The bedrock of the area consists almost wholly of a suite of sedimentary rocks deposited during the Devonian, a period named for the English county of Devon in which the western half of the park sits.
These rocks are exposed along the entire coast east from Hangman Point to Woody Bay and again from Countisbury Hill to Porlock Weir.
The highest point on Exmoor, Dunkery Beacon, is formed from the Hangman Sandstone whilst Dunster Castle is built upon a spur of the same rock.
[11] A small outlier of largely Jurassic age Lias strata lies at the northern margin of the Porlock Basin up against the southwesterly down-throwing fault which marks the northeastern edge of this half-graben.
[11] West of Porlock, the fold axis of the Lynton Anticline traces a curve beneath Culbone Hill, causing the Hangman Sandstones to dip moderately steeply to the north along the coastal strip.
Exmoor was to the south of the ice-sheet during the last glacial period though it has been postulated that a small glacier may have occupied the anomalously deep feature on the north side of Winsford Hill known as the Punchbowl.
[12] During the earlier Anglian glaciation, the British ice-sheet extended as far south as the present day north coast of Somerset and Devon.
Deposits of fragmentary rock material, the result of weathering and downslope movement of the underlying bedrock are known as ‘head’ and are widely recorded within the valleys of the national park.
Copper was mined near North Molton where it was found in association with lead, zinc, antimony and manganese ores.