The one durable Tibetan state to emerge in Amdo after the empire's collapse: a Buddhist theocratic kingdom in the Huang River valley east of Kokonor, built around a boy found in Gaochang and enthroned as btsan-po because he was said to descend from the Yarlung royal house. It brokered the Silk Road horse and Uighur caravan trade between Song China and the Tanguts.
Conquered by Song armies under Wang Hou in 1103-04; Qingtang was renamed Xining and the region was held by the Song from 1104 to 1119, which dispersed its commercial network.
Dunnell's 'The Hsi Hsia' contains the fullest English narrative of the rise and fall of the Tsongko/Gusiluo dynasty and its place in the Song-Tangut-Uighur trade triangle.
Contains Luciano Petech's 'Tibetan relations with Sung China and with the Mongols', the study Dunnell cites for the Tibetan-source side of Tsongkha.