Present-day Chongming first developed as two separate shoals—Xisha and Dongsha—during the Wude Era (AD 618–626) of the Tang dynasty. By the first year of Shenlong (705), a town was established on Xisha which was known as "Chongming". The number, size, and shapes of the islands fluctuated repeatedly over time and with the floods of the Yangtze River. In the 15th year of Jiading (1222) under the Southern Song, the islands were part of Tongzhou Prefecture. These early settlers were mostly fishermen and salt collectors, with its principal saltern at Tianci. In the 14th year of Zhiyuan (1277) under Kublai Khan of the Yuan, was organized within Yangzhou. Owing to the shifting sands of the islands, however, the district seat needed to be repeatedly moved.