Purcell and Rocky Mountain Trenches within the US

The Purcell Trench, also known as the Kootenay River Valley is a large valley on the western side of the northern part of North America's Rocky Mountains. The trench extends approximately from

Lake Pend Oreille Lake Pend Oreille ( ) in the northern Idaho Panhandle is the largest lake in the U.S. state of Idaho and the 38th-largest lake by area in the United States, with a surface area of . It is long, and deep in some regions, making it the fifth-deep ...

Idaho Idaho ( ) is a U.S. state, state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Monta ...

, down the

Kootenay River The Kootenay or Kootenai river is a major river in the Northwest Plateau, in southeastern British Columbia, Canada, and northern Montana and Idaho in the United States. It is one of the uppermost major tributary, tributaries of the Columbia Ri ...

(north) to

Kootenay Lake Kootenay Lake is a lake located in British Columbia, Canada. It is part of the Kootenay River. The lake has been raised by the Corra Linn Dam and has a dike system at the southern end, which, along with industry in the 1950s–70s, has changed ...

, up the north arm to Duncan Lake. It joins the

Rocky Mountain Trench The Rocky Mountain Trench, also known as the Valley of a Thousand Peaks or simply the Trench, is a large valley on the western side of the northern part of North America's Rocky Mountains. The Trench is both visually and cartographically a ...

another northward at the south tip of

Kinbasket Lake Kinbasket Lake (or Kinbasket Reservoir) is a reservoir on the Columbia River in southeast British Columbia, north of the city of Revelstoke and the town of Golden. The reservoir was created by the construction of the Mica Dam. The lake include ...

, in

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, for ...

.Doughty, P.T. & R.A. Price; Department of Geological Sciences, Queen’s University, Kingston, Ontario K7L 3N6, Canada; Geology of the Purcell Trench rift valley and Sandpoint Conglomerate: Eocene en echelon normal faulting and synrift sedimentation along the eastern flank of the Priest River metamorphic complex, northern Idaho; GSA Bulletin; September 2000; v. 112; no. 9; p. 1356–1374; 19 figures; 3 tables The trench bottom is wide and is above sea level. The trench is nearly a straight north or south line.W. C. ALDEN; Physiography and Glacial Geology of Western Montana and Adjacent Areas, Geological Survey Professional Paper 231; United States Government Printing Office, Washington, D.C.; 1953 Some of its

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sc ...

has been carved into U-shaped glacial valleys, it is primarily a product of geologic faulting. The trench splits the

Columbia Mountains The Columbia Mountains are a group of mountain ranges along the upper Columbia River in British Columbia, Montana, Idaho and Washington. The mountain range covers 135,952 km² (52,491 sq mi). The range is bounded by the Rocky Mountain T ...

between the

Purcell Mountains The Purcell Mountains are a mountain range in southeastern British Columbia, Canada. They are a subrange of the Columbia Mountains, which includes the Selkirk, Monashee, and Cariboo Mountains. They are located on the west side of the Rocky Moun ...

on the east and the

Selkirk Mountains The Selkirk Mountains are a mountain range spanning the northern portion of the Idaho Panhandle, eastern Washington, and southeastern British Columbia which are part of a larger grouping of mountains, the Columbia Mountains. They begin at Mica ...

on the west.Menounos, Brian & Osborn, Gerald & Clague, John & Luckman, Brian. (2009). Latest Pleistocene and Holocene glacier fluctuations in western Canada. Quaternary Science Reviews. 1877. 71-1. 10.1016/j.quascirev.2008.10.018.

Geology

The Priest River complex in northern

, northeastern

Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered o ...

, and southeastern

is a series of early and middle

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...

metamorphic core complexes that extend southward from the south-central Canadian Cordillera to the

Columbia Plateau The Columbia Plateau is a geologic and geographic region that lies across parts of the U.S. states of Washington, Oregon, and Idaho. It is a wide flood basalt plateau between the Cascade Range and the Rocky Mountains, cut through by the Col ...

and southeastward to the

Idaho Batholith The Idaho Batholith is a granitic and granodioritic batholith of Cretaceous- Paleogene age that covers approximately of central Idaho and adjacent Montana. The batholith has two lobes that are separate from each other geographically and geo ...

. The east side of the Priest River complex on forms the west wall of the Purcell Trench.

Purcell Trench Faults

The Purcell Trench is divided into three segments. The southern segment, which extends from

Lake Coeur d'Alene Lake Coeur d'Alene, officially Coeur d'Alene Lake ( ), is a natural dam-controlled lake in North Idaho, located in the Pacific Northwest region of the United States. At its northern end is the city of Coeur d'Alene. It spans in length and rang ...

through Sandpoint to north of Colburn, contains compelling evidence of the east-dipping Purcell Trench normal fault. The middle segment, which extends from Colburn and

Naples Naples (; it, Napoli ; nap, Napule ), from grc, Νεάπολις, Neápolis, lit=new city. is the regional capital of Campania and the third-largest city of Italy, after Rome and Milan, with a population of 909,048 within the city's adminis ...

, is unfaulted. The northern segment from Naples to

also contains evidence of a major eastdipping normal fault.

Glaciation

The width and depth of the inner valley of the

Kootenai River The Kootenay or Kootenai river is a major river in the Northwest Plateau, in southeastern British Columbia, Canada, and northern Montana and Idaho in the United States. It is one of the uppermost major tributaries of the Columbia River, the l ...

Boundary County, Idaho Boundary County is the northernmost county of the U.S. state of Idaho. As of the 2020 census, the population was 12,056. The county seat and largest city is Bonners Ferry. Boundary County was created by the Idaho Legislature on January 23, 1 ...

, when the first glaciers arrived is unknown. The valley floor higher than the present outlet from the west arm of

near

Nelson, British Columbia Nelson is a city located in the Selkirk Mountains on the West Arm of Kootenay Lake in the British Columbia Interior, Southern Interior of British Columbia, Canada. Known as "The Queen City", and acknowledged for its impressive collection of resto ...

, which is about above sea level (asl); it could have been as much as asl. There were at least three glacial invasions with their associated receding ice fronts. As of 1953 it was not possible to differentiate the effects of the earlier

Illinoian The Illinoian Stage is the name used by Quaternary geologists in North America to designate the period c.191,000 to c.130,000 years ago, during the middle Pleistocene, when sediments comprising the Illinoian Glacial Lobe were deposited. It precedes ...

and earlier glacial periods below

Bonners Ferry Bonners Ferry (Kutenai language: ʔaq̓anqmi) is the largest city and the county seat of Boundary County, Idaho, United States. The population was 2,543 at the 2010 census. History When gold was discovered in the East Kootenays of British ...

from those of the

Wisconsin glaciation The Wisconsin Glacial Episode, also called the Wisconsin glaciation, was the most recent glacial period of the North American ice sheet complex. This advance included the Cordilleran Ice Sheet, which nucleated in the northern North American Cor ...

period. It appears doubtful that the pre-Wisconsin ice sheets of the Cordilleran ice sheet reach to the valley of the

Pend Oreille River The Pend Oreille River ( ) is a tributary of the Columbia River, approximately long, in northern Idaho and northeastern Washington in the United States, as well as southeastern British Columbia in Canada. In its passage through British Columbia ...

References

{{reflist Geography of British Columbia Geography of Idaho

Geology

Purcell Trench Faults

Glaciation

See also

References