| Lithological Description: |
| Thinly interbedded limestone (laminated, nodular, or massive and persistent) and calcareous mudstone or siltstone (locally laminated). Individual limestones are typically 0.10-0.30m thick. In some areas, intervening mudstone units with relatively few limestone beds. Also includes littoral limestone facies of the Radstock Shelf - Mendip area and South Wales. |
| Definition of Lower Boundary: |
| At the base of grey limestone or mudstone sharply overlying irregular surface of pale grey or bluish and greenish-grey or reddish brown mudstone of the Cotham Member (Lilstock Formation) or eroded, commonly bored, locally conglomeratic surface of pale porcellanous limestone of Langport Member (Lilstock Formation). Coincides with the base of the Lias Group, which is commonly markedly non-sequential. |
| Definition of Upper Boundary: |
| At the base of the Charmouth Mudstone Formation. In any one section, this coincides with marked upward decrease in abundence of limestone beds, locally associated with marked decrease in their individual thickness and lateral persistence. In many cases, the horizon to be taken is obvious. Where there is doubt; the thickness and vertical frequency of limestone beds should be considered, and the following guidelines used: at top of a more or less persistent, thick (>0.10m) limestone above which the overlying mudstone (beneath any subsequent similar limestone) is at least twice the thickness typical of the underlying limestone-mudstone interbedded unit and above which is no return, within the Lias Group, to limestone-mudstone spacing on that previous scale. In type section, top formerly taken at top of Scipionianum Bed but characteristic interbedded limestone/mudstone sequence continues above that Bed, and that upper boundary in type area should be drawn at a higher stratigraphical level. In Dorset coastal sections, top is at top of Grey Ledge (Bed 49 of Lang, W D, 1924); in Somerset coastal sections, top is at the top of Bed 238 of Whittaker, A and Green, G W, 1983. |
| Thickness: |
| To c.140m |
| Geographical Limits: |
| Wessex Basin, Central Somerset Basin, Worcester Basin and adjoining marginal areas (Radstock-Bristol Shelf and South Wales), southern part of the East Midlands Shelf ie. Dorset coast to approximately Leicester. |
| Reference(s): |
| Donovan, D T and Kellaway, G A 1984. Geology of the Bristol district: Lower Jurassic rocks. Memoir of the British Geological Survey, Bristol Special Sheet (England and Wales). |
| Bristow, H B and Etheridge, R and Woodward, H B, 1873. Vertical sections of the Lower Lias and Rhaetic or Penarth Beds of Somerset and Gloucester- shires. No 9 Saltford. Vertical sections Sheet No 46. Geological Survey of England and Wales. |
| Cope, J C W and others, 1980. A correlation of Jurassic rocks in the British Isles. Part 1: Introduction and Lower Jurassic. Special Report of the Geological Society of London, No.14. |
| Old, R A, Sumbler, M G and Ambrose, K, 1987. Geology of the country around Warwick. Memoir of the British Geological Survey, Sheet 184 (England and Wales). |
| Donovan, D T, 1956. The zonal stratigraphy of the Blue Lias around Keynsham, Somerset. Proceedings of the Geologists' Association, Vol.66, p. 182-212. |
| Ivimey-Cook, H C, Warrington, G, Worley, N E, Holloway, S and Young, B, 1995. Rocks of Late Triassic and Early Jurassic age in the Carlisle Basin, Cumbria (north-west England). Proceedings of the Yorkshire Geological Society, Vol.50, 305-316. |
| Lang, W D, 1924. The Blue Lias of the Devon and Dorset coasts. Proceedings of the Geologists' Association, Vol.35, 169-185. |
| Hallam, A, 1960. A sedimentary and faunal study of the Blue Lias of Dorset and Glamorgan. Philosophical Transactions of the Royal Society, London, Vol.B243, 1-44. |
| Whittaker A and Green G W. 1983. Geology of the country around Weston-super-Mare. Memoir of the Geological Survey, England and Wales, Sheet 279 with parts of sheets 263 and 295. |
| Cox, B M, Sumbler, M G and Ivimey-Cook, H C, 1999. A Formational framework for the Lower Jurassic of England and Wales (Onshore Area). British Geological Survey Research Report No. RR/99/01. |
| Hesselbo, S P and Jenkyns, H C A, 1995. A comparison of the Hettangian to Bajocian successions of Dorset and Yorkshire. 105-150 in Taylor, P D (Editor). Field geology of the British Jurassic (London Geological Society). |
| Callomon, J H and Cope, J C W, 1995. The Jurassic geology of Dorset. 51-103 in Field Geology of the British Jurassic. Taylor, P D (Editor). (Bath: The Geological Society). |
| Torrens, H S and Getty, T A, 1980. The base of the Jurassic System. Special Report of the Geological Society of London, No.14, 17-22. |
