The Rocky Mountain Trench runs northwest-southeast through the western Rockies in British Columbia and connects to the Flathead and Bitterroot Valleys of Montana. Its fault zone bears some similarity to a rift valley, whereas the course of the Rio Grande in New Mexico actually follows a rift, the Rio Grande Rift. A rift valley is a trough between parallel faults which often forms a long, steep-sided valley, or sequence of valleys, with occasional side-faults. The causes are arguable, but they likely result from (1) a stretching tension in the Earth’s surface, which pulls the sides apart, leaving the center to subside; (2) compression from the sides, which thrusts outside masses higher than a central block, forcing it down; or (3) the rising of an elongated domed stratum, which eventually cracks down its length, allowing the central block to subside and form the valley ﬂoor. The most spectacular example of a rift valley is the one that runs from the ﬂoor of the Dead Sea through the Red Sea and Horn of Africa to the Zambezi River. In the United States, the run of the north-south–connected valleys North Warner and South Warner in southeastern Oregon and Surprise Valley in northeastern California, all lying on the western edge of the Great Basin, can be thought of as forming a minor rift valley.