On the footpath west of Kenilworth Castle, bootprints in the mud are filled with standing water the color of tomato soup made with milk. In this area of Warwickshire, the common surface stone is soft red Mercia Mudstone, having its origin in dust and sand deposited during the late Triassic period (about 230 million years ago). Around Kenilworth, Mercia Mudstone yielded a tough red clay which accounts for the remains of Roman tile kilns in the area.
Going deeper and earlier in geological time, below the Mercia Mudstone, lies a stratum of Permian Kenilworth Sandstone, deposited about 290 million years ago. It was this characteristically red sandstone that the builders of Kenilworth Castle took from a quarry just south of the castle beginning in the twelfth century. Go deeper and earlier still, and you reach a seam of coal from the Carboniferous period (350-300 million years ago), which north of Kenilworth has been thrust closer to the surface. The local availability of coal had a major impact on the development of industry in Coventry and Birmingham during the nineteenth century, and it was primarily to transport coal that the local canals were built. This map shows the surface geology of Warwickshire. The gray area extending north of Kenilworth is coal. The surrounding lighter terra cotta color is Mercia Mudstone. Notice the other seam of coal on the left of the map, centered on Birmingham.
The deeper and earlier you go, the more recent and significant the impact of local geology on the landscape of Warwickshire and the West Midlands. The first-century Romans scraped the surface for red clay to build their roof tiles; nineteenth-century West Midlanders mined deep for coal to fuel the industrialization of Britain and turn insignificant Birmingham into England's second largest city.
With its history of Roman occupation, its Anglo-Saxon origins recorded in the Domesday Book, and its Norman castle, Kenilworth is, of course, much older than Northfield, Minnesota, which was only settled in the 1850s. Northfield, however, stands on a foundation of much older rocks—limestones and dolomites from the Ordovician (approximately 490-445 million years ago).