Dip slope

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Schematic cross section of a cuesta, dip slopes facing left and harder rock layers in darker colors than softer ones Cuesta schematic1.PNG
Schematic cross section of a cuesta, dip slopes facing left and harder rock layers in darker colors than softer ones

A dip slope is a topographic (geomorphic) surface which slopes in the same direction, and often by the same amount, as the true dip or apparent dip of the underlying strata. [1] [2] A dip slope consists of the upper surface of a resistant layer of rock, often called caprock , that is commonly only slightly lowered and reduced in steepness by erosion. Dip slopes form the backslopes of cuestas, homoclinal ridges, hogbacks, and flatirons. The frontslopes of such ridges consist of either an escarpment, a steep slope, or perhaps even a line of cliffs. Generally, cuestas and homoclinal ridges are asymmetrical in that their dip slopes are less steep than their escarpments. In the case of hogbacks and flatirons, the dip of the rocks is so steep that their dip slope approaches the escarpment in their steepness. [1] [3] [4] [5]

Strike and dip terms used to denote the orientation of a geologic feature

Strike and dip refer to the orientation or attitude of a geologic feature. The strike line of a bed, fault, or other planar feature, is a line representing the intersection of that feature with a horizontal plane. On a geologic map, this is represented with a short straight line segment oriented parallel to the strike line. Strike can be given as either a quadrant compass bearing of the strike line or in terms of east or west of true north or south, a single three digit number representing the azimuth, where the lower number is usually given, or the azimuth number followed by the degree sign.

Caprock or cap rock is a harder or more resistant rock type overlying a weaker or less resistant rock type. Common types of caprock are sandstone and mafic rock types. An analogy of caprock could be the outer crust on a cake that is a bit harder than the underlying layer. Common caprock locations are escarpments, mesa formations, and salt domes. In the petroleum industry, caprock is generalized to any nonpermeable formation that may prevent oil, gas, or water from migrating to the surface.

Erosion Processes which remove soil and rock from one place on the Earths crust, then transport it to another location where it is deposited

In earth science, erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, animals, and humans. In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind (aeolic) erosion, zoogenic erosion, and anthropogenic erosion. The particulate breakdown of rock or soil into clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by its dissolving into a solvent, followed by the flow away of that solution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.

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The south-facing (right) side of Mount Rundle in Canada is a good example of a dip slope. The cap rock is Rundle Group dolomite. MountRundle.jpg
The south-facing (right) side of Mount Rundle in Canada is a good example of a dip slope. The cap rock is Rundle Group dolomite.

Dip slopes are the result of the differential erosion of strata of varying resistance to erosion that are dipping uniformly in one direction. In this case, strata, i.e. shale, mudstone, and marl, that are less resistant to erosion are preferentially eroded relative to stronger strata, i.e. sandstone, limestone, and dolomite, that are more resistant to erosion. As a result, the less resistant strata will be eroded away leaving the more resistant strata as a caprock forming the dip slope (backslope) of a ridge that slopes in the direction of caprock. When this happens to flat-lying beds, landforms such as plateaus, mesas, and buttes are formed. The erosion of tilted beds will form landforms called cuestas, homoclinal ridges, hogbacks, and flatirons. Plateaus, mesas, and buttes have flat tops, while cuestas and homoclinal ridges are asymmetrical (~flat) areas w/ridges. The less steep side (at the low point) is their dip slope (intersecting 'ground' surface, and disappearing underground) and the steeper other side (the opposite, and at the high point) is their escarpment. In case of hogbacks, the steepness of the dip slope and escarpment will be about the same. Dip slopes can also be formed by igneous structures such as sills. [3] [4] [5]

Shale A fine-grained, clastic sedimentary rock

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. It is the most common sedimentary rock.

Mudstone Fine grained sedimentary rock whose original constituents were clays or muds

Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.063 millimetres (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time, the platy clay minerals may become aligned, with the appearance of fissility or parallel layering. This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mud rocks such as mudstone and shale account for some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.

Marl Lime-rich mud or mudstone which contains variable amounts of clays and silt

Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and silt. The dominant carbonate mineral in most marls is calcite, but other carbonate minerals such as aragonite, dolomite, and siderite may be present. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under freshwater conditions; specifically an earthy substance containing 35–65% clay and 65–35% carbonate. It also describes a habit of coralline red alga. The term is today often used to describe indurated marine deposits and lacustrine (lake) sediments which more accurately should be named 'marlstone'. Marlstone is an indurated rock of about the same composition as marl, more correctly called an earthy or impure argillaceous limestone. It has a blocky subconchoidal fracture, and is less fissile than shale. The term 'marl' is widely used in English-language geology, while the terms Mergel and Seekreide are used in European references.

Landslides

Some dip slopes are quite prone to landslides, due to the dipping strata that underlie them. Large sheets of rock have a tendency to slide down dip slopes.

Landslide type of natural disaster, geological phenomenon

The term landslide or, less frequently, landslip, refers to several forms of mass wasting that include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows and debris flows. Landslides occur in a variety of environments, characterized by either steep or gentle slope gradients: from mountain ranges to coastal cliffs or even underwater, in which case they are called submarine landslides. Gravity is the primary driving force for a landslide to occur, but there are other factors affecting slope stability which produce specific conditions that make a slope prone to failure. In many cases, the landslide is triggered by a specific event, although this is not always identifiable.

See also

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Valley Low area between hills, often with a river running through it.

A valley is a low area between hills or mountains typically with a river running through it. In geology, a valley or dale is a depression that is longer than it is wide. The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys. Most valleys belong to one of these two main types or a mixture of them, at least with respect to the cross section of the slopes or hillsides.

Niagara Escarpment

The Niagara Escarpment is a long escarpment, or cuesta, in the United States and Canada that runs predominantly east/west from New York, through Ontario, Michigan, Wisconsin, and Illinois. The escarpment is most famous as the cliff over which the Niagara River plunges at Niagara Falls, for which it is named.

Escarpment Steep slope or cliff separating two relatively level regions

An escarpment, or scarp, is a steep slope or long cliff that forms as an effect of faulting or erosion and separates two relatively level areas having differing elevations. Usually scarp and scarp face are used interchangeably with escarpment.

The fluvial landforms of streams, stream beds, and river valleys have various landforms.

Inselberg Isolated rock hill or small mountain that rises abruptly from a relatively flat surrounding plain

An inselberg or monadnock is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In southern and south-central Africa, a similar formation of granite is known as a koppie, an Afrikaans word from the Dutch word kopje. If the inselberg is dome-shaped and formed from granite or gneiss, it can also be called a bornhardt, though not all bornhardts are inselbergs.

Landform A natural feature of the solid surface of the Earth or other planetary body

A landform is a natural feature of the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrangement in the landscape is known as topography. Typical landforms include hills, mountains, plateaus, canyons, and valleys, as well as shoreline features such as bays, peninsulas, and seas, including submerged features such as mid-ocean ridges, volcanoes, and the great ocean basins.

Spheroidal weathering

Spheroidal weathering is a form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock that is known as saprolite. When saprolite is exposed by physical erosion, these concentric layers peel (spall) off as concentric shells much like the layers of a peeled onion. Within saprolite, spheroidal weathering often creates rounded boulders, known as corestones or woolsack, of relatively unweathered rock. Spheroidal weathering is also called onion skin weathering,concentric weathering,spherical weathering, or woolsack weathering.

Butte Isolated hill with steep, often vertical sides and a small, relatively flat top

In geomorphology, a butte is an isolated hill with steep, often vertical sides and a small, relatively flat top; buttes are smaller landforms than mesas, plateaus, and tablelands. The word "butte" comes from a French word meaning "small hill"; its use is prevalent in the Western United States, including the southwest where "mesa" is used for the larger landform. Because of their distinctive shapes, buttes are frequently landmarks in plains and mountainous areas. In differentiating mesas and buttes, geographers use the rule of thumb that a mesa has a top that is wider than its height, while a butte has a top that is narrower than its height.

Mesa Elevated area of land with a flat top and sides that are usually steep cliffs

Mesa is the American English term for tableland, an elevated area of land with a flat top and sides that are usually steep cliffs. It takes its name from its characteristic table-top shape. It may also be called a table hill, table-topped hill or table mountain. It is larger than a butte, which it otherwise resembles closely.

Ridge A geological feature consisting of a chain of mountains or hills that form a continuous elevated crest for some distance

A ridge or a mountain ridge is a geological feature consisting of a chain of mountains or hills that form a continuous elevated crest for some distance. The sides of the ridge slope away from narrow top on either side.The line along the crest formed by the highest points, with the terrain dropping down on either side, is called the ridgeline. Ridges are usually termed hills or mountains as well, depending on size.

Cuesta A hill or ridge with a gentle slope on one side and a steep slope on the other

A cuesta is a hill or ridge with a gentle slope on one side, and a steep slope on the other. In geology the term is more specifically applied to a ridge where a harder sedimentary rock overlies a softer layer, the whole being tilted somewhat from the horizontal. This results in a long and gentle backslope called a dip slope that conforms with the dip of resistant strata, called caprock. Where erosion has exposed the frontslope of this, a steep slope or escarpment occurs. The resulting terrain may be called scarpland.

Terrace (geology) A step-like landform

In geology, a terrace is a step-like landform. A terrace consists of a flat or gently sloping geomorphic surface, called a tread, that is typically bounded one side by a steeper ascending slope, which is called a "riser" or "scarp." The tread and the steeper descending slope together constitute the terrace. Terraces can also consist of a tread bounded on all sides by a descending riser or scarp. A narrow terrace is often called a bench.

Pediplain An extensive plain formed by the coalescence of pediments

In geology and geomorphology a pediplain is an extensive plain formed by the coalescence of pediments. The processes through which pediplains forms is known as pediplanation. The concepts of pediplain and pediplanation were first developed by geologist Lester Charles King in his 1942 book South African Scenery. The concept gained notoriety as it was juxtaposed to peneplanation.

Hogback (geology) A long, narrow ridge or a series of hills with a narrow crest and steep slopes of nearly equal inclination on both flanks

In geology and geomorphology, a hogback or hog's back is a long, narrow ridge or a series of hills with a narrow crest and steep slopes of nearly equal inclination on both flanks. Typically, the term is restricted to a ridge created by the differential erosion of outcropping, steeply dipping, homoclinal, and typically sedimentary strata. One side of a hogback consists of the surface of a steeply dipping rock stratum called a dip slope. The other side is an erosion face that cuts through the dipping strata that comprises the hogback. The name "hogback" comes from the Hog's Back of the North Downs in Surrey, England, which refers to the landform's resemblance in outline to the back of a hog. The term is also sometimes applied to drumlins and, in Maine, to both eskers and ridges known as "horsebacks".

In structural geology, a homocline or homoclinal structure, is a geological structure in which the layers of a sequence of rock strata, either sedimentary or igneous, dip uniformly in a single direction having the same general inclination in terms of direction and angle. A homocline can be associated with either one limb of a fold, the edges of a dome, the coast-ward tilted strata underlying a coastal plain, slice of thrust fault, or a tilted fault block. When the homoclinal strata consists of alternating layers of rock that vary hardness and resistance to erosion, their erosion produces either cuestas, homoclinal ridges, or hogbacks depending on the angle of dip of the strata. On a topographic map, the landfroms associated with homoclines exhibit nearly parallel elevation contour lines that show a steady change in elevation in a given direction. In the subsurface, they characterize by parallel structural contour lines.

A homoclinal ridge or strike ridge is a hill or ridge with a moderate, generally between 10° to 30°, sloping backslope. Its backslope is a dip slope, that conforms with the dip of a resistant stratum or strata, called caprock. On the other side of the other slope, which is its frontslope, of a homoclinal ridge is a steeper or even cliff-like frontslope (escarpment) that is formed by the outcrop of the caprock. The escarpment cuts through the dipping strata that comprises the homoclinal ridge.

A pediment is a very gently sloping (.5°-7°) inclined bedrock surface. It typically slopes down from the base of a steeper retreating desert cliff, or escarpment, but may continue to exist after the mountain has eroded away. It is caused by erosion. It develops when sheets of running water wash over it in intense rainfall events. It may be thinly covered with fluvial gravel that has washed over it from the foot of mountains produced by cliff retreat erosion. It is typically a concave surface gently sloping away from mountainous desert areas.

Flatiron (geomorphology) A steeply sloping triangular landform created by the differential erosion of a steeply dipping, erosion-resistant layer of rock overlying softer strata.

Traditionally in geomorphology, a flatiron is a steeply sloping triangular landform created by the differential erosion of a steeply dipping, erosion-resistant layer of rock overlying softer strata. Flatirons have wide bases that form the base of a steep, triangular facet that narrows upward into a point at its summit. The dissection of a hogback by regularly spaced streams often resulted in the formation of a series of flatirons along the strike of the rock layer that formed the hogback. As noted in some, but not all definitions, a number of flatirons are perched upon the slope of a larger mountain with the rock layer forming the flatiron inclined in the same direction as, but often at a steeper angle than the associated mountain slope. The name flatiron refers their resemblance to an upended, household flatiron.

Scarp retreat

Scarp retreat is a geological process through which the location of an escarpment changes over time. Typically the cliff is undermined, rocks fall and form a talus slope, the talus is chemically or mechanically weathered and then removed through water or wind erosion, and the process of undermining resumes. Scarps may retreat for tens of kilometers in this way over relatively short geological time spans, even in arid locations.

References

  1. 1 2 Jackson, JA, J Mehl and K Neuendorf (2005) Glossary of Geology. American Geological Institute, Alexandria, Virginia. 800 pp. ISBN   0-922152-76-4
  2. ↑ Allaby, M (2008) A Dictionary of Earth Science. Oxford University Press, New York, New York. 654 pp. ISBN   978-0-19-921194-4
  3. 1 2 Cotton, CA (1952) Geomorphology An Introduction to the Study of Landforms, 6th ed. John Wiley and Sons, New York. 505 pp.
  4. 1 2 Fairbridge, RW (1968) Hogback and Flatiron. In RW Fairbridge, ed., pp. 524-525, The Encyclopedia of Geomorphology (Encyclopedia of Earth Sciences, Volume III), Reinhold, New York, 1296 pp. ISBN   978-0879331795
  5. 1 2 Simonett, SD (1968) Cuesta. In RW Fairbridge, ed., pp. 233, The Encyclopedia of Geomorphology (Encyclopedia of Earth Sciences, Volume III), Reinhold, New York, 1296 pp. ISBN   978-0879331795
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