The area thus occupied by the two countries was of no very great extent, and it was even less than would appear from a modern map of the Tigris and Euphrates valley. For not only was the head of the Persian Gulf some hundred and twenty, or hundred and thirty, miles distant from the present coast-line, but the ancient course of the Euphrates below Babylon lay considerably to the east of its modern bed. In general character the lands of Sumer and Akkad consist of a flat alluvial plain, and form a contrast to the northern half of the Tigris and Euphrates valley, known to the Greeks as Mesopotamia and Assyria. These latter regions, both in elevation and geological structure, resemble the Syro-Arabian desert, and it is only in the neighbourhood of the two great streams and their tributaries that cultivation can be carried out on any extensive scale. Here the country at a little distance from the rivers becomes a stony plain, serving only as pasture-land when covered with vegetation after the rains of winter and the early spring. In Sumer and Akkad, on the other hand, the rivers play a far more important part. The larger portion of the country itself is directly due to their action, having been formed by the deposit which they have carried down into the waters of the Gulf. Through this alluvial plain of their own formation the rivers take a winding course, constantly changing their direction in consequence of the silting up of their beds and the falling in of the banks during the annual floods. Of the two rivers the Tigris, owing to its higher and stronger banks, has undergone less change than the Euphrates. It is true that during the Middle Ages its present channel below Kût el-'Amâra was entirely disused, its waters flowing by the Shatt el-Hai into the Great Swamp which extended from Kûfa on the Euphrates to the neighbourhood of Kurna, covering an area fifty miles across and nearly two hundred miles in length.[3] But in the Sassanian period the Great Swamp, the formation of which was due to neglect of the system of irrigation under the early caliphs, did not exist, and the river followed its present channel.[4] It is thus probable that during the earlier periods of Babylonian history the main body of water passed this way into the Gulf, but the Shatt el-Hai may have represented a second and less important branch of the stream.[5]