Twin Sisters Mountain
Encyclopedia
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 6940 ft (2,115 m) (its elevation is given by Peakbagger.com as "between 7000 and 7040 feet (40-foot closed contour)"). North Twin is 6660 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.
. 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.
, 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.
U.S. state
A U.S. state is any one of the 50 federated states of the United States of America that share sovereignty with the federal government. Because of this shared sovereignty, an American is a citizen both of the federal entity and of his or her state of domicile. Four states use the official title of...
of Washington. Part of the Cascade Range
Cascade Range
The Cascade Range is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, and the notable volcanoes known as the High Cascades...
, it lies just southwest of Mount Baker
Mount Baker
Mount Baker , also known as Koma Kulshan or simply Kulshan, is an active glaciated andesitic stratovolcano in the Cascade Volcanic Arc and the North Cascades of Washington State in the United States. It is the second-most active volcano in the range after Mount Saint Helens...
. Of its two main peaks, South Twin is higher, at 6940 ft (2,115 m) (its elevation is given by Peakbagger.com as "between 7000 and 7040 feet (40-foot closed contour)"). North Twin is 6660 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.
Summits
Both of the summits are located within Mount Baker WildernessMount Baker Wilderness
Mount Baker Wilderness in northern Washington, USA,contains . Its eastern border is shared with the boundary of the North Cascades National Park for a distance of 40 miles . The Wilderness extends from State Route 20 north to the Canadian border...
. Twin Sisters Mountain as a whole is located southwest of Mount Baker, separated from the volcano by the Middle Fork of the Nooksack River
Nooksack River
The Nooksack River is a river in the northwest part of the U.S. state of Washington. It drains an area of the Cascade Range around Mount Baker, near the Canadian border. The lower river flows through a fertile agricultural area before emptying into Bellingham Bay and, via the Strait of Juan de Fuca...
. 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.
Geology
Twin Sisters Mountain is composed of duniteDunite
Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...
, an ultramafic rock that is mostly olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
. 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.