Great Oolite Group, Middle Jurassic, Minchinhampton, Bath—Cotswolds Province
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Lower Fuller’s Earth
The Lower Fuller’s Earth sequence, up to 30 m thick, is much like that farther south, except that another prominent, un-named shell bed rich in P. acuminata appears about 2 m below the Acuminata Bed. This shell bed persists north-eastwards into Oxfordshire where it passes into the basal bed of the Stonesfield Slate Beds.
The uppermost beds of the Lower Fuller’s Earth, above the Acuminata Bed, pass laterally into the Througham Tilestones, which make their first appearance at Minchinhampton and thicken to a maximum of about 7 m near Bisley. They consist mainly of fissile sandy limestones which were once widely exploited for roofing ‘slates’. The tilestones in their turn pass laterally eastwards into the Taynton Stone.
‘Shelly Beds and Weatherstones’; Dodington Ash Rock
The Minchinhampton area is important because it marks the transition to the North Cotswold type of succession. The area was made famous by the monographs of Morris and Lycett in the 1850s, which describe the finely preserved mollusca collected from the freestone quarries on Minchinhampton Common. Although the main quarries are closed, quarrying still continues on a reduced scale. The present survey interpretation of the sequence hereabouts is given in P948998 (col. 5).
The ‘Shelly Beds and Weatherstones’, comprising 7.5 m of cream, cross-bedded oolite freestones with scattered shell debris succeed the Througham Tilestone. They probably represent a north-north-west-trending carbonate bar separating the offshore, open-shelf limestones of the Dodington Ash Rock from the lagoonal limestones of the White Limestone. The overlying 4 m of white, yellowish-weathering, fine-grained, detrital, non-shelly limestone recorded in the old quarries is thought to represent the Dodington Ash Rock, the lower part of which has passed laterally into the ‘Shelly Beds and Weatherstones’.
No fewer than 11 species of ammonites ascribed to the subcontractus and morrisi zones have been recorded from the Minchinhampton area but unfortunately none is adequately localised and their stratigraphical position remains uncertain because their matrix, which is mainly fine grained and nonoolitic, cannot be matched with the quarry descriptions. However, the limited field evidence suggests that the ammonites may have been collected from the ‘Shelly Beds and Weatherstones’, perhaps in the transition zone to the Dodington Ash Rock at the western margin of Minchinhampton Common.
The Athelstan Oolite consists mostly of massive, white and creamy-grey, shell-detrital oolite. At the base there is an impersistent marly oyster bed, the ‘Scroff’, overlain by about 6 m of coarse, shelly, bedded, sparry oolite known to quarrymen as the ‘Planking’. The formation passes eastwards into the Ardley Member of the White Limestone.