The BGS Lexicon of Named Rock Units — Result Details

Dowhills Mudstone Formation

Computer Code: DHLS Preferred Map Code: notEntered
Status Code: Full
Age range: Brigantian Substage (CX) — Marsdenian Substage (CZ)
Lithological Description: Dark grey to black, rather soft, pyritous mudstones with scattered thin siltstone and sandstone beds (less than 10%). There are some thin chert beds on the north crop of the Culm synclinorium. The Ashton Mudstone Member is the lateral equivalent of the Formation on the south crop; it is thought to be characterised by more abundant and thicker sandstones. Further work is needed to confirm this.
Definition of Lower Boundary: In the north crop of the Culm synclinorium, between Brushford and Ashbrittle, the lower boundary is taken at the incoming of dark grey predominantly unfossiliferous finely laminated silty mudstones with scattered siltstones of the Dowhills Mudstone Formation above the highest limestone bed of the underlying Bampton Limestone Formation. Between Ashbrittle and Wellington the lower boundary is taken above the highest limestone bed of the underlying Westleigh Limestone Formation. West of South Molton the lower boundary is taken at the incoming of dark grey to black pyritic mudstones above the streaky laminated cherts and mudstones of the underlying Coddon Hill Chert Formation. The boundary is taken at the top of the highest chert bed. These pyritic mudstones, the Limekiln Beds of Prentice (1960) were included by Edmonds et al. (1985) in the Crackington Formation. Further work is needed in this area. In the south crop of the Culm synclinorium between Exeter and the Teign valley, the lower boundary is taken at the incoming of bluish grey micaceous mudstones with scattered thin sandstones and siltstones of the Ashton Mudstone Member above the fossiliferous interbedded cherts, limestones and mudstones (Posidonia Beds) of the Teign Chert Formation.
Definition of Upper Boundary: The upper boundary is gradational and taken at the incoming of more abundant and relatively thicker sandstones of the overlying Crackington Formation above the mudstones with less than 10% sandstone in the underlying Dowhills Mudstone Formation.
Thickness: 210 to 430m on south crop, 30m on north crop.
Geographical Limits: On the north crop of the Teign Valley Group between Bideford (SS 45 27), north Devon and Wellington (ST 13 20), Somerset. Between the Bideford area (SS 45 27) and Brushford (SS 92 26) west Somerset, the formation has not been mapped by BGS. In this area it probably equates with the Limekiln Beds (Prentice 1960), included in the Crackington Formation by Edmonds et al. (1979). Further work is needed in this area to resolve this. Due to thrusting along the south crop of the Teign Valley Group, the formation is seen only between the Teign valley (SX 84 84) and Exeter (SX 90 92), Devon. It may be present in the Okehampton area (SX 59 94) but has not been mapped, probably being included in the Crackington Formation. Further work is needed.
Parent Unit: Teign Valley Group (TEVY)
Previous Name(s): Ashton Shale Member (*260)
Limekiln Beds [Obsolete Name And Code: See DHLS And CKF] (LMK)
Crackington Formation (Pars) (-861)
Ashton Formation (-644)
Dowhills Beds (-4607)
Alternative Name(s): Ashton Shale Member
Stratotypes:
Partial Type Section  Stream section 300m to westsouthwest of Dowhills Farm, 3km east of Bampton, northeat Devon. Intermittant exposure of up to 15m of dark grey fissile mudstone. 
Reference Section  Northwest face of Kiln Cottage Quarry, Bampton, northeast Devon. 5 to 6m exposure of basal part of formation, comprising unfossiliferous, finely laminated mudstone and thin siltstone, locally micaceous and rich in plant debris. Underlain by 23m of mudstone and limestone at the Bampton Limestone Formation. 
Reference(s):
Webby, B D. and Thomas, J M. 1965. Whitsun field meeting: Devonian of west Somerset and the Carboniferous of north-east Devon. Proceedings of the Geologists Association, Vol. 76, 179-194. 
Whiteley, M J. 2004. Culm Trough. 477-504 in British Lower Carboniferous Stratigraphy. Cossey, P J, Adams,A E, Purnell, M A, Whiteley, M J, Whyte, M A and Wright, V P (editors). Geological Conservation Review Series, No 29. (Peterborough: Joint Nature Conservation Committee). 
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. 
Chesher, J A, 1968. The succession and structure of the Middle Teign Valley. Proceedings of the Ussher Society, Vol.2, 15-17. 
Edwards, R A, Scrivener, R C, Barton, C M, Bristow, C R, Williams, B J, Goode, A J J, Cornwell, J D, Royles, C P, Self, S J, Warrington, G, Riley, N J, Turner, N, Jones, N S, Smith, S A, Marks, R J, Fortey, N J, Morton, R D, Strong, G E, Kemp, S J, Forster, A, Ault, L, Haslam, H W and Chesley, J T. 1999. Geology of the country around Exeter. Memoir of the British Geological Survey, Sheet 325 (England and Wales). 
Edmonds, E A, Whittaker, A, and Williams, B J. 1985. Geology of the country around Ilfracombe and Barnstaple. Memoir of the British Geological Survey, Sheets 277 and 293 (England and Wales). 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E310