The magnificent site-plans which we owe—with so much else—to Sir Leonard Woolley's brilliant and painstaking work at Ur show on the northern and western edges of the mound two harbours, opening out upon the white unknown outside the plan and thus tacitly posing the question of how much we can ever know about the great canal-system which once linked Ur and the other cities of Sumer in a net of intercommunication and—even more important—distributed to them the irrigation waters without which no city or other permanent human settlement could have existed.
The question is certainly legitimate, but not altogether an easy one; for it is well known that the major rivers of Iraq have substantially changed their courses since antiquity and also that in large sections of the country heavy deposits of new soil now securely hide all ancient river- and canal-beds deep under the present surface out of reach of investigation. Special methods must therefore be brought to bear if the ancient system is eventually to be reconstructed and we should like to call attention here to one such method, that of Ceramic Surface Survey, which has already been applied with good results and moderately extensively.