The BGS Lexicon of Named Rock Units — Result Details

Yeolmbridge Formation

Computer Code: YF Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Famennian Age (DA) — Courceyan Substage (CF)
Lithological Description: Dark green-grey, black and green mudstones with variably abundant siltstone laminae and scattered thin- to medium-bedded sandstones, limestone nodules and units of nodular cephalopod limestone. Lavas and thin tuff beds are sporadically present.
Definition of Lower Boundary: The lower boundary is taken at the gradational incoming of silt laminated dark green-grey and black mudstones of the Yeolmbridge Formation above the dark green-grey and black mudstones with siliceous thin beds and nodules of the underlying Stourscombe Formation. This contact is not well documented and needs further work.
Definition of Upper Boundary: The upper boundary is not seen; everywhere tectonic.
Thickness: Not known due to structure, but unlikely to exceed 100m.
Geographical Limits: Launceston area (SX 33 84). Mapped as ?part of the Transition Group on the Holsworthy sheet (323). Might extend westwards to the north Cornwall coast where it has been mapped as Transition Group and eastwards to west Dartmoor, but further work is needed.
Parent Unit: Tamar Group (TAMA)
Previous Name(s): Transition Group (Pars) (-3150)
Yeolmbridge Beds (-3782)
Alternative Name(s): none recorded or not applicable
Partial Type Section  Yeolmbridge quarry, 3km north of Launceston. The quarry is partially flooded and only 15m in the lower part of the Formation is exposed around the rim of the quarry. (Grid reference provides for southwest and northeast parts), The basal junction is thought to have been exposed at one time on the basis of late Famennian faunas from Museum collections. The section comprises silt-laminated mudstones with sparse sandstones. Thin limestones and limestone nodules are present locally. Whitely, 2004. 
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). 
Selwood, E B. 1960. Ammonoids and trilobites from the Upper Devonian and Lower Carboniferous of the Launceston area, Cornwall. Palaeontology, Vol.3, 153-185. 
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). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable