Читать книгу The Reivers Way онлайн

14 страница из 34

Sometimes the rivers formed vast deltas, which allowed strange, fern-like trees to gain rootholds, only to be toppled by floods and buried beneath more mud and sand. The plant matter was compressed over time to form coal measures. These are Northumberland’s predominant rock types, forming rocky edges in some places, but breaking down to form sandy soils in others, with enough coal to support a little mining activity.

The Carboniferous rocks were laid down in layers, and that helps to explain what happened next, around 295 million years ago. A great mass of molten dolerite was squeezed, under enormous pressure, between the layers of rock – rather like jam between two slices of bread. This rock is always prominent wherever it outcrops, and is referred to as the Whin Sill. It forms some of Northumberland’s most striking landscapes. The highest parts of Hadrian’s Wall, for example, were built along the crest of the Whin Sill, so as a geological feature it has shaped the political landscape of Britain! The Whin Sill outcrops all the way across Northumberland and is notable along the coast, where its higher parts are crowned with castles at Bamburgh and Lindisfarne, while its lower parts form low cliffs and the Farne Islands.