Arkell.1956.Jurassic..

7°
BORDERS OF THE MASSIF CENTRAL
of the massif. At the south end of the Cevennes they turn a corner and
fill a gulf in the southern side of the massif, forming dry limestone
plateaux called the Causses (Pfrom Latin calx). The chief limestones
and dolomites building the Causses are Bajocian and Bathonian, but a
similar facies continues in places to the Tithonian. The special vegetation
nourishes sheep from whose milk the celebrated Roquefort cheese has
been made since Roman times in underground caverns in the limestone.
The Pliensbachian and Toarcian are exceptionally well developed as
marls with a rich and perfectly-preserved Mediterranean ammonite
fauna, and the Lower Lias forms a lower limestone plateau, the Avant-
Causses.
The Tithonian outcrops begin at the spectacular rock on which the
Chateau de Crussol is perched, on a precipice above the Rhone opposite
Valence. They continue southwards through Bas-Vivarais, Gard and
Languedoc, to the Mediterranean coast. In Bas-Vivarais are the classic
localities of Privas and Berrias (after which is named the lowest stage
of the Cretaceous), where the ammonite succession is critical for world
correlation.
The Jurassic of the area formed the subject of detailed studies by
Oppel (1865; 1866, posthumous). Little more than thirty years later
was published the thesis by Prof. F. Roman (1897) on the Bas-Languedoc,
which began a lifetime's study of the stratigraphy and palaeontology of
the area. Far more on the Jurassic than can be condensed here will be
found in both his books, the 'Geologie Lyonnaise' (1926) and the posthumous
'Bas-Vivarais' (1950). Between these three were issued a long
series of monographs on individual stages, written often in collaboration
with colleagues or pupils, and published in the Travaux du Laboratoire
de Geologie de Lyon, which he financed out of his own resources. Among
these is the standard work on the Montagne de Crussol (Riche & Roman,
1921), the source of the marvellous Tithonian and Kimeridgian material
a n Q l
figured in the early monographs of Fontannes (1879) Dumortier &
Fontannes (1876). Still earlier Dumortier (1864-74) published a fourvolume
monograph on the Lias of the Rhone basin, and the Middle and
Upper Lias ammonites of Aveyron have been profusely illustrated in a
series of monographs by Monestier (1921-34). The Causses have been
well described stratigraphically by Agadele (1939).
Thanks to these and other workers, so much is now known of this
region that a mere tabulation of the ammonite horizons present is all
that need be attempted here for most of the stages. The detail can
easily be filled in by consulting the works of Roman, which give copious
references to the other literature. There is even an account in English
(Roman, 1936) of the Lyons-Crussol area, prepared for a memorable
excursion of the Geologists' Association, in which I was fortunate to
take part. The Lower and Middle Jurassic of the northern part of the
area have also been ably summarized and analysed by Mouterde (1953)
in the volume so often drawn on in the previous section.
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