Hogback (geology)

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Hogback west of Denver, Colorado. It is formed by the more erosionally resistant Lytle and South Platte Formations protecting the softer, slope-forming Morrison Formation.
Oblique air photo of a hogback located between Gallup and Ramah in western New Mexico.

A hogback is a homoclinal ridge, formed from a monocline, composed of steeply tilted strata of rock protruding from the surrounding area. The name comes from the ridge resembling the high, knobby spine between the shoulders of a hog. In most cases, the two strata that compose a hogback are different types of sedimentary rock with differing weathering rates. The softer rock erodes more quickly than overlying hard rock. Over time, the softer rock retreats to a point where the hard and soft rock strata are adjacent. This creates cliffs that steepen as the softer rock continues to erode. Hogbacks are often found as ridges along the "eroded flanks of large, tightly folded anticlines and synclines."[1] The defining characteristic of a hogback is a steep dip slope that is greater than 30° - 40° with a near symmetric slope on each ridge face. A cuesta is a homoclinal ridge with a more gentle dip slope.[2][3]

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Dome-encircled hogbacks

Black Hills hogback

While most hogbacks snake along a surface in a relatively straight line, few, such as those in Sundance, Wyoming, encircle a dome. The Dakota Sandstone Hogback encircles the Black Hills, an elliptical dome spanning from northwestern South Dakota to northeastern Wyoming. The Black Hills are approximately 125 miles long and 65 miles wide. The Dakota Hogback ridge formed when the resistant layers of the Dakota Sandstone and underlying layers were thrust upward near the center of the present-day Black Hills due to a granite intrusion, approximately 60 million years ago. The Black Hills are "the farthest outlying segment of a great mid-continental uplift known as the Laramide orogeny" (Raventon, 33). The Dakota hogback rim separates the surrounding flat plains from the two mile wide Red Valley trench of the Black Hills. The ridge "presents a steep face towards the valley and rises several hundred feet above it" (Cleland, 355).

Green Mountain hogbacks

Green Mountain (Google Maps), also known as the Little Sundance Dome, is found just east of Sundance, Wyoming. It is a circular dome about 1800m across and 1400m wide surrounded by a rim of triangluar hogbacks (similar in appearance to flatirons). Green Mountain itself, much like the Black Hills, is a laccolith formed by the intrusion of magma into the Earth's crust (Cleland, 354-355). The hogbacks surrounding the mountain are steep (with dip slopes of approximately 50°) and point upward towards the center of the mountain.

References

  1. ^ "Hogback". The Columbia Encyclopedia, Sixth Edition. http://www.encyclopedia.com/doc/1E1-hogback.html. Retrieved 2008-11-14. "Hogbacks are commonly formed along the eroded flanks of large, tightly folded anticlines and synclines." 
  2. ^ Divener, V.. "Structural Control of Fluvial Landscapes". Crustal Structures and Landforms (course notes). Long Island University C.W. Post Campus. http://myweb.cwpost.liu.edu/vdivener/notes/structure_landforms.htm. Retrieved 2008-03-16. "Homoclinal ridges are called cuestas where bedding dip is gentle [and] hogbacks where steep (>30-40 degree)." 
  3. ^ "Cuesta, or homoclinal ridge (geology)". Britannica Online Encyclopedia. http://www.britannica.com/eb/topic-145944/cuesta. Retrieved 2008-03-16. "Cuestas with dip slopes of 40°–45° are usually called hogback ridges."