Great Zab
By the end of the 13th century, although isolated East Syriac outposts persisted to the southeast of the Great Zab, the districts of northern Mesopotamia included in the metropolitan provinces of Mosul and Nisibis were clearly regarded as the heartland of the Church of the East. When the monks Bar Sawma and Marqos (the future patriarch Yahballaha III) arrived in Mesopotamia from China in the late 1270s, they visited several East Syriac monasteries and churches: