Arkell.1956.Jurassic..

28 THE BRITISH ISLES
Series (up to 60 m.), and in the Hebrides by the Great Estuarine Series
(120 m.), containing Viviparus scoticus and pelecypods.
UPPER BATHONIAN
Zone of Clydoniceras discus (Sowerby). Lower Cornbrash (up to 3 m.).
This comprises a variety of rubbly and flaggy limestones and marls,
sometimes only a line of small nodules; sometimes shelly, with masses
of Trigonia angulata, Astarte hilpertonensis, Meleagrinella echinata, etc.,
sometimes almost barren. The brachiopods Ornithella obovata (above)
and Cererithyris intermedia (below), abound and are useful as subzonal
indices. The only ammonites known are Clydoniceras discus (Sow.),
C. thrapstonense (Arkell) and other spp., Delecticeras delectum Arkell and
spp., Choffatia subbakeriae (d'Orb.), C. homoeomorpha (Buckman) and spp.
Below the Cornbrash is the Upper Forest Marble (Wychwood Beds),
a shallow-water formation (up to 42 m.) comprising oolitic flaggy and
shaly limestones, with ripple-marks and current-bedding and subordinate
clay lenticles. Locally there are sands and doggers (Hinton Sands).
The only ammonites ever found in these beds are two specimens of
Clydoniceras hollandi (Buckman) from the Bradford Clay, and two small
Siemiradzkia sp.
Zone of Oppelia aspidoides (Oppel). Next below the Bradford Clay
come the Kemble Beds, which are developed in Forest Marble facies in
Oxfordshire and include there the typical development in Wychwood
Forest, but southward pass laterally into the Bath Stone and Lower
Rags. Near Bath the Lower Rags contain a ferruginous oolite (Twinhoe
Ironshot) with Oppelia aspidoides, Wagnericeras arbustigerum (d'Orb.)
and other Procerites-like Perisphinctids. Below is Upper Fuller's Earth
clay, which reaches a thickness of 27 m. at Lansdown near Bath (Lansdown
Clay) but has not yielded ammonites and soon wedges out northwards.
MIDDLE BATHONIAN
Zone of Tulites subcontractus (Morris & Lycett). (Fuller's Earth
Rock and main Great Oolite, up to 38 m.). The Fuller's Earth Rock of
Somerset (6-10 m.) splits north of Bath into two, divided in the South
Cotswolds by the Hawkesbury Clay (9 m.). Each half passes northward
into Great Oolite, comprising a 'White Limestone' facies above and a
shelly, flaggy oolite facies below. The only ammonite known from the
upper division(Tresham Rock) is Bullatimorphites bullatimorphus Buckman,
from Tiltups End, near Nailsworth. The lower division (Cross Hands
Rock and Minchinhampton Beds) contains a rich ammonite fauna, both
in the Great Oolite of Minchinhampton and in the lower Fuller's Earth
Rock of Somerset. The principal ammonites are Tulites subcontractus
(M. & L.), T. cadus (Buck.) and spp., T. (Rugiferites) rugifer (Buck.),
Morrisiceras morrisi (Oppel) and spp., Lycetticeras lycetti Arkell, Krumbeckia
reuteri Ark., Berbericeras schwandorfense Ark., B. sekikense Roman
and occasional Wagnericeras and Siemiradzkia.
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