shallowed, land-locked areas developed, and ultimately com- munication with the open ocean was cut off. The district became converted into ' an immense delta or fenland, including many large lagoons and wide channels, surrounded by swamps which were never much above the level of the sea.' * These delta deposits are our Coal Measures. Thus the Millstone Grit and Carboniferous Limestone are alike unrepresented, and the only Carboniferous rocks present on the surface in the county are the Coal Measures of the Warwickshire coalfield. The Coal Measures form a narrow belt of country extending for about fifteen miles from Bedworth on the south-east, past Nuneaton and Atherstone, to Tamworth on the north-west, where the outcrop attains its greatest breadth of about four miles. They rest unconformably on the Cambrian, and are succeeded with every appearance of perfect 1 A. J. Jukes-Browne, The Building of the British Isles, ed. 2 (1892), p. 133. 10