Gloucestershire Geology Trust


Forest Marble

Middle Jurassic (Bathonian)

Great Oolite
Forest Marble

Silicate-mudstone, greenish grey, variably calcareous and in the south notably sandy, with lenticular typically cross-bedded limestone units that form banks and channel-fills, especially in lower part. A variety of limestone types occur, of which grey, weathering brown and flaggy, variably sandy medium to coarsely bioclastic grainstone or less commonly packstone predominates, especially at the base, which is increasingly ooidal north from Bath. Other types include fissile sandy limestone, grading to calcareous sandstone, and oyster-limestone. Bivalves and brachiopods dominate the fauna, and lignite debris and fish scales and teeth are common, but infauna and signs of bioturbation are rare.

Upper Boundary
Generally mudstone in the upper part of the Formation, overlain sharply and non-sequentially by ooidal shelly wackestone/packstone of the Cornbrash Formation.

Lower Boundary
Cotswold region: base of silicate-mudstone, greenish grey, or limestone, typically grey to brown variably sandy medium to coarsely bioclastic grainstone or packstone, resting with erosive, commonly channelled, disconformity on white to yellow, peloidal, ooidal or lime mud-rich, less silici-muddy limestone of the White Limestone Formation or the Chalfield Oolite Formation, or on a bored and oyster-encrusted hardground surface of the ooidal limestone of the Athelstan Oolite Formation.

Representative Sites
Daglingworth Quarry
Kemble Railway Cutting

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

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