Arkell.1956.Jurassic..

THE BRITISH ISLES 27
by clay, and in the south it is represented by the lower part of the
Kellaways Clay only, as Buckman perceived in 1927.
The lower subzone of M. macrocephalus comprises the southern Upper
Cornbrash, which in addition to various Macrocephalitids yields Choffatia
spp. and, very rarely, Kepplerites cerealis (Buckman) and Paracadoceras
breve (Blake), which confirm its essentially Callovian age. The upper
part of the subzone is often characterized by Ornithella {Microthyridina)
lagenalis and O. (M.) calloviensis, the lower part by Ornithella siddingtonensis
and O. arenaria. Pebble beds occur locally (Douglas & Arkell,
1932. 1935)-
BATHONIAN (Great Oolite Series including Lower Cornbrash, plus
Crackment Limestone, up to 150 m.)
The last stage of the English Jurassic to yield to ammonite 'zoning',
the Bathonian has only lately been reduced to order, chiefly by mapping.
The Fuller's Earth Rock, which in the South Cotswolds underlies 'The
Great Oolite', has proved to pass northwards into the part of the Great
Oolite which at Minchinhampton yields the same ammonite fauna as
the lower part of the Fuller's Earth Rock in Somerset. This result
vindicates a correlation published by Buckman in 1901, which seemed
impossible. (Arkell & Donovan, 1952; ammonites monographed,
Arkell, 1951- (in progress); other mollusca revised by Cox & Arkell,
1948-50; brachiopods by Muir-Wood, 1936 and McKerrow, 1953.) The
Bathonian of the Cotswolds comprises four sedimentary cycles of clay,
sand or false-bedded oolite, and shelly compact limestone, as follows
(Arkell & Donovan, 1952, p. 251). The middle parts of the third and
fourth cycles together comprise the Forest Marble of Wychwood Forest
and the Oxford district.
FORMATIONS AMMONITES
Cycle 4—
3. Lower Cornbrash . . . . . Clydoniceras discus
2. Wychwood Beds and Hinton Sands
1. Bradford Clay
Cycle 3—
. . . . . . Clydoniceras hollandi
3. Petty France White Limestone
2. Kemble Beds, Bath Stone, Lower Rags .
1. Lansdown Clay (Upper F.E. Clay)
Cycle 2—
. Oppelia aspidoides
3. Tresham Rock and White Limestone
2. Hens' Cliff Oolite
1. Hawkesbury Clay (Middle F.E. Clay)
Cycle 1—
. . Bullatimorphites bullatimorphus
3. Minchinhampton White Limestone . . Tulites subcontracts
2. Minchinhampton Weatherstones and Stonesfield
Slates . . . . . . Gracilisphinctes progracilis
1. Stroud Clay (Lower F.E. Clay)
Top of Inferior Oolite : rubbly beds . . . Zigzagiceras zigzag
North-east of the Cotswolds the Bathonian becomes much thinner, most,
of these subdivisions disappear, and ammonites become scarce. In
Yorkshire the stage is represented by deltaic beds, the Upper Estuarine
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