By the time of the Uruk period (c. 4100–2900 BC calibrated), the volume of trade goods transported along the canals and rivers of southern Mesopotamia facilitated the rise of many large, stratified, temple-centered cities (with populations of over 10,000 people) where centralized administrations employed specialized workers. It is fairly certain that it was during the Uruk period that Sumerian cities began to make use of slave labor captured from the hill country, and there is ample evidence for captured slaves as workers in the earliest texts. Artifacts, and even colonies of this Uruk civilization have been found over a wide area—from the Taurus Mountains in Turkey, to the Mediterranean Sea in the west, and as far east as central Iran.