The Ubaid period (c. 6500–3700 BC) is a prehistoric period of Mesopotamia. The name derives from Tell al-'Ubaid where the earliest large excavation of Ubaid period material was conducted initially in 1919 by Henry Hall and later by Leonard Woolley. In South Mesopotamia the period is the earliest known period on the alluvial plain although it is likely earlier periods exist obscured under the alluvium. In the south it has a very long duration between about 6500 and 3800 BC when it is replaced by the Uruk period.