Gates of the Mountains Wilderness in Montana is a protected area within Helena National Forest, named for the Missouri River passage Meriwether Lewis called “the gates of the rocky mountains” in 1805. After unsuccessful consideration for national park status in 1922, 1935, and the 1960s, Congress designated it wilderness in 1964 under the Wilderness Act. Its defining features include the Missouri canyon’s pale gray Madison Limestone, the Eldorado thrust fault, and a river course that cut through rising land rather than turning away. Holter Dam raised water levels there in 1918, and the area is also linked to the tragic 1949 Mann Gulch fire.