"Possibly in this period, the delta sedimentation—which now extends the coastline by three miles a century—remained insufficient, and the sea still reached too far inland, to support a population even as large as the modest population of the late precolonial era. Whatever the condition of the coast, all scholars accept that during the 11th century, Pagan established its authority in Lower Burma and this conquest facilitated growing cultural exchange, if not with local Mons, then with India and with Theravada stronghold Sri Lanka. From a geopolitical standpoint, Anawrahta's conquest of Thaton checked the Khmer advance in the Tenasserim coast"
From the Wikipedia page on the Kingdom of Thaton and Anawrahta the founder of Imperial Pagan.
Drawing of the projected coast on a map.
