List of Sites of Special Scientific Interest in County Durham

In the west of the county, outcrops of Dinantian limestones are found in the river valleys, while the upland areas are underlain by the younger Namurian Millstone Grit Series.

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.

[4][5] Although each SSSI designated for geological reasons has its own distinctive features, they fall into a small number of broad categories, as indicated in the individual citation sheets:[3] From its inception, English Nature classified sites using the 1974–1996 county system.