'IRAQ AND THE PERSIAN GULF' [‎21r] (46/862)

CHAPTER II
GEOLOGY AND DESCRIPTION OF THE LAND
GEOLOGY
T hough much was written, little was known about the geology and
structure of Iraq before the War of 1914-1918. Since then a
flood of light has been shed on the subject, as the result of the search
for oil and for more adequate water-supplies in certain parts. Most
of the State has now been well surveyed topographically, and much of
the geology has been carefully mapped (fig. 2).
Structurally and geologically Iraq may be divided into three parts:
(i) The plains of the western deserts rising almost imperceptibly
from the banks of the Euphrates and underlain by strata tilted
slightly towards the east, but virtually free from folds.
(ii) The mountain districts in the east and north, where folded,
contorted, and even broken layers of rock build up the ranges,
(iii) The silt-filled depression of Mesopotamia between these two
parts, the prehistoric head of the Persian Gulf The historical term used to describe the body of water between the Arabian Peninsula and Iran. .
These three parts, though markedly affecting each other through
out the ages, have had very different histories. The first has been
controlled by its underlying block of worn-down Arabian land—part
of the ancient African continent ending just west of the Euphrates—
which has formed a resistant foundation covered by sedimentary
layers, thin on the west but thickening as the river is approached.
The second was the southern part of a long-enduring ocean between
the African and Eurasian continents, in which a thick accumulation
of sediment was deposited and has been folded later into mountains.
The third and central unit covers the junction of the first two and is
young compared with them.
At certain periods in the history of the earth parts of its crust have
suffered deformation by contraction. At such times the larger con
tinental masses have sometimes behaved as rigid blocks, which, though
they may break and shear, will not readily bend; sometimes they have
moved bodily towards each other, the long ocean belts between them
have been narrowed, and their pliable sedimentary floors have been
forced to fold between the continents. This is precisely what has
happened in south-western Persia and along northern and eastern
Iraq, where the rock-sheets laid horizontally on the floor of the sea