The whole is overlain by a suite of unconsolidated deposits of Quaternary age arising from glaciation and from other processes operating during the post-glacial period to the present.
[1] The geological interest of the west of the county was recognised by the designation in 2003 of the North Pennines Area of Outstanding Natural Beauty as a European Geopark.
[2]: 19 It forms a part of the larger 'North of England Plutonic Suite', whose members (which include the granites at Shap and Skiddaw) were emplaced during the Caledonian Orogeny.
Further east again the Carboniferous succession is overlain by younger rocks but they are present at increasing depth to the North Sea coast and beyond.
The 'Millstone Grit' of County Durham (minus the 'Great Limestone Member') has been renamed as the 'Stainmore Formation', having also been referred to somewhat confusingly as the Stainmore Group at one time.
[7] A feature of much of the Carboniferous succession in northern England is its cyclicity which has involved regular changes between marine deposition and sedimentation from rivers.
The Frosterley Marble, a bituminous coraliferous limestone once worked at Harehope Quarry in Weardale is used as a decorative stone and can be found in many churches in the region.
These are overlain by the thin bituminous limestones of the 'Marl Slate' representing the start of a series of inundations of the area by rising sea levels.
The Magnesian Limestone forms a broken west-facing scarp running from the western edge of Sunderland southwards through Houghton-le-Spring and Hetton-le-Hole to Coxhoe where its outcrop is offset to the west by the Butterknowle Fault.
English Nature has defined this part of the county east of the scarp where the sequence is exposed at or near the surface, as the 'Durham Magnesian Limestone Plateau' in its assessment of 'character areas'.
[12] The occurrence of gypsum and anhydrite reflects the extreme nature of the evaporation of the shallow Zechstein Sea on occasion though such evaporites are economically valuable.
The sill underlies the larger part of the county at depth and appears to extend beneath the North Sea to the east.
[2]: 20 The presence of the Whin Sill gives rise to High Force where the River Tees drops in spectacular fashion over this erosion-resistant rock.
The Little Whin Sill is a thinner, more geographically restricted intrusion of dolerite of similar age outcropping in upper Weardale.
A quarry near High Force in upper Teesdale works the dolerite (or 'whinstone') of the Whin Sill for roadstone, aggregate and as larger blocks for coastal protection purposes.
This intrusion of basaltic andesite which is up to 30m wide in places has been dated to 55.8+/- 0.9Ma[16] The broad structure of the Alston Block has been described in the Devonian section above.
[17] To the east of the block the cover of late Palaeozoic and early Mesozoic rocks dip into the North Sea Basin.
[19] Some locations in County Durham are afforded statutory legal protection against adverse developments through being designated as one or more of the following: A geodiversity audit of County Durham produced many locations which are now afforded recognition as 'Local geological sites' (formerly referred to as 'Regionally Important Geodiversity Sites' or simply 'RIGS'.
Bodies such as the North Pennines AONB Partnership and Durham County Council have published local geodiversity action plans (or 'LGAPs') which summarise an area's geological interests and propose various measures to conserve what is perceived to be valuable.
[22] The North Pennines AONB was designated as Britain's first European Geopark in 2003 partly in acknowledgement of its geological and geomorphological interest.