The Salton Trough is an active tectonic pull-apart basin, or graben. It lies within the Imperial, Riverside, and San Diego counties of southeastern California, United States and extends south of the Mexico-United States border into the state of Baja California, Mexico. The northwestern end of the trough starts at the San Gorgonio Pass in Riverside County, and extends southeast to the Gulf of California. Major geographical features located in the trough include the Coachella Valley, the Salton Sea, and the Imperial Valley, in the United States, and the western side of the Mexicali Valley, and the Colorado River Delta in Mexico.