Westrift is a fjord which separates the rest of the Borean Tundra from the mountainous region of Coldarra.
Just above the Borean Tundra region’s southwest tip is Westrift, a tremendous tear in the ice that stretches for miles. The only place to cross is a lone bridge near the center. (LoM 93)
The bridge can be seen destroyed in Wrath of the Lich King, and the only way to reach Coldarra, without a flying mount, is through flight from Amber Ledge to the Transitus Shield.
It is called "the Westrift Valley" on the official WotLK site.
The collapsed tunnel into Coldarra