The southwestern route is believed to be the Ganges/Brahmaputra Delta, which has been the subject of international interest for over two millennia. Strabo, the 1st-century Roman writer, mentions the deltaic lands: Regarding merchants who now sail from Egypt...as far as the Ganges, they are only private citizens... His comments are interesting as Roman beads and other materials are being found at Wari-Bateshwar ruins, the ancient city with roots from much earlier, before the Bronze Age, presently being slowly excavated beside the Old Brahmaputra in Bangladesh. Ptolemy's map of the Ganges Delta, a remarkably accurate effort, showed that his informants knew all about the course of the Brahmaputra River, crossing through the Himalayas then bending westward to its source in Tibet. It is doubtless that this delta was a major international trading center, almost certainly from much earlier than the Common Era. Gemstones and other merchandise from Thailand and Java were traded in the delta and through it. Chinese archaeological writer Bin Yang and some earlier writers and archaeologists, such as Janice Stargardt, strongly suggest this route of international trade as Sichuan-Yunnan-Burma-Bangladesh route. According to Bin Yang, especially from the 12th century the route was used to ship bullion from Yunnan (gold and silver are among the minerals in which Yunnan is rich), through northern Burma, into modern Bangladesh, making use of the ancient route, known as the 'Ledo' route. The emerging evidence of the ancient cities of Bangladesh, in particular Wari-Bateshwar ruins, Mahasthangarh, Bhitagarh, Bikrampur, Egarasindhur, and Sonargaon, are believed to be the international trade centers in this route.