This article describes the geology of Dartmoor National Park in Devon, in south-west England. Dartmoor gained national park status in 1951 but the designated area of extends beyond the upland of Dartmoor itself to include much of the surrounding land, particularly in the northeast. The geology of the national park consists of a core of granite intruded during the early Permian period into a sequence of sedimentary rocks originating in the Devonian and Carboniferous periods. These rocks were faulted and folded, sometimes, intensely, during the Variscan orogeny. Thermal metamorphism has also taken place around the margins of the granite pluton altering the character of the sedimentary rocks whilst mineral veins were emplaced within the granite. A small outlier of Palaeogene sediments occurs on the eastern boundary of the national park.
The area was not subject to glaciation during the Quaternary ice ages but periglacial processes have contributed to the character of the modern landscape. Tin mining and the quarrying of granite were some of the area's significant extractive industries in the past whilst tourism based in large part upon the perceived quality of the area's landscape is important for the modern economy.
Devonian and Carboniferous sedimentary rocks
Several depositional basins stretched east-west across south-west England during the Devonian and Carboniferous periods and, over millions of years, each acquired a mix of sand, mud and silt which would eventually become the sandstones, mudstones and siltstones of Devon and Cornwall . This series of sedimentary basins were subject to broadly north-south compression during the course of the Variscan orogeny leading to the complexly folded and faulted geology which characterises the area today.
Detail of the depositional basins
Those sedimentary basins relevant to Dartmoor's story are i) the South Devon Basin, the rocks formed within which now stretch around the southern margins of the national park from Padstow via Plymouth to Torquay, ii) the Tavy Basin stretching from Tintagel through Tavistock to Newton Abbot and iii) the Culm Basin which corresponds to the whole of mid Devon. The South Devon Basin comprises a middle and upper Devonian sequence of mudstones, siltstones, sandstones and limestones together with rocks of igneous origin; tuffs and lavas. The Tavy Basin comprises an upper Devonian and Carboniferous sequence of mudstones and sandstones, together with chert. The Kate Brook Slate Formation is of Famennian age and consists of greenish-grey and black slates representing an outer shelf facies. The Crackington Formation which is a Namurian age turbidite deposit consisting of dark grey shales with sandstone layers. The Culm Basin comprises a Carboniferous sequence also of mudstones, siltstones and sandstones together with chert and tuffs.
Tectonic stratigraphy
Around the margins of the granite, the original stratigraphy – the order in which the layers of sedimentary rock were initially deposited – is complicated by the fact that thrust faulting has led to older rocks coming to lie on top of younger rocks in many areas, the contact between the two being a slide plane. If the underlying rocks have not moved substantially from their original position then they are referred to as an autochthon and the overlying rocks, which have moved into that position as an allochthon, allochthonous block or nappe. To the west of the granite, the Greystone Nappe is seen to have over-ridden the rocks of the autochthonous Kate Brook Slate Formation (also known as the Tavy Formation). The Greystone Nappe is formed from numerous formations of Carboniferous age. This nappe has been over-ridden in turn by the Blackdown Nappe consisting of the Bealsmill Formation. East of the granite in the eastern margins of the park, the Liverton tectonic unit is over-ridden by the Ilsington and then the Teign Valley units east of Haytor whilst further south it is over-ridden by the Kate Brook and Ashburton Down units.
Devonian rocks
The Kate Brook Slate Formation (a.k.a. Tavy Formation) outcrops is of Frasnian to Famennian age and consists of greenish-grey and black slates representing an outer shelf facies. These rocks are found along the southwestern margin of the park from Tavistock southwards and on the southeastern margin south from Ashburton. The grey Givetian (Famennian – Eifelian) age Chercombe Bridge Limestone Formation (393-383mya) outcrops at Buckfastleigh and also along the boundary of the park to the northeast between Ashburton and Goodstone.