'IRAQ AND THE PERSIAN GULF' [‎21v] (47/862)

I 4 GEOLOGY AND DESCRIPTION OF THE LAND
have been rucked into ridges and hollows, as a result of the P ressur e
exerted from both sides when the Afncan-Arabian mass in the south
and the Eurasian continent in the north moved towards each other.
Only part of this action occurred in Iraq. Here the earth-waves are
highest, most tightly packed, and most distorted in Kurdistan and
along the Persian borderland; they decrease in size and complexity
towards the centre of the country. But the Arabian block and its
sedimentary mantle in south-western Iraq is almost undisturbed.
The three parts will now be considered in more detail.
(i) The Western Desert Plains
The geological history of this region is long but simple. Events
have caused a succession of layers to be deposited on the rigid con
tinental block and on its gentle eastern slope, beyond which the block
ends at a hidden edge. Sometimes the block has been above sea-level
and subject to desert conditions; always the sea has lapped its edge,
ebbing and flowing over its surface, because of changes in the
relative level of land and sea. It must be emphasized that the surface
of the block is so nearly horizontal that it has provided a delicate
record of even small rises and falls of sea-level, for a slight rise has
meant the flooding of a considerable area of land by a shallow sea.
The process is exactly analogous to that of the ebb and flow of the
tides on a gently sloping foreshore, though the interval between the
‘tides’ is measured by millions of years.
The ancient basement of the continental mass is only revealed far
to the west, outside Iraq, where great cracks have occurred and a rift
of the earth’s crust has appeared along the gulf of Aqaba and the Red
Sea; but this basement, here seen to be made up of old crystalline
rocks pierced by granitic intrusions, is believed to underlie the whole
region eastwards. Over the western part there is a thick mantle of
red, yellow, white, and grey sandstone—the ‘Nubian sandstone’—
resulting from air-borne accumulations during a long period of hot
desert conditions which probably endured for a few hundred million
years between the beginning of the Cambrian until the beginning of
the Lower Cretaceous. This sheet of aeolian deposits reaches east
wards as far as Rutba in western Iraq and possibly beyond. Farther
east it either thins out, or was never laid down, or was later bevelled
off by the coastal erosion of the eastern sea. This immersion of
the gently inclined continental slope east of the Nubian sandstone
occurred during the Jurassic—possibly in the preceding Triassic—
when a pile of marly, massive, thin-bedded limestones was laid down