Arkell.1956.Jurassic..

THE JURA MOUNTAINS 8l
of ruckles was evidently determined by the frame of resistant horsts—
Black Forest, Vosges, Serre, Cremieu—within which the Mesozoic cover
was able to crease.
Attempts to draw scale-sections through the Jura anticlines show that
such tight folding cannot be continued at depth: there is no room for
ancient core rocks inside the looped Jurassic formations. Tunnels have
FIG. 9.—The Jura Mountains. Jurassic outcrops in black, but including numerous
narrow elongate outcrops of Cretaceous and Tertiary sediments in the synclines.
The division between the folded or chain Jura and the plateau Jura is shown by a
dot-dash line (simplified).
confirmed this: none has revealed any rocks older than Trias inside the
anticlines. Thus the Jura ranges have long been renowned as text-book
examples of disharmonic folding of cover rocks torn bodily from their
resistant basement above a plane of lubrication—here Triassic marls
and salt (fig. 10).
In essence this interpretation is unchallengeable: the disharmony
(decollement) is manifest. That the basement is flat and totally unfolded,
however, by no means follows. The implication that the thrust was
transmitted horizontally through the Mesozoic cover for thirty or forty
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