Twin Sisters Mountain

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Twin Sisters Mountain
Twin Sisters Mountain 3433.JPG
As seen from southern flank of Mount Baker
Highest point
Elevation7,000+ feet (2,130+ m)[1]
Prominence3,520 ft (1,070 m)[1]
Coordinates48°42′16″N 121°59′15″W / 48.7045599°N 121.9873731°W / 48.7045599; -121.9873731Coordinates: 48°42′16″N 121°59′15″W / 48.7045599°N 121.9873731°W / 48.7045599; -121.9873731[2]
LocationWhatcom County, Washington, U.S.
Parent rangeCascade Range
Topo mapUSGS Twin Sisters Mountain
Mountain typeIgneous dunite
Twin Sisters range

Twin Sisters Mountain (Nooksack: Kwetl’kwítl’ Smánit, "red mountain"),[3] 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, with a summit elevation above 7,004 feet (2,135 m).[a][1] The summit of North Twin is above 6,644 feet (2,025 m).[b][4] 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.[5]


Twin Sisters Mountain is composed of relatively unaltered dunite,[6] an ultramafic rock that is mostly olivine. The Twin Sisters Range is a massive slab of relatively dense rock that was uplifted from major thrust fault activity in the form of a nappe. The magnesium and iron rich rock was forced upwards from the tectonic collision between the Bell Pass Mélange and the Chilliwack River terrane during the Mesozoic era. The reddish color of the mountain range may be due to the oxidation of iron and magnesium in the dunite that is exposed to chemical weathering. Petrology of the Twin Sisters dunite consists of Enstatite rich rocks that contain, along with olivine, significant amounts of chromite, clinopyroxene, and serpentinite. These minerals are commonly associated with ultramafic rocks that crystallize below the Earth's crust within the mantle (geology). The serpentinized minerals are thought to have formed during later stages of the uplift where water could penetrate through fault fractures and initiate metamorphism.[7][8]


Twin Sisters west aspect

Twin Sisters are located in the marine west coast climate zone of western North America.[9] Most weather fronts originate in the Pacific Ocean, and travel northeast toward the Cascade Mountains. As fronts approach the North Cascades, they are forced upward by the peaks of the Cascade Range (Orographic lift), causing them to drop their moisture in the form of rain or snowfall onto the Cascades. As a result, the west side of the North Cascades experiences high precipitation, especially during the winter months in the form of snowfall. During winter months, weather is usually cloudy, but, due to high pressure systems over the Pacific Ocean that intensify during summer months, there is often little or no cloud cover during the summer[9] Because of maritime influence, snow tends to be wet and heavy, resulting in high avalanche danger.[9]


  1. ^ lists the NAVD 88 summit elevation of South Twin between 7004 ft. and 7044 ft
  2. ^ lists the NAVD 88 summit elevation of North Twin between 6644 ft. and 6684 ft.


  1. ^ a b c "South Twin, Washington".
  2. ^ "South Twin". Geographic Names Information System. United States Geological Survey.
  3. ^ "Cultural Resources Department". NookSack Indian Tribe. Retrieved 2013-09-16.
  4. ^ "North Twin, Washington".
  5. ^ Washington Road & Recreation Atlas. Benchmark Maps. 2000.
  6. ^ "Generalized Geological Map of the Mount Baker 1:100,000 Quadrangle". North Cascades geological map. USGS. Retrieved 2007-07-16.
  7. ^ Ragan, Donal M. (1963). "Emplacement of the Twin Sisters Dunite" (PDF). American Journal of Science. 261 (6): 549–565. doi:10.2475/ajs.261.6.549. Retrieved 2013-10-09.
  8. ^ "Rocks of the Western Domain". USGS Geology in the Parks. USGS. Archived from the original on 2007-07-16. Retrieved 2007-07-16.
  9. ^ a b c Beckey, Fred W. Cascade Alpine Guide, Climbing and High Routes. Seattle, WA: Mountaineers Books, 2008.

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