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27-604: 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 . It is generally faulted along its western side, and varies in height, with gaps in numerous locations where rivers exit

54-438: A stratum , is the smallest formal unit in the hierarchy of sedimentary lithostratigraphic units and is lithologically distinguishable from other layers above and below. Customarily, only distinctive beds, i.e. key beds , marker beds , that are particularly useful for stratigraphic purposes are given proper names and considered formal lithostratigraphic units. In case of volcanic rocks, the lithostratigraphic unit equivalent to

81-415: A bed can be defined by thickness where a bed is a coherent layer of sedimentary rock, sediment, or pyroclastic material greater than 1 cm thick and a lamina is a coherent layer of sedimentary rock, sediment, or pyroclastic material less than 1 cm thick. This method of defining bed versus lamina is frequently used in textbooks, e.g., Collinson & Mountney or Miall. Both definitions have merit and

108-526: A bed is a flow . A flow is “...a discrete, extrusive, volcanic rock body distinguishable by texture, composition, order of superposition, paleomagnetism, or other objective criteria.” A flow is a part of a member as a bed of sedimentary rock is a part of a member. In geotechnical engineering a bedding surface often forms a discontinuity that may have a large influence on the mechanical behaviour (strength, deformation, etc.) of soil and rock masses in tunnel , foundation , or slope construction. These are

135-509: A combination of local deposition on the inclined surfaces of ripples or dunes , and local erosion . Graded beds show a gradual change in grain or clast sizes from one side of the bed to the other. A normal grading occurs where there are larger grain sizes on the older side, while an inverse grading occurs where there are smaller grain sizes on the older side. Bed thickness is a basic and important characteristic of beds. Besides mapping stratigraphic units and interpreting sedimentary facies,

162-442: A result of changes in environmental conditions. As a result, a bed is typically, but not always, interpreted to represent a single period of time when sediments or pyroclastic material accumulated during uniform and steady paleoenvironmental conditions. However, some bedding surfaces may be postdepositional features either formed or enhanced by diagenetic processes or weathering . The relationship between bedding surfaces controls

189-491: A typical regional topographic expression of outcrops of steeply dipping strata, commonly sedimentary strata, that consist of alternating beds of hard, well-lithified strata, i.e. sandstone and limestone , and either weak or loosely cemented strata, i.e. shale , mudstone , and marl . The surface of a hard, erosion-resistant layer forms the back slope (dip-slope) of the hogback where weaker strata have been preferentially stripped off of it by erosion. The opposite slope that forms

216-694: Is to the west. North of Denver its major gaps are I-80 in southern Wyoming, U.S. Highway 34 at Loveland , and U.S. 36 to Rocky Mountain National Park . Interstate 70 passes through a highway cut, revealing the numerous layers making up the ridge. South of Denver, the major gaps are U.S. Route 24 in Colorado Springs , U.S. Route 50 in Pueblo , and finally in Colorado, U.S. Route 160 in Walsenburg . Before

243-737: The Colorado Plateau to the west and the Southern Rocky Mountains to the east. While most hogbacks snake along a surface in a relatively sinuous line, a 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 (201 km) long and 65 miles (105 km) wide. The Dakota Hogback ridge formed when

270-542: The Black Hills. The ridge "presents a steep face towards the valley and rises several hundred feet above it. Green Mountain, also known as the Little Sundance Dome , is found just east of Sundance, Wyoming . It is a circular dome about 5,900 feet (1,800 m) across and 4,600 feet (1,400 m) wide surrounded by a rim of triangular hogbacks (similar in appearance to flatirons ). Green Mountain itself, much like

297-533: The analysis of bed thickness can be used to recognize breaks in sedimentation, cyclic sedimentation patterns, and gradual environmental changes. Such sedimentological studies are typically based on the hypothesis that the thicknesses of stratigraphic units follows a log-normal distribution . Differing nomenclatures for the bed and laminae thickness have been proposed by various authors, including McKee and Weir, Ingram, and Reineck and Singh. However, none of them have been universally accepted by Earth scientists. In

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324-422: The choice of which one to use will depend on the focus of the specific study on a case by case basis. Typically, but not always, bedding surfaces record changes in either the rate or type of accumulating sediment that created the underlying bed. Typically, they represent either a period of nondeposition, erosional truncation, shift in flow or sediment regime, abrupt change in composition, or combination of these as

351-496: The front of a hogback, which is its escarpment or scarp, consists of a slope that cuts across the bedding of the strata. Because of the steeply dipping nature of the strata that forms a hogback, a slight shift in location may take place as the landscape is lowered by erosion, but it will be a matter of feet rather than miles, as might happen with cuestas . All gradations occur between hogbacks, homoclinal ridges , and cuestas . The differences between these landforms are related to

378-530: The gross geometry of a bed. Most commonly, the bottom and top surfaces of beds are subparallel to parallel to each other. However, some bedding surfaces of a bed are nonparallel, e.g., wavy, or curved. Differing combinations of nonparallel bedding surfaces results in beds of widely varying geometric shapes such as uniform-tabular, tabular-lenticular, curved-tabular, wedge-shaped, and irregular beds. Types of beds include cross-beds and graded beds . Cross-beds, or "sets," are not layered horizontally and are formed by

405-545: The high mountains. But on the edge of the foothills, where the layers were scraped and pushed only slightly upward, the western edge of their remnant still stands at about a 45-degree angle - the Dakota Ridge. The top layer is a hard sandstone, the Dakota Sandstone, from which the ridge gets its name. It protects the softer shales and limestones beneath it from weathering and erosion. Hogback (geology) Hogbacks are

432-572: The hogback was created during the Laramide Orogeny, the geologic strata comprising the hogback are much older. For example, fossilized data such as dinosaur footprints have been observed in the exposed strata, created by dinosaurs which lived during the Jurassic Period approximately 150 million years ago. Some of these footprints were attributed to the Diplodocus dinosaur and could be seen on

459-466: The hogback west of Denver, Colorado as recently as the 1980s. The ridge forms a barrier between the high plains and the Rocky Mountain foothills . The ridge is pierced by a few water-cut gaps, which have been used to provide road access between the mountains and the plains. The ridge is paralleled by I-25 from north of Cheyenne, Wyoming , through Colorado, into northern New Mexico . The ridge

486-579: The less erosion-resistant and interbedded mudstone, siltstone, and thinly-bedded sandstones of the Morrison Formation of Jurassic age. Dinosaur Ridge is only a short segment of the Dakota Hogback that extends the length of the Front Range from Wyoming to southern Colorado. The Grand Hogback is a 70-mile long (110 km) ridge located in western Colorado. It marks part of the boundary between

513-501: The mountains, the region was a shallow sea. As sediments fell to the bottom of the water, they were compressed into soft sedimentary rock. Thus, oyster and clam shells, sand , and mud built slowly into layers of sandstone , shale , limestone , and "mudstone." As the Rocky Mountains rose over the last 67 million years, up to nearly 30,000 feet above sea level, the soft sedimentary rocks were quickly weathered and washed away from

540-573: The mountains. The ridge takes its name from the Dakota Formation , a formation with resistant sandstone beds that cap the ridge. The hogback was formed during the Laramide orogeny , approximately 50 million years (50 my) ago, when the modern Rockies were created. The general uplift to the west created long faulting in the North American Plate , resulting in the creation of the hogback. While

567-460: The nearby Black Hills, is a laccolith formed by the intrusion of magma into the Earth 's crust . Bed (geology) In geology , a bed is a layer of sediment , sedimentary rock , or volcanic rock "bounded above and below by more or less well-defined bedding surfaces". A bedding surface is three-dimensional surface , planar or curved, that visibly separates each successive bed (of

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594-710: The practice of engineering geology, a standardized nomenclature is used for describing bed thickness in Australia, the European Union, and the United Kingdom. Examples of widely used bed thickness classifications include Tucker (1982) and McKee and Weir (1953). According to both the North American Stratigraphic Code and International Stratigraphic Guide, a bed is the smallest formal lithostratigraphic unit that can be used for sedimentary rocks. A bed,

621-459: The resistant sandstones of the Dakota Sandstone and underlying strata were uplifted near the center of the present-day Black Hills because of a granite intrusion , approximately 60 million years ago. The Black Hills are the easternmost segment of the Laramide orogeny . The Dakota hogback rim separates the surrounding flat plains from the two-mile wide (3.2 km) Red Valley trench of

648-489: The same or different lithology ) from the preceding or following bed. Where bedding surfaces occur as cross-sections, e.g., in a 2-dimensional vertical cliff face of horizontal strata, are often referred to as bedding contacts . Within conformable successions, each bedding surface acted as the depositional surface for the accumulation of younger sediment. Specifically in sedimentology , a bed can be defined in one of two major ways. First, Campbell and Reineck and Singh use

675-553: The scientific literature. It also can be difficult to distinguish immediately adjacent members of this series of landforms. Dinosaur Ridge is a well known hogback that is part of Morrison Fossil Area National Natural Landmark within Morrison, Colorado . It is a hogback formed by the differential erosion of well-cemented sandstones of the Cretaceous Dakota Formation , which form the dip slope of this hogback, overlying

702-448: The steepness in dip of the resistant beds from which they have been eroded and to their geographic extent. Where each type occurs depends upon whether the local rock attitudes are either nearly vertical, moderately dipping, or gently dipping. Because of their gradational nature, the exact angle of dip and slope that separates these landforms is arbitrary and some differences in the specific angles used to define these landforms can be found in

729-403: The term bed to refer to a thickness-independent layer comprising a coherent layer of sedimentary rock, sediment, or pyroclastic material bounded above and below by surfaces known as bedding planes. By this definition of bed, laminae are small beds that constitute the smallest (visible) layers of a hierarchical succession and often, but not always, internally comprise a bed. Alternatively,

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