Mesopotamia

MESOPOTAMIA. Although the boundaries of the modern kingdom of 'Iraq, which occupies the land of ancient Mesopo tamia, have recently been clearly defined the term Mesopotamia has been loosely used. In this article the area included is the great depression shut in between the escarpment of the Arabian desert and the mountains, which form the western boundary of the plateau of Iran on the west and east respectively, and bounded on the north by the mountains of Armenia and Asia Minor, and on the south by the Persian gulf. In the north there is a belt of stony country varying between 4o and ioo miles broad which extends as far as Hit, which stands on the site of the old coast line before the formation of the alluvial plain of the twin rivers, the Tigris and the Euphrates. This alluvial plain extends south wards, falling nearly ioo feet between Baghdad and Basra, and forms the fertile basin in which grew up the ancient civilizations of Sumer and Akkad. The plain is everywhere extremely flat and has a transverse slope away from the rivers and from the Persian hills to the Tigris. This flatness and the form of the slope combined to allow perennial irrigation on a large scale and so made the ancient civilizations possible. The alluvial plain itself is of comparatively small extent, about 35 thousand square miles, rather over a sixth of the whole area of Mesopotamia.

Geology.

South of the Eurasiatic massif there are three great plateaux, the remains of a tropical continent. Between these plateaux, India, Arabia and Africa south of the Atlas there are a series of basins, Mesopotamia itself forming the basin between the plateau of Arabia and the fold mountains of Persia. The latter parallel ranges, which bound it on the east are of many different ages. The south-western boundary is the Archaean rocks of the Arabian plateau. This contrast between the eastern and western boundaries is enhanced by the difference of climates. The rainfall of the Persian mountains has been accompanied by rapid rock decay and numerous canyons have carried down to the plain a rich and fertile alluvium, but the valley bottom lacking the rain fall of the Indo-Gangetic plain, to which it corresponds physio graphically, has not been able to support the dense population of the latter area. On the west the plateau is arid, streams are

few, and the denudation has been of the desert type, contributing but little to the plain.

The history of this condition seems to be as follows. The earliest direct information on the geological history of upper Mesopotamia is found at Ana on the Euphrates where upper Cretaceous limestones were formed at a time when upper Mesopo tamia was all submerged. A volcanic period in Oman seems to have occurred at the beginning of Cenozoic times, when most of Arabia and Persia were dry land, and coal seams were forming in Mesopotamia. The lower Eocene sea covered parts of the Persian gulf, and during the middle Eocene there was another advance of the sea. At this time there was a deposition of a nummulitic limestone, which is succeeded in upper Mesopotamia by beds which contain fish teeth and marine shells of the Upper Eocene. This period is not widely developed, and in the succeed ing Oligocene there was a land period of which at present only a few fossil remains have been discovered. During the Miocene the sea extended from northwest India to Asia Minor. A further ad vance appears to have followed which resulted in the deposition of red clays, which are interbedded with rocksalt, oil beds and gyp sum. The deposition of these beds was interrupted by an uplift which eventually converted the whole area into a continuous land surface. This uplift was associated with foldings of the greatest significance. Previous foldings had already built the ranges of northern Persia and to a certain extent the Zagros range, but the main folding, which built up the mountain boundary to the northeast of Mesopotamia is probably Pliocene.