In geology and geomorphology, a tessellated pavement is a relatively flat rock surface that is subdivided into polygons by fractures, frequently systematic joints, within the rock. This type of rock pavement bears this name because it is fractured into polygonal blocks that resemble tiles of a mosaic floor, or tessellations . [1]
Four types of tessellated pavements are recognized: tessellated pavements formed by jointing; tessellated pavements formed by cooling contraction; tessellations formed by mud cracking and lithification; and tessellated sandstone pavements of uncertain origin. [1]
The most common type of tessellated pavement consists of relatively flat rock surfaces, typically the tops of beds of sandstones and other sedimentary rocks, that are subdivided into either more or less regular rectangles or blocks approaching rectangles by well-developed systematic orthogonal joint systems. The surface of individual beds, as exposed by erosion, are typically divided into either squares, rectangles, and less commonly triangles or other shapes, depending on the number and orientation of the joint sets that comprise the joint system. This relatively flat surface of individual beds of sedimentary rocks are frequently altered by weathering along joints as to cause the bedrock along the joints to be either raised or recessed as the result of differential erosion. This type of tessellated pavement is commonly observed along shorelines where wave action has created relatively flat and extensive wave-cut platforms that expose jointed bedrock and keeps the surfaces of these platforms relatively clear of debris. [1] [2]
The second type of tessellated pavement consists of a bedrock surface that exhibits joints that form polygons that are typically regular in size, spacing, and junctions. [1] Typically, these polygons represent the cross-sections of polygonal, typically hexagonal joints, called columnar jointing, that formed as the result of the cooling of basaltic lava. This type of surface can be seen at the Giant's Causeway in Northern Ireland. [3] [4] [5]
The third type of tesselation recognized by Branagan [1] is associated with the shrinkage and cracking of fine-grained, either clayey or calcareous, sediments. They consist of polygonal cracking, often associated with individual 'plates' that tend to be concave upward, that characterizes the formation of mudcracks in fine-grained sediments. [6] [7] Often, the outlines of the polygons formed by this type of cracking are preserved and accentuated by the infilling of the cracks with material of a different composition from that of either the clayey or calcareous sediments in which the cracks form. The infilling of the cracks by sediments of a different character often preserved the polygonal pattern of the cracking where it can be exhumed by erosion as a patterned pavement after the sediment becomes lithified into a sedimentary rock. [7] [8]
The final type of tessellated pavement consists of relatively flat, sandstone surfaces that typically exhibit a complex pattern of five- or six-sided polygons. Typically, these polygons vary greatly in size from 0.5 to 2 m in width. These polygons are defined by well-developed fractures that sometimes have raised rims. They are found within exposures of the Hawkesbury Sandstone within the Sydney, New South Wales, Australia region, exposures of the Precipice Sandstone at the Kenniff Cave Archaeological Site in Queensland, Australia, and in exposures of Upper Cretaceous sandstones of the Boulder, Colorado, U.S., region. The origin of this type of tessellated pavement remains uncertain. The size and shape of these polygons appears to be dependent to a large extent on the grain size, texture, and coherence of the rock. This polygonal tessellation is best developed in relatively fine-grained, uniform, and siliceous or silicified sandstones. [1]
The most well known example of a tessellated pavement is the Tessellated Pavement that is found at Lufra, Eaglehawk Neck on the Tasman Peninsula of Tasmania. This tessellated pavement consists of a marine platform on the shore of Pirates Bay, Tasmania. This example consists of two types of formations: a pan formation and a loaf formation. [1] [2]
The pan formation is a series of concave depressions in the rock that typically forms beyond the edge of the seashore. This part of the pavement dries out more at low tide than the portion abutting the seashore, allowing salt crystals to develop further; the surface of the "pans" therefore erodes more quickly than the joints, resulting in increasing concavity. [2]
The loaf formations occur on the parts of the pavement closer to the seashore, which are immersed in water for longer periods of time. These parts of the pavement do not dry out so much, reducing the level of salt crystallisation. Water, carrying abrasive sand, is typically channelled through the joints, causing them to erode faster than the rest of the pavement, leaving loaf-like structures protruding. [2]
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
Weathering is the deterioration of rocks, soils and minerals through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs in situ, and so is distinct from erosion, which involves the transport of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity.
Landforms are categorized by characteristic physical attributes such as their creating process, shape, elevation, slope, orientation, rock exposure, and soil type.
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.
A tessellation or tiling is the covering of a surface, often a plane, using one or more geometric shapes, called tiles, with no overlaps and no gaps. In mathematics, tessellation can be generalized to higher dimensions and a variety of geometries.
A concretion is a hard, compact mass formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin concretio "(act of) compacting, condensing, congealing, uniting", itself from con meaning "together" and crescere meaning "to grow".
In geology, a dike or dyke is a sheet of rock that is formed in a fracture of a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma flows into a crack then solidifies as a sheet intrusion, either cutting across layers of rock or through a contiguous mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack.
The Bimini Road, sometimes called the Bimini Wall, is an underwater rock formation near the island of North Bimini in the Bimini chain of islands. The Road consists of a 0.8 km (0.50 mi)-long northeast-southwest linear feature composed of roughly rectangular limestone blocks. Various claims have been made for this feature being either a wall, road, pier, breakwater, or other man-made structure. However, credible evidence or arguments are lacking for such an origin.
A joint is a break (fracture) of natural origin in a layer or body of rock that lacks visible or measurable movement parallel to the surface (plane) of the fracture. Although joints can occur singly, they most frequently appear as joint sets and systems. A joint set is a family of parallel, evenly spaced joints that can be identified through mapping and analysis of their orientations, spacing, and physical properties. A joint system consists of two or more intersecting joint sets.
A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity.
The geology of Hong Kong is dominated by igneous rocks formed during a major volcanic eruption period in the Mesozoic era. It made up 85% of Hong Kong's land surface and the remaining 15% are mostly sedimentary rocks located in the northeast New Territories. There are also a very small percentage of metamorphic rocks in the New Territories, formed by deformation of pre-existing sedimentary rocks (metamorphism).
Columnar jointing is a geological structure where sets of intersecting closely spaced fractures, referred to as joints, result in the formation of a regular array of polygonal prisms, or columns. Columnar jointing occurs in many types of igneous rocks and forms as the rock cools and contracts. Columnar jointing can occur in cooling lava flows and ashflow tuffs (ignimbrites), as well as in some shallow intrusions. Columnar jointing also occurs rarely in sedimentary rocks, due to a combination of dissolution and reprecipitation of interstitial minerals by hot, hydrothermal fluids and the expansion and contraction of the rock unit, both resulting from the presence of a nearby magmatic intrusion.
A tableland is an area containing elevated landforms characterized by a distinct, flat, nearly level, or gently undulating surface. They often exhibit steep, cliff-like edges, know as escarpments, that separate them from surrounding lowlands. Depending on either their size, other physical characteristics, or geographic location, the landforms comprising a tableland are individually referred to by a number of names including either butte, mesa, plateau, potrero, tepui, or tuya. Table Mountains are also a type of tableland. A homologous landform under the sea is called either a tablemount or guyot.
Mudcracks are sedimentary structures formed as muddy sediment dries and contracts. Crack formation also occurs in clay-bearing soils as a result of a reduction in water content.
A bedrock river is a river that has little to no alluvium mantling the bedrock over which it flows. However, most bedrock rivers are not pure forms; they are a combination of a bedrock channel and an alluvial channel. The way one can distinguish between bedrock rivers and alluvial rivers is through the extent of sediment cover.
The Organ Pipes National Park, abbreviated as OPNP, is a national park located in the Central region of Victoria, Australia. The 121-hectare (300-acre) protected area was established with the focus on conservation of the native flora and fauna, and preservation of the geological features in the Jacksons Creek, a part of the Maribyrnong valley, north-west of Melbourne. It is situated in a deep gorge in the grassy, basalt Keilor Plains.
The Bass Formation, also known as the Bass Limestone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The Bass Formation erodes as either cliffs or stair-stepped cliffs. In the case of the stair-stepped topography, resistant dolomite layers form risers and argillite layers form steep treads. In general, the Bass Formation in the Grand Canyon region and associated strata of the Unkar Group-rocks dip northeast (10°–30°) toward normal faults that dip 60+° toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area. In addition, thick, prominent, and dark-colored basaltic sills intrude across the Bass Formation.
The geology of the Isle of Skye in Scotland is highly varied and the island's landscape reflects changes in the underlying nature of the rocks. A wide range of rock types are exposed on the island, sedimentary, metamorphic and igneous, ranging in age from the Archaean through to the Quaternary.
A fin is a geological formation that is a narrow, residual wall of hard sedimentary rock that remains standing after surrounding rock has been eroded away along parallel joints or fractures. Fins are formed when a narrow butte or plateau develops many vertical, parallel cracks. There are two main modes of following erosion. The first is when water flows along joints and fractures and opens them wider and wider, eventually causing erosion. The second is where the rock type (stratum) is harder and more erosion resistant than neighboring rocks, causing the weaker rock to fall away.
The Catoctin Formation is a geologic formation that expands through Virginia, Maryland, and Pennsylvania. It dates back to the Precambrian and is closely associated with the Harpers Formation, Weverton Formation, and the Loudoun Formation. The Catoctin Formation lies over a granitic basement rock and below the Chilhowee Group making it only exposed on the outer parts of the Blue Ridge. The Catoctin Formation contains metabasalt, metarhyolite, and porphyritic rocks, columnar jointing, low-dipping primary joints, amygdules, sedimentary dikes, and flow breccias. Evidence for past volcanic activity includes columnar basalts and greenstone dikes.
