For millennia, Sichuan's rugged and riverine landscape presented enormous challenges to the development of transportation infrastructure, and the lack of roads out of the Sichuan Basin contributed to the region's isolation. Since the 1950s, numerous highways and railways have been built through the Qinling in the north and the Bashan in the east. Dozens of bridges across the Yangtze and its tributaries to the south and west have brought greater connectivity with Yunnan and Tibet.