Arkell.1956.Jurassic..

THE BRITISH ISLES 31
BAJOCIAN (Inferior Oolite Series, less Crackment Limestone; up to
240 m.)
For the most part richly-fossiliferous limestones, except in Yorkshire
where largely replaced by deltaic sandstones and shales.
UPPER BAJOCIAN (up to c. 15 m.)
In the south of England the Upper Bajocian is the most constant part
of the stage in distribution and thickness. It constitutes the 'Top Beds',
which are disconformable on the Middle and Lower Bajocian and Lias
in different parts of Dorset, Somerset, the Cotswolds and Oxfordshire,
and transgressive on Carboniferous Limestone in the planed Variscan
structures of the Mendips. It consists of varying combinations of rubbly
and bedded limestones, of which the most characteristic is the Clypeus
Grit, full of Clypeus ploti, brachiopods and myacean casts. In the
Lincolnshire-Rutland basin of deposition it probably contains some of
the best Jurassic building-stones in Britain, the Barnack, Weldon, Ketton
and Clipsham stones in the Upper Lincolnshire Limestone. In Yorkshire
it is probably represented by the upper argillaceous part of the Scarborough
Beds; but no ammonites of definitely Upper Bajocian age are
known from either area. In western Scotland and the Inner Hebrides
are up to 9 m. of shale, clay and sandstone with ammonites of the two
lowest zones. The zonal sequence is as follows :
Zone of Parkinsonia parkinsoni (Sow.). To this zone belong the Clypeus
Grit and, farther south, the equivalent Doulting Stone, Anabacia Limestones,
Upper Coral Bed, Microzoa and Sponge Beds, and Truellei Bed.
The commonest ammonites are Parkinsonia parkinsoni (Sow.) and P.
dorsetensis (Wright). The former abounds from the Dorset coast to the
Evenlode valley in Oxfordshire, but has not been found beyond. A
common form in the upper part of the zone was figured by Buckman as
P. schloenbachi and made index of a special zone; but it was not correctly
named, and the true P. parkinsoni occurs with it (see Arkell, 1951, Mon.
Engl. Bathonian Am., p. 9). At the base in south Dorset is often (but
not always distinguishable) a bed 1 to 2 ft. thick, containing Strigoceras
truellei (d'Orb.), which Buckman regarded as another separate zone.
The ammonite is not known anywhere else in Britain and its distribution
abroad does not warrant recognition of a separate truellei zone. For
purely local work in Dorset, however, the truellei bed is a useful datum.
Various Cadomites and Oppeliidae (Oxycerites, etc.) occur.
Zone of Garantiana garantiana (d'Orb.). This zone is never more
than 1 ft. thick in south Dorset, and usually a mere seam with average
thickness of 4 inches. In north Dorset it expands into the Sherborne
building-stone and Hadspen Stone. Farther north it forms the Dundry
Freestone and Upper Trigonia Grit, which are conspicuously transgressive
beds in the Mendips and Cotswolds, sometimes with a thin
basal conglomerate (e.g. Maes Knoll conglomerate of Dundry Hill).
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