Arkell.1956.Jurassic..

THE BRITISH ISLES 35
TOARCIAN (Upper Lias, up to 135 m. in Yorkshire)
The Toarcian in the definition here accepted corresponds to the Upper
Lias with the Cotswold and Bridport Sands and at the top the Cephalopod
Bed of the South Cotswolds. The Upper Lias clay of the Northants-
Lincoln area reaches 60 m. in thickness but represents only the lower
half of the stage. Sands began to form at the time of the Bifrons Zone
in the Cotswolds, but not until the Moorei Subzone on the Dorset coast,
where the Bridport Sands, of Moorei and Aalensis date, with the Opalinum
bed, reach 42 m.* The limestone Junction Bed on the Dorset coast
(1-2 m.) at the base of the Upper Lias overlaps in date with the Cephalopod
Bed at the top of the formation in the Cotswolds. The most complete
development at any one place is found in Yorkshire (Dean, 1954). In
Raasay, western Scotland, the bulk (21 m.) of the Upper Lias, the Dun
Caan Shales, is Upper Toarcian; below is the Raasay Ironstone (2-4 m.)
of the Subcarinata Subzone.
The zonal sequence is as follows :—
ZONES SUBZONES
Lytoceras jurense (Zieten) .
Hildoceras bifrons (Brug.) . . -J
Harpoceras falcifer (Sow.) f . -j
Dactylioceras tenuicostatum (Y. "1
&B.) . . . )
Pleydellia aalensis
Dumortieria moorei
Dumortieria levesquei
Phlyseogrammoceras dispansum
Grammoceras struckmanni
Grammoceras striatulum
Haugia variabilis
Pbymatoceras lilli
Dactylioceras braunianum
Peronoceras fibulatum
Frechiella subcarinata
Ovaticeras pseudovatum
Harpoceras falcifer
Harpoceras exaratum
Dactylioceras tenuicostatum
Tiltoniceras acutum
With these and the other zones of the Lias, the succession of ammonite
faunas is sufficiently well illustrated by the list of subzones.
UPPER PLIENSBACHIAN (Middle Lias or Domerian, up to 125 m. in Dorset)
In its typical development the Middle Lias consists of relatively thin
ironstone or marly limestone at top, corresponding to the Zone of
• On p. 163 of my Jurassic System (1933) the thickness was misprinted, 40 ft. for
140 ft.
f Although prior as zonal index, Hildoceras (?) serpenlinum (Schloth.) is too doubtful
for it to be used : see Bull. Zool. Nomencl., 1951, vol. ii, parts 6-8, p. 192.
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