LOWER Mesopotamia, the Sumer of the ancient world,is no more nor less than the river-valley of the Tigrisand the Euphrates; it does not include the high-lying Syrian desert to the west, because that is deserta waterless expanse of gravel barren for most of the year at least-where the wandering Bedouin may pitch their tents for a brief space but no man claiming to be civilized could make his home; and it does not include the Persian mountains that fringe it on the east because always those mountains were held by warlike tribes more ready to raid the cultivated fields of the valley people than to submit to their sway. And it is a land of recent formation. Originally that arm of the sea which we call the Persian Gulf extended far inland, to the north of modern Baghdad, and it was only at a relatively late date in human history that salt water gave place to dry land, a change due not to any sudden cataclysm but to the gradual deposit of river silt filling the great rift between mountain and desert. If the Tigris and the Euphrates alone had been concerned the formation of the delta would have followed the normal pattern; starting in the extreme north it would have pushed southwards very gradually, and man's occupation of the newly-made soil would have been conditioned by that slow progress so that only after centuries or indeed millennia could he have settled in the south country where Ur lies. But as a matter of fact this was not the case at all. The people of Sumer themselves believed that the oldest of their cities was Eridu, which lies about twelve miles south ofUr, and excavation there by an Iraqi Government expedition has gone far towards confirming this belief; nowhere in Lower Mesopotamia proper

have there been found traces of a settlement so ancient as that at Eridu. Clearly this requires explanation, and we must look again at the physical geography of our area.