Geology of Exmoor National Park

Each of the individual sandstones and mudstones of Devonian age within the park are ascribed to the Exmoor Group.

Unconformably overlying the Hangman Sandstone in the Porlock Basin is the Luccombe Breccia Formation, a presumed Permian age deposit consisting of calcareous breccia, sandstone and conglomerate which may be up to 650m thick.[9]

A further presumed unconformity separates the breccia from the mudstones of the overlying Mercia Mudstone Group which underlie the larger part of the low ground between Exmoor and North Hill. At the top of the group is a 25m thickness of mudstones with gypsum referred to as the Blue Anchor Formation. Above this are around 12m thickness of mudstones and limestone assigned to the Penarth Group.[10]

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. Perhaps as much as a 100m thickness of the Blue Lias overlies the Penarth Group. The lowermost part of the Lias may be latest Triassic in age. In character it is grey mudstones and shales with thin limestones.[10]

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. A series of broadly east–west oriented folds affect the Hangman Sandstones at North Hill and south of Minehead.[10]

The Lynmouth-East Lyn Fault is a reverse or thrust fault dipping south at about 45 degrees which brings Lynton Slates over the Hangman Sandstones.

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.[11] During the earlier Anglian glaciation, the British ice-sheet extended as far south as the present day north coast of Somerset and Devon. Exmoor would have been subject to intense periglacial processes at this time and during other glacial periods.

Tidal flat deposits occupy the low ground at Porlock, landward of which are river terraces and the modern day alluvial deposits associated with the Horner Water and its tributaries. 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. A number of areas of peat are recorded on the upland surface of Exmoor. Small areas of talus (or scree) are found either side of Heddon Mouth. A variety of marine deposits are to be found along the shoreline including sand and cobbles whilst storm beach deposits are to be seen at Lynmouth.[12]

Iron ore was formerly mined in the Brendon Hills where mineral-bearing lodes provided lens-shaped bodies of ore. The highest production levels were achieved in 1877 when almost 50,000 tons were mined. Operations reduced thereafter and all work had ceased in 1910. Copper was mined near North Molton where it was found in association with lead, zinc, antimony and manganese ores. These and some gold were emplaced by fluids associated with the emplacement of the Cornubian batholith of which the Dartmoor granite is the largest exposed part.[13]

Silver and lead were worked at Combe Martin during the thirteenth century and again during the Elizabethan era.[13]