'IRAQ AND THE PERSIAN GULF' [‎22v] (49/862)

l 6 GEOLOGY AND DESCRIPTION OF THE LAND
the composition and succession of the strata beneath the surface of
the earth and to be able to deduce if possible the position of under
lying unbroken anticlines and domes.
The north and east of Iraq were for long periods of time covered
by the sea which ebbed and flowed over the basement to the west.
But here there is no rigid foundation on which the sediments have
been deposited, and the sea-bottom has often sunk during the period
of deposition. The beds are therefore generally much thicker than
those of the western region, while the later contortions they have
undergone during the process of mountain-building and the denuda
tion to which they have been subjected have added to their com
plexity.
Nevertheless their history can be partly read in the walls of the
gorges which have been carved through the outer ranges of Persia
and in Iraq. Details of the stratigraphy are reserved for Appendix A,
and only a summarized history is given below.
In Cambrian times, when the Arabian basement was exposed, the
waters of the shallow sea that lapped its edges were so concentrated
in this north-eastern region that they deposited salt and gypsum.
This concentrate was covered by a thick layer of sand and mud.
Occasional masses of marine shells in Upper Cambrian times have
built up reefs of limestone among the beds of silt. Then for a period
the sea retreated and the region appears to have been covered by
swamps bordered by forest—possibly in Devonian times near Zakho
in the north, and certainly during the Middle Cretaceous period
farther to the south-east.
In the early Carboniferous period the sea invaded this land from
the south and east, and by Upper Carboniferous times moderately
deep water covered the whole area. Gritty red sandstones mark the
coming of this sea, and thick deposits of fossiliferous dark-coloured
limestone show the establishment of deeper and clearer water. The
sea persisted throughout the whole of the Mesozoic era, though its
depth and extent, and the nature of its bordering lands, varied con
siderably. After a long spell of deep water the ocean contracted at the
end of the Permian period to a shallow gulf. The purple, blue, and
yellow muds and sands now exposed in the lowest horizon of the bed
of the Sirwan (Diyala), on the borders of Iraq and Persia, are tokens
of this withdrawal. The sea soon expanded again, and became exten
sive and moderately deep in the Triassic period, though freely sup
plied with mud. In the Jurassic it was stagnant and the scanty debris
falling to the sea-floor gradually decomposed, without an adequate