Dyrham Formation

Early Jurassic (Pliensbachian)

Lithology
Pale to dark grey and greenish grey, silty and sandy mudstone, with interbeds of silt or very fine-grained sand (locally muddy or silty), weathering yellow. Variably micaceous. Impersistent beds or doggers of ferruginous limestone (some ooidal) and sandstone, which tend to occur at the top of sedimentary cycles. Sporadic large cementstone nodules.

Upper Boundary
At base of ferruginous limestone or ironstone of the Marlstone Rock Formation.

Lower Boundary
Marked or gradational downward change from silty mudstones to smoother argillaceous sediments of the Charmouth Mudstone Formation; commonly coincides with negative change of slope and/or line of seepage that may correspond with a sandy bed (e.g. Capricornus Sandstone in the Cotswolds; Sumbler, M G, Barron, A J M and Morigi, A N, 2000).

Representative Sites
Robinswood Hill
Stonehouse Brickpit

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