In the period 55004000 B.C., much of Mesopotamia shared a common culture, called Ubaid after the site where evidence for it was first found. Characterized by a distinctive type of pottery, this culture originated on the flat alluvial plains of southern Mesopotamia (ancient Iraq) around 6200 B.C. Indeed, it was during this period that the first identifiable villages developed in the region, where people farmed the land using irrigation and fished the rivers and sea (Persian Gulf). Thick layers of alluvial silt deposited every spring by the flooding rivers cover many of these sites. Some villages began to develop into towns and became focused on monumental buildings, such as at Eridu and Uruk. The Ubaid culture spread north across Mesopotamia, gradually replacing the Halaf culture. Ubaid pottery is also found to the south, along the west coast of the Persian Gulf, perhaps transported there by fishing expeditions. Baked clay figurines, mainly female, decorated with painted or appliqué ornament and lizardlike heads, have been found at a number of Ubaid sites. Simple clay tokens may have been used for the symbolic representation of commodities, and pendants and stamp seals may have had a similar symbolism, if not function. During this period, the repertory of seal designs expands to include snakes, birds, and animals with humans. There is much continuity between the Ubaid culture and the succeeding Uruk period, when many of the earlier traditions were elaborated, particularly in architecture.