The BGS Lexicon of Named Rock Units — Result Details

Holsworthy Group

Computer Code: HOWY Preferred Map Code: notEntered
Status Code: Full
Age range: Namurian Stage (CN) — Bolsovian Substage (CC)
Lithological Description: Grey mudstones with abundant thin- to thick-bedded sandstones, subordinate siltstones and a few conglomerate beds. Mudstones and subordinate thin- to medium-bedded sandstones dominate the lower part and medium- to thick-bedded sandstones with subordinate mudstones and siltstones dominate the upper part. Scattered slumped units occur in the upper part.
Definition of Lower Boundary: Due to thrusting along the south crop of the group, the base is seen only between Okehampton (SX 59 94) and the Teign Valley (SX 84 84), east Dartmoor, Devon. The base is gradational and taken at the incoming of dark grey mudstones with abundant thin- to medium-bedded turbidite sandstones of the Holsworthy Group (Crackington Formation) above the dark grey mudstones with sparse scattered thin turbidite sandstones of the underlying Teign Valley Group (Dowhills Mudstone Formation). Between the Bideford area (SS 45 27) north Devon and the Westleigh area (ST 06 17), west Somerset, the base is taken at a similar level. However, in the Bideford area, the base needs further investigation as the Limekiln Beds (Prentice 1960) included in the Crackington Formation by Edmonds et al. (1979) may be part of the Dowhills Mudstone Formation. South of the main crop, the Group occurs in tectonic slices between Bodmin and Dartmoor and no base is seen. The base here is thought to be unconformable on the underlying Teign Valley Group.
Definition of Upper Boundary: The upper boundary is taken at an angular unconformity, where breccias of the Exeter Group rests upon sandstone and subordinate mudstone at the Bude Formation.
Thickness: c.3500 to 4000m.
Geographical Limits: Culm Synclinorium, between the north Cornwall (SX 15 97) and north Devon (SS 45 27) coast and eastwards to Exeter (SS 93 92) and Tiverton (SS 95 13) in east Devon. To the south tectonic slices occur in the area south of Launceston (SX 32 77) between Bodmin Moor and Dartmoor.
Parent Unit: Culm Supergroup (CU)
Previous Name(s): Upper Culm (-2852)
Upper Culm Group [Obsolete Name And Code: Use HOWY] (UCU)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  The Culm synclnorium centred on Holsworthy, mid Devon. McKeown et al. 1973. 
Reference(s):
Waters, C N, Brown, M A E, Dean, M T and Powell, J H. In press. Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore). British Geological Survey Research Report, RR/05/06. 
Edmonds, E A, Williams, B J and Taylor, R T. 1979. Geology of Bideford and Lundy Island. Memoir of the Geological Survey of Great Britain, Sheets 292 with 275, 276, 291, and part of 308 (England and Wales). 
Sedgwick, A and Murchison, R I. 1840. On the physical structure of Devonshire, and on the subdivisions and geological relations of its older stratified deposits. Transactions of the Geological Society of London, Vol.5, 633-705. 
Prentice, J E. 1960. The Dinantian, Namurian and Lower Westphalian rocks of the region of southwest of Barnstaple, north Devon. Quarterly Journal of the Geological Society, London, Vol.115, 261-290. 
McKeown, M C, Edmonds, E A, Williams, M, Freshney, E C and Masson Smith, D J. 1973. Geology of the country around Boscastle and Holsworthy. Memoir of the British Geological Survey, sheets 322 and 323 (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable