 To the east, these rocks dip beneath the Westphalian Coal Measures, which underlay a broad swathe in the centre of the county.Further east, the Coal Measures are overlain by Permian Magnesian Limestone, which forms a low plateau across the eastern part of the county.Igneous intrusions are a prominent feature of the geology and gave rise to extensive mining for lead and other minerals, especially in the upper Weardale area in the west of the county.