Elevation : 7,020 ft (2,140 m)
Twin Sisters Mountain, commonly called the Twin Sisters, is a mountain in the U.S. state of Washington. Part of the Cascade Range, it lies just southwest of Mount Baker. Of its two main peaks, South Twin is higher, at 6,940 ft (2,115 m) (its elevation is given by Peakbagger.com as "between 7000 and 7040 feet (40-foot closed contour)"). North Twin is 6,660 ft (2,030 m) high (its elevation is given by Peakbagger.com as "between 6640 and 6680 feet (40-foot closed contour)"). There are several glaciers on the northeast slopes of Twin Sisters Mountain.
Both of the summits are located within Mount Baker Wilderness. Twin Sisters Mountain as a whole is located southwest of Mount Baker, separated from the volcano by the Middle Fork of the Nooksack River. The South Fork Nooksack River flows around the eastern end of Twin Sisters Mountain. Other streams that drain Twin Sisters Mountain include Skookum Creek, Sister Creek, Green Creek, and Howard Creek. All the streams eventually end up in the Nooksack River.
Twin Sisters Mountain is composed of dunite, an ultramafic rock that is mostly olivine. This dunite is part of the so-called Bell Pass Mélange: the rock originated deep in the mantle, and then was smashed up against the North American plate in a complex process when island arcs and ocean floor were accreted because of a subduction zone.