A
hogback is a homoclinal ridge, formed from a
monoclineA monocline is a step-like fold consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence.-Formation:Monoclines may be formed in several different ways...
, composed of steeply tilted
strataIn geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. They may extend over...
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 rockSedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
with differing weathering rates. The softer rock
erodesErosion is a gravity driven process that moves solids in the natural environment or their source and deposits them elsewhere...
more quickly than overlying hard rock.
A
hogback is a homoclinal ridge, formed from a
monoclineA monocline is a step-like fold consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence.-Formation:Monoclines may be formed in several different ways...
, composed of steeply tilted
strataIn geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. They may extend over...
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 rockSedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
with differing weathering rates. The softer rock
erodesErosion is a gravity driven process that moves solids in the natural environment or their source and deposits them elsewhere...
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
cliffIn geography and geology, a cliff is a significant vertical, or near vertical, rock exposure. Cliffs are formed as erosion landforms due to the processes of erosion and weathering that produce them. Cliffs are common on coasts, in mountainous areas, escarpments and along rivers. Cliffs are...
s 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." The defining characteristics of a hogback a steep
dip slopeA dip slope is a geological formation often created by erosion of tilted strata. Dip slopes are found on homoclinal ridges with one side that is steep and irregular and another side, the dip slope, that is generally planar with a dip parallel to the bedding...
that is greater than 30° - 40° with a near symmetric slope on each ridge face. A
cuestaIn structural geology and geomorphology, a cuesta is a ridge formed by gently tilted sedimentary rock strata in a homoclinal structure. Cuestas have a steep slope, where the rock layers are exposed on their edges, called an escarpment or, if more steep, a cliff...
is a homoclinal ridge with a more gentle dip slope.
Black Hills hogback
While most hogbacks snake along a surface in a relatively straight line, few, such as those in
Sundance, WyomingSundance is a town in Crook County, Wyoming, United States. The population was 1,161 at the 2000 census. The town is named for the Sun Dance ceremony practiced by several North American Indian Nations.-Sundance in popular culture:...
, encircle a
domeIn structural geology, a dome is a deformational feature consisting of symmetrically-dipping anticlines; their general outline on a geologic map is circular or oval...
. The
Dakota Sandstone HogbackThe Dakota Hogback is a long hogback ridge at the eastern fringe of the Rocky Mountains that extends north-south from southern Wyoming through Colorado and into northern New Mexico in the United States. The ridge is prominently visible as the first line of foothills along the edge of the Great Plains...
encircles the
Black HillsThe Black Hills are a small, isolated mountain range rising from the Great Plains of North America in western South Dakota and extending into Wyoming, USA. Set off from the main body of the Rocky Mountains, the region is something of a geological anomaly—accurately described as an "island of trees...
, an elliptical dome spanning from northwestern
South DakotaSouth Dakota is a state located in the Midwestern region of the United States of America. It is named after the Lakota and Dakota Sioux American Indian tribes. South Dakota was carved out of the southern half of the Dakota Territory and admitted to the Union on November 2, 1889...
to northeastern
WyomingWyoming is a state in the Western United States. The majority of the state is dominated by the mountain ranges and rangelands of the Rocky Mountain West, while the easternmost section of the state includes part of a high elevation prairie region known as the High Plains. While the tenth largest...
. The Black Hills are approximately 125 miles long and 65 miles wide. The Dakota Hogback
ridge300px|thumb|right|Stratigraphic ridge found in Northeastern [[Tennessee]]A ridge is a geological feature that features a continuous elevational crest for some distance. Ridges are usually termed hills or mountains as well, depending on size...
formed when the resistant layers of the Dakota
SandstoneSandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow,...
and underlying layers were
thrust upwardTectonic uplift is a geological process most often caused by plate tectonics which increases elevation. The opposite of uplift is subsidence, which results in a decrease in elevation. Uplift may be orogenic or isostatic.-Orogenic uplift:...
near the center of the present-day Black Hills due to a
graniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their...
intrusionAn intrusion is any formation of intrusive igneous rock; rock formed from magma that cools and solidifies within the crust of a planet. In contrast, an extrusion consists of extrusive rock; rock formed above the surface of the crust....
, 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
plainIn geography, a plain is a land with relatively high relief, as well as flat. Prairies and steppes are types of plains, and the archetype for a plain is often thought of as a grassland, but plains in their natural state may also be covered in shrublands, woodland and forest, or vegetation may be...
s 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
eastEast is a direction in geography. It is one of the four cardinal directions or compass points, opposite of west and at right angles to north and south. East is the direction toward which the Earth rotates about its axis, and therefore the general direction from which the Sun appears to rise...
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
flatironsThe Flatirons are rock formations near Boulder, Colorado. There are five large, numbered Flatirons ranging from north to south along the east slope of Green Mountain, and the term "The Flatirons" sometimes refers to these five alone...
). Green Mountain itself, much like the Black Hills, is a
laccolithA laccolith is an igneous intrusion that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.Laccoliths tend to form at...
formed by the intrusion of
magmaMagma [from Greek μάγμα, paste] is molten rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma often collects in a magma chamber inside a volcano...
into the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's
crustIn geology, a crust is the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in...
(Cleland, 354-355). The hogbacks surrounding the mountain are steep (with
dip slopeA dip slope is a geological formation often created by erosion of tilted strata. Dip slopes are found on homoclinal ridges with one side that is steep and irregular and another side, the dip slope, that is generally planar with a dip parallel to the bedding...
s of approximately 50°) and point upward towards the center of the mountain.