Channeled Scablands

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The Channeled Scablands are a barren, relatively soil-free landscape in eastern Washington, scoured clean by a flood unleashed when a large glacial lake drained.[1] They are a geologically unique erosional feature in the U.S. state of Washington. They were created by the cataclysmic Missoula Floods that swept periodically across eastern Washington and down the Columbia River Plateau during the Pleistocene epoch. Geologist J Harlen Bretz coined the term in a series of papers in the 1920s. Debate over the origin of the Scablands raged for four decades and is one of the great debates in the history of earth science. The Scablands are also important to planetary scientists as perhaps the best terrestrial analog for the Martian outflow channels.[2]

History[edit]

Bretz conducted research and published many papers during the 1920s describing the Channeled Scablands. His theories of how they were formed required short but immense floods (500 cubic miles of water), for which Bretz had no explanation. Bretz's theories met with vehement opposition from geologists of the day, who tried to explain the features with uniformitarian theories.

J.T. Pardee first suggested in 1925 to Bretz that the draining of a glacial lake could account for flows of the magnitude needed. Pardee continued his research over the next 30 years, collecting and analyzing evidence that eventually identified Lake Missoula as the source of the Missoula Floods and creator of the Channeled Scablands.

Pardee's and Bretz's theories were accepted only after decades of painstaking work and fierce scientific debate. Research on open channel hydraulics in the 1970s put Bretz's theories on solid scientific ground. In 1979 Bretz received the highest medal of the Geological Society of America, the Penrose Medal, to recognize that he had developed one of the great ideas in the earth sciences.

Geology[edit]

The term scabland refers to an area that has experienced fluvial erosion resulting in the loss of loess and other soils, leaving the land barren.[3] River valleys formed by erosional downcutting of rivers create V-shaped valleys, while glaciers carve out U-shaped valleys. The Channeled Scablands have a rectangular cross section, with flat plateaus and steep canyon sides, and are spread over immense areas of eastern Washington. The morphology of the scablands is butte-and-basin.[4] The area that encompasses the Scablands has been estimated between 1,500 and 2,000 square miles (3,900 and 5,200 km2), though those estimates still may be too conservative.[5]

They exhibit a unique drainage pattern that appears to have an entrance in the northeast and an exit in the southwest. The Cordilleran Ice Sheet dammed up Glacial Lake Missoula at the Purcell Trench Lobe.[6] A series of floods occurring over the period of 18,000 and 13,000 years ago swept over the landscape when the ice dam broke. The eroded channels also show an anastomosing, or braided, appearance.

There are also immense potholes and ripple marks, much larger than those found on ordinary rivers. When first studied, no known theories could explain the origin of these features. The giant current ripples are between 3.3 and 49.2 feet (1 and 15 m) high and are regularly spaced, relatively uniform hills.[7] Vast volumes of flowing water would be required to produce ripple marks of this magnitude, as they are larger-scale versions of the ripple marks found on streambeds that are typically only centimeters high. Large potholes were formed by swirling vortexes of water called kolks scouring and plucking out the bedrock.[8]

The Scablands are littered with large boulders called glacial erratics that rafted on glaciers and were deposited by the glacial outburst flooding. The lithology of erratics usually does not match the rock type that surrounds it, as they are often carried very far from their origin.[9]

See also[edit]

Notes[edit]

  1. ^ Marshak, Stephen, 2009, Essentials of Geology, W. W. Norton & Company, 3rd ed. ISBN 978-0393196566
  2. ^ Carr, M. H. (2006), The Surface of Mars. Cambridge Planetary Science Series, Cambridge University Press.
  3. ^ Baker (2008)
  4. ^ Baker (2008)
  5. ^ Foster (2011)
  6. ^ Foster (2011)
  7. ^ Baker (2008)
  8. ^ Foster (2011)
  9. ^ Foster (2011)

References[edit]

  • Allen, John Eliot; Marjorie Burns; and Sam C. Sargent. (1986) Cataclysms on the Columbia. Portland, Oregon: Timber Press, ISBN 0-88192-215-3
  • Baker, Victor R.. "The Channeled Scabland: A Retrospective." Reviews in Advance. Annual Reviews, 30 Dec 2008. Web. 9 Oct 2013. <http://www2.ess.ucla.edu/~jewitt/Baker08.pdf>.
  • Bretz, J Harlen (1923), "The Channeled Scabland of the Columbia Plateau." Journal of Geology, v.31, p.617-649.
  • Bretz, J Harlen (1925), "The Spokane flood beyond the Channeled Scablands." Journal of Geology, v.33, p.97-115, 236-259.
  • Bretz, J H.; Smith, H.T.U.; and Neff, G.E., (1956) "Channeled Scabland of Washington — New Data and Interpretations." Geological Society of America Bulletin, v.67, p.957-1049.
  • "Channeled Scablands Theory." Spokane Outdoors. N.p., n.d. Web. 9 Oct 2013. <http://www.spokaneoutdoors.com/scabland.htm>
  • Foster, Tom. "Channeled Scabland." Huge Floods. N.p., n.d. Web. 9 Oct 2013. <http://www.hugefloods.com>
  • Soennichsen, John (2008), "Bretz's Flood: The Remarkable Story of a Rebel Geologist and the World's Greatest Flood", Seattle, Washington, Sasquatch Books, ISBN 978-1-57061-505-4

External links[edit]

Coordinates: 47°33′44″N 119°32′22″W / 47.5622°N 119.5395°W / 47.5622; -119.5395