The geology of Northumberland in northeast England includes a mix of sedimentary, intrusive and extrusive igneous rocks from the Palaeozoic and Cenozoic eras.
The geology of the rest of the county is characterised largely by a thick sequence of sedimentary rocks of Carboniferous age.
These are intruded by both Permian and Palaeogene dykes and sills and the whole is overlain by unconsolidated sediments from the last ice age and the post-glacial period.
The county's geology contributes to a series of significant landscape features around which the Northumberland National Park was designated.
Greywackes originating during the Wenlock epoch of the Silurian Period are assigned to the Riccarton Group and occupy an area either side of the border to the north of Byrness.
Rocks of the Border Group extend in a broken and faulted belt south from Berwick towards Alnwick then southwest via Rothbury then onward to the county boundary west of Kielder Water.
The overlying Alston Formation extends south from the Scottish border along the coastal strip before turning inland through Alnwick towards Greenhead.
The sandstones, siltstones, mudstones and coals of the Pennine Coal Measures Group overlie the Stainmore Formation and, from a line roughly between Amble and Derwent Reservoir via Morpeth, Ponteland and Stocksfield, extend eastwards to the coast with a gentle regional dip towards the North Sea basin.
The sill provides for characterful topography at Dunstanburgh, Bamburgh, Lindisfarne and neighbouring districts including the Farne Islands, offering several good sites for the construction of castles.
There are also a number of areas of glacio-fluvial sands and gravels representing glacially derived material re-worked by rivers.