Arkell.1956.Jurassic..

THE ATLAS RANGES 257
northern and eastern ring of Jurassic marine outcrops, was above water
level throughout the Jurassic and can be regarded as a source of supply
for clastic sediments. In the north the Sahara plateau has been defined
as 'the part of Africa where Jurassic sediments are absent' (Savornin,
1931; Menchikoff, 1949). The sole exception to this definition is a small
area in southern Tunisia and northern Tripolitania.
THE ATLAS RANGES IN BARBARY
The boundary between the Sahara plateau and folded Barbary runs
in an almost straight line, with only two very gentle sinuosities, from the
Atlantic coast of Morocco at Agadir to the Syrtis gulf at Gabes in Tunisia,
a distance of about 1200 miles. The boundary is determined by a monoclinal
fold, which is the most continuous of the many parallel anticlines
FIG. 31.—The Jurassic outcrops of Barbary (black) and their probable continuation
underground (shaded). Based on Savornin, 1031.
and periclines that constitute the Atlas ranges. On the Sahara plateau
there is no Jurassic; in the cores of all the anticlines Jurassic is well
developed. The line where the Jurassic wedges out southwards is usually
concealed by a Cretaceous cover in Algeria; it is probably often a line of
bevel at the plane of Cretaceous transgression, as in Egypt and southern
England (Dorset-Devon border). In parts of eastern Morocco, however,
a Liassic shoreline is uncovered. Cliffs of Palaeozoic sandstones, slates
and more rarely Visean limestones are exposed, with a Jurassic beach at
the foot. Reef corals grew on the rocks or in the shallows of the Middle
Lias sea offshore. In some places there are lenticular deposits of sandstone
and clay, representing estuaries of Jurassic wadis, forming small gulfs
or lagoons (Menchikoff, 1934, 1949; Choubert, 1952, pp. 142-3 and pi. 11).
The southern boundary of Barbary is therefore at least as old as the
Jurassic, although the main period of uplift of the Atlas ranges, for which
the boundary became the southern hinge, occurred in the Upper Eocene,
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