Arkell.1956.Jurassic..

20 THE BRITISH ISLES
Portland the Roach bed is full of the turreted gastropod Aptyxiella portlandica
(Sowerby). The succession is:—
Zone of Titanites giganteus (up to 35 m.). Portland Freestone Beds
above, Cherty Beds below. Inland the Freestone Beds are represented
by rubbly creamy limestones, the Cherty Beds by sands and sandstones
(Swindon Sands and Stone) with rubbly limestones (Cockly bed) at base,
the latter with abundant Titanites and Kerberites.
Zone of Glaucolithites gorei (Salfeld). These evolute Pavlovia-like
ammonites occur in the upper part of the Portland Sand in Dorset, in
the glauconitic Tisbury Stone in the Vale of Wardour (of which Salisbury
Cathedral is mainly built) and in thin glauconitic beds (1-3 m.) at Swindon
and near Oxford and Aylesbury, at all of which places there is a basal
lydite bed (Upper Lydite Bed).
Zone of Zaraiskites albani (Arkell) (1935, p. 339, pi. 26, fig. 2). So far
found only in Purbeck, in the lowest 11 m. of the Portland Sand (Emmit
Hill Marls and basal Massive Bed). With the Zaraiskites are poorlypreserved
Pavloviae.
KIMERIDGIAN (Kimeridge Clay*, max. 495 m.) (Plates 1, 2)
In Dorset the whole formation consists of clays and shales, with some
cementstone bands and septarian nodules, but inland, at Swindon and
about Oxford and Aylesbury, the Pectinatus Zone in the Upper
Kimeridgian is developed as sands with sandstone doggers. The maximum
thickness is reached in Purbeck, around the type locality of Kimeridge.
Twenty miles farther west, in the Weymouth district, the thickness is
nearly halved. Inland it is reduced to 90 m. at Swindon and about
30-45 m. at Oxford, although Lower, Middle and Upper Kimeridgian
are all represented. The highest Jurassic in Yorkshire probably belongs
to the Wheatleyensis Zone. The ammonite zones recognized are as
follows:
UPPER KIMERIDGIAN
Zone of Pavlovia pallasioides (Neaverson). In Dorset c. 21 m. of
marls and clays with badly-preserved crushed Pavloviae. The type
locality is Hartwell, near Aylesbury, where the Hartwell Clay yields
P. pallasioides, P. hartwellensis and other species (Neaverson, 1925,
pi. ii, figs. 3-5, pi. iii, figs. 5, 6) and also Dorsoplanites ultimum (Neaverson,
pi. i, fig. 11).
Zone of Pavlovia rotunda (Sowerby). In Dorset 80 m. of clays and
shales with Pavlovia spp., mostly crushed, except in a line of nodules
near the middle, which dips to the beach at Chapmans Pool and yields
perfect specimens of P. rotunda and Buchia spp. Inland, phosphatized
fragments of Pavloviae from these two zones are common in lydite beds
* The neologism 'Kimmeridge', with two 'm's' has unfortunately come to be widely
adopted of late. From Domesday Book (1085) to 1892 only one'm' was generally used.
See Arkell, 1947, p. 68, footnote.
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