A hogback is a homoclinal ridge, formed from a monocline

Monocline

A monocline is a step-like fold in rock strata 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 strata

Stratum

In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...

 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

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition 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 erodes

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials 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 cliff

Cliff

In 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 usually...

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 characteristic of a hogback is a steep dip slope

Dip slope

A 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 cuesta

Cuesta

In 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, Wyoming

Sundance, Wyoming

Sundance 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 dome

Dome (geology)

In 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 Hogback

Dakota Hogback

The 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 Hills

Black Hills

The 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...

, an elliptical dome spanning from northwestern South Dakota

South Dakota

South Dakota is a state located in the Midwestern region of the United States. It is named after the Lakota and Dakota Sioux American Indian tribes. Once a part of Dakota Territory, South Dakota became a state on November 2, 1889. The state has an area of and an estimated population of just over...

 to northeastern Wyoming

Wyoming

Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...

. The Black Hills are approximately 125 miles long and 65 miles wide. The Dakota Hogback ridge

Ridge

A ridge is a geological feature consisting of a chain of mountains or hills that form a continuous elevated crest for some distance. Ridges are usually termed hills or mountains as well, depending on size. There are several main types of ridges:...

 formed when the resistant layers of the Dakota Sandstone

Sandstone

Sandstone 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 colour, but the most common colours are tan, brown, yellow,...

 and underlying layers were thrust upward

Tectonic uplift

Tectonic 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 granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

 intrusion

Intrusion

An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...

, 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 plain

Plain

In geography, a plain is land with relatively low relief, that is flat or gently rolling. 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...

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 east

East

East is a noun, adjective, or adverb indicating direction or geography.East is one of the four cardinal directions or compass points. It is the opposite of west and is perpendicular to north and south.By convention, the right side of a map is 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

Flatiron (geomorphology)

A flatiron in geomorphology is a steeply sloping wedge-shaped landscape feature created by differential erosion of a resistant rock layer which is inclined in the same direction as, but at a steeper angle than the exposed mountain slope. Flatirons are associated with hogback ridges...

). Green Mountain itself, much like the Black Hills, is a laccolith

Laccolith

A laccolith is a sheet 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 relatively...

 formed by the intrusion of magma

Magma

Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 into the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's crust

Crust (geology)

In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

 (Cleland, 354-355). The hogbacks surrounding the mountain are steep (with dip slope

Dip slope

A 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.