Devon County Geological Sites
Site reference no. SX76NW1 Name: Holne Road
District: Dartmoor National Park Parish: Buckfastleigh (Teignbridge District) National grid ref: 73556668-73826628 OS sheets: 1:50k 202 1:25k OL28 1:10k SX76NW GS 1:50k 349
GridRef x y Lat Long SX736664 273600 66400 50.483742 -3.7830105
Locality description (address): Holne Road, Buckfastleigh
Nature of site: Road cuttings on Holne Road and the Mardle Way – Holne Road Buckfastleigh relief road.
Geological / geomorphological features: Upper Devonian grey Kate Brook Slate intruded by Permian lamprophyre and purple and green Gurrington Slate intruded by Devonian basic rock. The northern cuttings on the east side of Holne Road exposes uniform well cleaved greenish grey-weathering Kate Brook Slate with cleavage generally dipping about 300 SE and folded by a set of small kink-band folds with steep axial planes trending together with some more open warping of the cleavage. A lamprophyre intrusion, probably a minette, cuts the slates [73586660]. The section of the intrusion exposed at the base of the cutting is approximately horizontal over a distance of 5.5m varying in thickness from 1.0-1.3m. At the SE end the intrusion is truncated by a fault. At the NW end the intrusion starts to turn into a southerly dipping attitude and lamprophyre debris is found in the drainage ditch at the top of the cutting. The intrusion has dark chilled margins 10mm wide. A second lamprophyre dyke [73596658] 35m SE cuts across the slaty cleavage and dips 450. It narrows upwards from 0.5m - 0.2m and does not appear in the drainage ditch above. Both intrusions strike 0300. The lamprophyres of this suite of intrusions have a radiometric age of about 295 million years. The southern cutting between Church Hill and Mardle Way exposes Gurrington Slate intruded by dolerite. The upper surface of the intrusion is vesicular suggesting that it was intruded into wet unconsolidated sediment. The tilting and inversion of the vesicular top at the NW end of the intrusion shows that it is deformed by a large recumbent northerly facing fold to which the cleavage in the slates is related. The unexposed junction between the Kate Brook Slate and Gurrington slate in the vicinity of Church Hill is a thrust continuing a structure shown on the Geological Survey Sheet 338.
Reasons for registration as a County Geological Site: The Holne road lamprophyre intrusions are of unweathered clean rock, showing clear relationships to the country rock and easily accessible. The site also displays the relationships of basic igneous rocks to the folding and exposes two formations of Upper Devonian slates.
Site sensitivity: None Safety: 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: Parking with care on Holne Road. There is direct access to the cuttings.
Date of assessment (V = visited) : V September 1997 R T Taylor
Site owner : Devon County Council
Other comments: Lamprophyre rock excavated during the road widening has been placed beside to the northern dyke. It is hoped that those wishing to examine the rock in detail will collect from this material, rather than from the rock in situ, to prevent damage to the exposures and the side of the cutting.
References: RUNDLE, C.C. 1980. K-Ar ages for Lamprophyre dykes from SW England. Report of the Institute of Geological Sciences, Isotope Geology Unit, No. 80/9. SELWOOD, E.B., and others. 1984. Geology of the Country around Newton Abbot. Memoirs of the British Geological Survey, Sheet 339. London. HMSO. WILCOCK, A.D. 1982. An introduction to the geology of the area between Buckfastleigh and Ivybridge. Proceedings of theUssher Society. Vol.5, (3), pp. 289-295.