Gloucestershire Geology Trust


Blue Lias

Early Jurassic (Rhaetian-Hettangian)

Blue Lias

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.

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.

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.

Representative Sites
Hock Cliff
Hagloe Shoreline

Information on lithology and boundaries taken from the British Geological Survey Lexicon of Named Rock Units

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