Nan Shan

The journey from Tun-Huang to Lan-Chow took the First Doctor, Susan, Barbara, and Ian (whilst travelling with Marco Polo's caravan) through the foothills of the Nan Shan mountain range. As they travelled east from Su-Chow, the snow-capped mountains could be seen on the horizon to the south. (PROSE: Marco Polo)

Behind the scenes
Nan Shan translates literally to "southern mountains", and unsurprisingly there are many ranges in China known by this name. The ones in this instance are better known as the, as can be deduced from their location described in the story.