Arkell.1956.Jurassic..

CHAPTER 4
THE JURA MOUNTAINS
The folded ranges of the Jura form an almost perfectly symmetrical arc
or crescent, about 245 miles long on the outer side and 212 miles long on
the inner, with a maximum width in the centre of 40 miles. The southern
horn of the crescent is joined to the outer front of the Alps near Grenoble;
the north-eastern horn narrows to a point in the Tertiary lowland as the
single anticline of Lagern, near Baden. From the two extremities inwards
the folds multiply fanwise in text-book virgations, until in the middle
of the arc there are about twenty anticlines and as many synclines. Most
of the folds are formed of Jurassic rocks, but many long and narrow
outliers of Cretaceous and even Tertiary sediments are pinched in
the synclines; for simplicity these are not shown on the sketch-map
(fig- 9)-
On the convex rim of the arc, except on the north-west side, the outer
fold range abuts against unfolded Jurassic; this constitutes the plateau
Jura (Jura tabulaire, Tafeljura) as opposed to the folded Jura (Kettenjura,
Jura plisse). The boundary is shown approximately by a dot-dash line
on fig. 9. At no great depth under the plateau Jura lies crystalline
basement. In the north the basement is an extension of the Vosges and
Black Forest massifs; in the centre is another massif which comes to the
surface only in a small inlier of crystallines overlain by Permo-Trias,
the La Serre massif; in the south another small inlier of Pre-Cambrian
basement crops out at the north-west edge of the He Cremieu, which is
an isolated remnant of plateau Jura. This southern outcrop of basement
rocks belongs to the Massif Central, from which it is severed by the rift
valley of the Rhone-Saone. The rifting occurred at the same time as
that of the Rhine valley between the Vosges and Black Forest and before
the main Jura folding. The plateau Jura bordering the Black Forest
on the south and east is riven by a system of faults parallel to the Rhine
valley rift-faults, and this system is overriden by the outer thrusts and
asymmetric folds of the Kettenjura.
The air traveller from England to Geneva who is lucky enough to pick
a day on which there is a low cloud-sea, hiding the plateau Jura and the
midland valley of Switzerland, sees the arcuate fold ranges of the Kettenjura
with their forested tops rising above the cloud like a barrier-reef
cutting the ocean. Beyond the cloud-filled midland valley stand the
Alps, their bare white peaks raised high in the blue sky and glittering in
the sun. This unforgettable sight impresses on the mind a vivid picture
of the relation of Jura to Alps. The Jura folds are ruckles in front of
the great pile of nappes. The size and arcuate shape of the whole crescent
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