Devon County Geological Sites

Site reference no. SX58NW1 Name: Sourton Tors

District: Dartmoor National Park, Parish: Sourton West Devon National grid ref: 5398894 - 5449904 OS sheets: 1:50k 191 1:25k OL28 1:10k SX GS 1:50k 338 GridRef x y Lat Long SX542898 254200 89800 50.68953 -4.0654694

Locality description (address): Sourton Tors

Nature of site: Twin ridge tors of volcanic agglomerate and dolerite in the metamorphic aureole of the Dartmoor granite.

Geological / geomorphological features: Lower Carboniferous, Meldon Shale and Quartzite Formation. The two tors are aligned NE-SW and are separated by a more gently sloping shelf. The higher ridge is formed by the outcrop of massive volcanic agglomerate and tuff. Some of the fragments in the agglomerate are large and are mainly of lava (keratophyre), with some sedimentary fragments. Traces of bedding are visible in the finer-grained tuffs. The dip is about 300 to the NW and the bedding is inverted on the overturned limb of a large fold. Below the agglomerates there is no exposure of rock on the shelf but some fragments of hornfelsed shale occur. The lower tor is formed of porphyritic dolerite with feldspar crystals up to 20 mm long. Some secondary fibrous amphibole can be seen. The dolerite intrudes the rocks of the Meldon Shale and Quartzite Formation. The whole sequence is affected by the thermal metamorphism of the Dartmoor Granite. To the north east [546900] the embankments of the ponds of the old ice works provide a view towards Meldon reservoir. The West Okement river, which is controlled by the NW-SE aligned Prewley Fault in its upper reaches, Can be seen to make a sharp turn to the NE at Vellake corner as a result of river capture. The depression in the ridge at Prewley Farm marks the continuation of the fault line and the original line of the West Okement.

Reasons for registration as a County Geological Site: The site provides excellent and accessible exposures of typical agglomerate and dolerite in the Lower Carbonifeous succession north of Dartmoor. Good views are obtained of river capture and fault control geomorphological features to the north-east.

Site sensitivity: None

Safety: Generally safe.

Interest groups: Schools. Years 5-11: Years 12-16: Years 17-18. University. Undergraduate - Research Professional geologists - Amateur geologists - General public.

Access and Parking: Limited parking for cars opposite Sourton Church (probably restricted at times on Sundays) Access by bridleway, south-east, to the tors.

Date of assessment (V = visited) : V 17th March 1998 R T Taylor

Site owner : Duchy of Cornwall, common. Access land.

References: EDMONDS, E.A. and others. geology of the Country around Okehampton. Memoirs of the Geological Survey of Great Britain. London, Her Majesty’s Stationary Office, 1968. DEARMAN, W.R.and BUTCHER, N.E. 1959, The Geology of the Devonian and Carboniferous rocks of the north-west border of the Dartmoor Granite, Devonshire. Proceedings of the Geologists Association. Vol. 70, pp. 51-92