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61-560: The Shinumo Quartzite is a middle member of the Unkar Group. The Unkar Group is about 1,600 to 2,200 m (5,200 to 7,200 ft) thick and composed of, in ascending order, the Bass Formation , Hakatai Shale , Shinumo Quartzite, Dox Formation , and Cardenas Basalt . The Unkar Group consists of a sequence of sedimentary rocks that accumulated in a variety of environments ranging from fluvial to shallow- marine . Overall,

122-671: A bivalve of unknown affinity, which were reported from the Bass Formation, are now judged to be rounded mud-flakes or pellets that are likely oncolites of algal origin. The lithology and sedimentary structures observed in the Bass Limestone indicate that, except for the Hotauta Member, it accumulated beneath a sea that transgressed from the west. The Hotauta Member consists of fluvial sands and gravels that accumulated within valleys on an eroded surface of

183-412: A marine transgression from the west. The dolomite of the Bass Formation likely was originally deposited as limestone and subsequently altered to dolomite by later diagenesis . This limestone largely accumulated in clear, relatively warm, shallow marine waters by both biological and abiological processes. During the maximum, deepest, incursion of marine waters, limestone and deep-water mud accumulated in

244-436: A shoreline is lacking. Various types of other fossils, i.e. jellyfish , sponges , worm trails, and bivalves have been reported from the Bass Formation. Critical examinations of these reported fossils have concluded that the fossil sponges are inorganic silica concretions ; the jellyfish are either gas escape structures or algal colonies; and the worm trails are inorganic sedimentary structures. In addition, fossils of

305-523: A dark-reddish brown, structureless, ferruginous sediment that is usually a few centimeters to 30 cm (0.98 ft) thick. This contact is regarded as a classic example of an ancient peneplain . In the eastern part of the Grand Canyon, the contact between the Bass Formation and the overlying Hatakai Shale is typically gradational over an interval of a meter or so. For example, in Red Canyon,

366-418: A geometric pattern. In uncompleted non-orthogonal cracks they form as a single three-point star shape that is composed of three cracks. They could also form with more than three cracks but three cracks in commonly considered the minimum. In completed non-orthogonal cracks, they form a very geometric pattern. The pattern resembles small polygonal shaped tiles in a repetitive pattern. Mud curls form during one of

427-550: A large river system. The age of the Shinumo Quartzite is approximately known. Based upon recent dates of volcanic ash layers, detrital zircon grains, and detrital mica flakes, geologists agree that the Unkar Group was deposited between about 1254 and 1100 Ma. The Hakatai Shale, Shinumo Quartzite, and Dox Formation samples yield clusters of zircon as young as 1170 Ma. Thus, these strata must be younger than 1170 Ma. The Dox Formation, which overlies

488-576: A relief of about 6 m (20 ft) in the Shinumo 15-minute topographic quadrangle and 15 m (49 ft) in the Bright Angel and Vishnu 15-minute topographic quadrangles. In Hotauta Canyon, and at Granite Narrows, this surface is extremely smooth with a relief of only a few meters. The greatest relief on this surface can be seen opposite the mouth of Shinumo Creek where low rounded hills of Vishnu Basement Rocks rise 6 m (20 ft) above

549-447: A result of a reduction in water content. Naturally forming mudcracks start as wet, muddy sediment dries up and contracts. A strain is developed because the top layer shrinks while the material below stays the same size. When this strain becomes large enough, channel cracks form in the dried-up surface to relieve the strain. Individual cracks spread and join up, forming a polygonal, interconnected network of forms called "tesselations." If

610-516: A shallow, near-shore marine environment, coastal plain mudflats , and deltas that marked the beginning of Hakatai Shale deposition – dominated the Grand Canyon area. The uranium-lead dating of zircons from an ash bed in the Bass Formation, argon–argon dating of overlying igneous rocks, and thermochronologic studies of the underling Vishnu Basement Rocks – constrain the age of the Bass Formation and associated Unkar Group . Zircons from air-fallen, volcanic ash beds in its lower part yielded

671-404: A uranium-lead date of 1,254.8 ± 1.6 Ma. This date is consistent with radiometric ages of Precambrian strata interpreted to be correlative with the Bass Formation and its age estimated from earlier paleomagnetic studies. In addition, this date is consistent with the younger Cardenas Basalt having been erupted about 1,104 Ma. Finally, the uranium-lead dating of zircons is consistent with studies of

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732-405: Is 80 m (260 ft). It is 100 m (330 ft) thick at Phantom Creek (north side of Isis Temple , Cheops Pyramid, & Utah Flats), and 57 m (187 ft) thick at Crystal Creek. The thin section of Bass Formation at Crystal Creek likely reflects the presence of a Vishnu Basement Rocks topographic high in the paleosurface on which it accumulated. The base of the Bass Formation

793-551: Is a major unconformity that also forms the base of the Grand Canyon Supergroup and the Unkar Group. This unconformity separates the underlying and deeply eroded crystalline basement , which consists of granites, gneisses, pegmatites, and schists of the Vishnu Basement Rocks, from stratified Proterozoic rocks of the Grand Canyon Supergroup. This contact is a remarkably smooth surface that has

854-411: Is composed of "rusty red" quartzite and overlies the lower-middle member. The "rusty-red" quartzite of the upper-middle member grades downward into the lower-middle member through an interval of interbedded purple and red-brown quartzite. Their colors and cementation appear to be the result of ground or connate water alteration after their deposition. Finally, the upper member of the Shinumo Quartzite

915-505: Is composed of reddish brown and locally purple sandstone and an overlying well-cemented gray quartzite. The Shinumo Quartzite is characterized as a quartzite, but the gray quartzite within the upper member of the Shinumo Quartzite contains mudstone -rich intervals. In addition, the gray quartzite was subjected to cementation by silica and bleaching that removed its original reddish brown and purple hematite pigments, after its deposition. Basaltic sills and dikes intrude all formations of

976-415: Is inferred to have accumulated in the delta plain , delta front, and mouth-bar of an ancient delta . The deformed strata of the upper member are interpreted as evidence of reoccurring earthshocks along regional fault and fold systems that were active during its deposition. The contact between the Shinumo Quartzite and Dox Formation indicates a shift from deposition in coastal deltas to fluvial deposition by

1037-560: Is marked by the Hotauta Member of the Bass Formation. It is a prominent discontinuous, conglomerate, initially named the Hotauta Conglomerate and later redesignated the Hotauta Member. This conglomerate consists of rounded, pebble to cobble -sized clasts of chert , granite, quartz , plagioclase crystals , and micro pegmatites in a quartz sand matrix. About 80% of the gravel-size clasts consist of granite and quartzite . The quartzite gravel lacks any local equivalents in

1098-634: Is part of the Great Unconformity . The differential erosion of the Unkar Group left resistant beds of the Cardenas Basalt and Shinumo Quartzite as topographic highs, (today seen as ancient & weathered monadnocks ), that are now buried by sandstones, shales, and conglomerates of the Tapeats Sandstone. These monadnocks served locally as sources of coarse-grained sediments during the marine transgression that deposited

1159-580: Is the lowermost formation, overlies deeply eroded granites , gneisses , pegmatites , and schists that comprise Vishnu Basement Rocks . There has been some discussion about the nomenclature of the Bass Formation. It was originally named the Bass Limestone in 1914 for Bass Canyon, where it is typically exposed. The Bass Limestone has been reclassified as the Bass Formation by geologists because it consists of heterogeneous mixtures of clastic and carbonate sedimentary strata of which dolomite

1220-609: Is the predominant rock type and limestone is only a minor rock type. In addition, its Hotauta Member was originally designated as the Hotauta Conglomerate in 1914 as a separate unit of formation status for Hotauta Canyon. It was later included in the Bass Formation as a member where it has remained. The Bass Formation consists of interbedded sandstone ( arkose ), and silty sandstone, prominent interbeds of conglomerate and dolomite, and subordinate interbeds of argillite and limestone. Dolomite and sandy dolomite are

1281-541: Is thermodynamically favored like columnar jointing and polygonal patterned ground . Syneresis cracks are broadly similar features that form from underwater shrinkage of muddy sediment caused by differences in salinity or chemical conditions, rather than aerial exposure and desiccation. Syneresis cracks can be distinguished from mudcracks because they tend to be discontinuous, sinuous , and trilete or spindle-shaped. Mudcracks are generally polygonal when seen from above and v-shaped in cross section. The "v" opens towards

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1342-415: The facies change, from quartz arenite, to mudstone and fine-grained arkose – is gradational. Between the three-member Tonto Group (above) and the Shinumo Quartzite, and the rest of the folded and faulted Unkar Group, is a prominent angular unconformity , which is part of the Great Unconformity . Though this surface is typically a plane, differential erosion (also areal erosion) of the tilted strata of

1403-576: The marine transgression to form the Tonto Group. No fossils have been reported from the Shinumo Quartzite. The sedimentary strata comprising the Shinumo Quartzite are inferred to have accumulated in a variety of environments. The lower member is regarded to have been deposited by ancient river systems and later, in coastal tidal flats . The lower-middle, and upper-middle members are argued to have accumulated in nearshore marine environments and coastal tidal and supratidal flats . The upper member

1464-679: The strata within it are conformable with the exception of a significant disconformity between the Hakatai Shale and Shinumo Quartzite. The Unkar Group is overlain in ascending order by the Nankoweap Formation , about 113 to 150 m (371 to 492 ft) thick; the Chuar Group , about 1,900 m (6,200 ft) thick; and the Sixtymile Formation , about 60 m (200 ft) thick. The Unkar Group, as

1525-440: The Bass Formation so far. The environment, in which these stromatolites grew, as judged by the associated sediments, was one of quiet, shallow marine waters. The common presence of ripple-marks and mud-cracks suggest intermittent desiccation. Thin layers of flake-breccia associated with them indicate occasional periods of turbulence of brief duration. However, direct evidence specifically indicating an intertidal environment close to

1586-416: The Bass Formation, Hakatai Shale , Shinumo Quartzite , Dox Formation , and Cardenas Basalt . The Unkar Group is overlain in ascending order by the Nankoweap Formation , about 113 to 150 m (371 to 492 ft) thick; the Chuar Group , about 1,900 m (6,200 ft) thick; and the Sixtymile Formation , about 60 m (200 ft) thick. The Grand Canyon Supergroup , of which the Bass Formation

1647-462: The Bass Formation, metasomatism and recrystallization have produced chrysotile asbestos , both above and below the sills. Veins of asbestos with fibers up to 10 cm (3.9 in) in length, often commonly occur within 3 m (9.8 ft) of the upper and lower contacts of the sills. The Bass Limestone shows generally, an increase in thickness to the northwest ranging from 37 (121 ft) to 104 m (341 ft) thick. Its average thickness

1708-522: The Bass formation have been affected by post-depositional alteration including: dolomitization , recrystallization, stylolitization , and silicification . Basaltic sills and dikes intrude all strata of the Unkar Group that underlie the Cardenas Lava. Only sills, whose feeder dikes are not exposed, can be observed intruding the Bass Formation. Where the sills have intruded siliceous dolomites in

1769-483: The Dox Formation directly overlying Shinumo Quartzite consists of predominantly dark green to black, fissile, slope-forming shale that contains thin sandstone beds. This shale makes a distinct notch between the resistant cliff-forming quartzites of the Shinumo Quartzite underlying them and resistant cliff-forming arkosic sandstones of the Dox Formation overlying them. The change in topographic expression, color, and

1830-667: The Grand Canyon, indicating a distant source. Excellent exposures of the Hotauta Member occur at the Colorado River level near Hance Rapids (river mile 77) and along the South Kaibab and North Kaibab Trails . A variety of sedimentary structures have been reported from the Bass Formation. They include ripple marks ; mudcrack covered surfaces; cone-in-cone structures ; interformational breccias/conglomerates; both normal and reversed small-scale, graded bedding ; and local channel fills. The dolomite and limestone within

1891-516: The Hakatai Shale. The relief at this unconformity is small, being about 10 m (33 ft). The Shinumo Quartzite overlying this unconformity consists of arkosic conglomeratic sandstone that contains quartzite and granite pebbles. Within the Unkar Group, the upper contact of the Shinumo Quartzite with the Dox Formation appears to be gradational and is marked by a change in topographic expression and color. The basal 12 m (39 ft) of

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1952-411: The Shinumo Quartzite is estimated to have accumulated between 1150 and 1140 and 1100 Ma. Thus, the Shinumo Quartzite must date to between 1170 and 1150 to 1140 Ma. This would suggest that the disconformity at the base of the Shinumo Quartzite is a minor one. Bass Formation The Bass Formation , also known as the Bass Limestone , is a Mesoproterozoic rock formation that outcrops in

2013-528: The Tapeats Sandstone and other members of the Tonto Group . The contact between the Bass Formation and the Tapeats Sandstone forms part of a relative flat surface that lies between the monadnocks. Fossil stromatolites occur within the Bass Formation. Diagnostic “columnar forms” of stromatolites are uncommon. Of these forms, only Collenia undosa Walcott, Collenia symmetrica Fenton & Fenton, and Collenia frequens Walcott, have been reported from

2074-646: The Unkar Group below the Cardenas Lava. The Hakatai Shale, Shinumo Quartzite, and Dox Formation are intruded by these dikes. They can be traced, discontinuously, to within a few meters of the base of the Cardenas Lava. The Shinumo Quartzite exhibits a variety of sedimentary structures. The sandstones of the lower and lower-middle members exhibit centimeter- to meter-scale planar tabular cross-stratification and trough cross-beds. These cross beds record north-directed paleocurrents along with subordinate bidirectional paleocurrents. The upper-middle member exhibits abundant cross beds, clay galls, and mudcracks . The sandstones of

2135-551: The Unkar Group left resistant beds of the Cardenas Basalt and Shinumo Quartzite as ancient hills, called monadnocks , that are up to 240 m (790 ft) tall. Thin drapes of Tapeats Sandstone of the Tonto Group now cover most of these ancient monadnocks. However, a few of these monadnocks protrude further up into the Bright Angel Shale ( Isis Temple prominence example). These monadnocks served locally as sources of coarse-grained sediments that accumulated during

2196-415: The Vishnu Basement Rocks. The quartzite gravel indicates that the conglomerates of the Hotauta Member were deposited by river systems that extended some unknown distance outside of the Grand Canyon region. Marine sediments buried the fluvial deposits of the Hotauta Member as a smooth surface, with a local relief of probably no more than 150 feet (46 m) of the Vishnu Basement Rocks, being submerged by

2257-499: The asbestos deposits of the Sierra Ancha - Salt River Canyon region of Gila County, Arizona . As in the case of the Bass Formation, these chrysotile asbestos deposits are of the contact metamorphic type that occur in magnesium-bearing Mesoproterozoic dolomites and limestones altered by basaltic dikes and sills. As in the case of the Grand Canyon asbestos deposits, dolomite and limestone reacted with silica-bearing fluids, heated by

2318-764: The basalt intrusions, forming the serpentine mineral chrysotile. Much like the Grand Canyon asbestos deposits, these basaltic sills and dikes range in age from 1,050 to 1,140 Ma. Quite unlike the Grand Canyon asbestos deposits, the asbestos deposits of the Sierra Ancha-Salt River Canyon region have been extremely productive. 36°06′29″N 112°07′15″W  /  36.1080°N 112.1209°W  / 36.1080; -112.1209 Mudcrack Mudcracks (also known as mud cracks , desiccation cracks or cracked mud ) are sedimentary structures formed as muddy sediment dries and contracts. Crack formation also occurs in clay-bearing soils as

2379-474: The base of the Shinumo Quartzite near the South Kaibab trail contains jasper pebbles . Typically, the Shinumo Quartzite has been subdivided into four poorly defined, unnamed members. First, the basal lower member consisting of purplish arkosic conglomeratic sandstone. Unlike the rest of the Shinumo Quartzite, it contains quartzite and granite gravels up to 5 cm (2.0 in) in diameter. As in

2440-432: The base of the formation. These layers are characterized by sharp basal contacts, some graded-bedding , and a diverse silicate mineral assemblage that differs from the carbonate or calcsilicate dominated mineral assemblages of adjacent strata. Zircons from one of these volcanic ash beds has been dated using Uranium-lead dating techniques. The base of both the Bass Formation and Unkar Group in eastern Grand Canyon

2501-688: The base section of the Grand Canyon Supergroup , overlies deeply eroded granites , gneisses , pegmatites , and schists that comprise Vishnu Basement Rocks . In sharp contrast to argillaceous strata above and below it, the Shinumo Quartzite consists predominately of beds of red, brown or purple sedimentary quartzites that are firmly, to extremely well cemented by silica , and characterized by poorly developed bedding. It also contains beds of massive white, red or purple sandstone and conglomeratic sandstone. One conglomeratic sandstone layer that lies about 21 m (69 ft) above

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2562-414: The case of the Hotauta Conglomerate, the quartzite gravel of the lower member lacks any known equivalents in the Grand Canyon region. Second, the lower-middle member is composed of purple cross-bedded quartzite, sometimes is subdivided into two units, and overlies the lower member. The purple quartzites of the lower-middle member exhibit crisp, well-defined reduction spots. Third, the upper-middle member

2623-406: The central part of the Grand Canyon, while the sandstone, conglomerate, and argillite dominate the Bass Formation in the eastern part of the Grand Canyon. The Bass Formation typically becomes generally finer grained toward the top. Volcanic ash layers occur in the Bass Formation. They consist of layers of white, very fine-grained tephra , that are interbedded with dolomite and argillite toward

2684-501: The contact consists of an interval in which stromatolitic limestone of the Bass Formation is intimately interbedded with coarse clastic sedimentary rock of the overlying Hakatai Shale. In the eastern part of the Grand Canyon, the contact is sharp, but conformable. The contact between the Tapeats Sandstone and Bass Formation and the rest of the folded and faulted Unkar Group is a prominent angular unconformity , which

2745-554: The deposition of the Shinumo Quartzite. These deformed beds can be seen at river level in the vicinity of Nevilles Rapid, ( List of Colorado River rapids ), at river mile 75. The Shinumo Quartzite ranges in thickness from 330 m (1,080 ft) in the Bright Angel Canyon area to 345 m (1,132 ft) at Papago Creek and 405 m (1,329 ft) in the Shinumo Creek area. In the Bright Angel Canyon area,

2806-550: The early 1900s, William Wallace Bass mined asbestos in the Shinumo Creek-Hakatai Canyon area and John Hance mined asbestos in Asbestos Canyon. Although these deposits had good quality asbestos, little asbestos, at most a few tons, was produced from them because these deposits were fairly small and difficult to access. These asbestos mines are now abandoned and protected as historical sites. In

2867-546: 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

2928-517: The final stages in desiccation . Mud curls commonly occur on the exposed top layer of very thinly bedded mud rocks. When mud curls form, the water that is inside the sediment begins to evaporate causing the stratified layers to separate. The individual top layer is much weaker than multiple layers and is therefore able to contract and form curls as desiccation occurs. If transported by later currents, mud curls may be preserved as mud-chip rip-up clasts. Naturally occurring mudcracks form in sediment that

2989-437: The general level of a relatively flat surface. The Vishnu Basement Rocks underlying this surface are often deeply weathered to an average depth of 3 m (9.8 ft) below it. Where it has not been removed by erosion, prior to, and during the deposition of the overlying Bass Formation, a residual regolith developed by subaerial weathering of the underlying basement rocks – is present. Typically, this regolith consists of

3050-681: The mid-1960s, Gregory H. Billingsley found several veins of asbestos up to 7.6 cm (3.0 in) thick at the mouth of Tapeats Creek . These veins outcrop upstream along the Colorado River for nearly two miles up to Stone Creek. Green chlorite , green garnet , and talc are associated with the asbestos veins, which occur in a 0.6-to-0.9 m (2-to-3 ft) thick contact-metamorphism -zone associated with basaltic sills. For unknown reasons, claims were never filed on these veins and no attempts were made to mine them. The Grand Canyon asbestos deposits are very similar in origin and nature to

3111-419: The predominant lithologies. Beds of intraformational breccia are also found throughout the Bass Formation. The dolomites and limestones vary in color from gray to red-gray and sometimes contain biscuit-form and biohermal stromatolite beds. The conglomerates, breccias, sandstones, and argillites vary in color from purple-brown to dark red and reddish brown. Dolomite and limestone dominate the Bass Formation in

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3172-525: The same region or location as the other cracks. Orthogonal intersections can have a preferred orientation or may be random. In oriented orthogonal cracks, the cracks are usually complete and bond to one another forming irregular polygonal shapes and often rows of irregular polygons. In random orthogonal cracks, the cracks are incomplete and unoriented therefore they do not connect or make any general shapes. Although they do not make general shapes they are not perfectly geometric. Non-orthogonal mudcracks have

3233-467: The southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area (below East Rim). In addition, thick, prominent, and dark-colored basaltic sills intrude across the Bass Formation. The Bass Formation is the basal part of the Unkar Group . The Unkar Group is about 1,600 to 2,200 m (5,200 to 7,200 ft) thick and composed of, in ascending order,

3294-428: The strain continues to build, the polygons start to curl upwards. This characteristic can be used in geology to understand the original orientation of a rock. Cracks may later be filled with sediment and form casts over the base. Typically, the initial crack pattern is dominated by T-shaped junctions. If a mudfield is repeatedly wetted and dried, it can be annealed to a pattern dominated by Y-shaped junctions, as it

3355-551: The top of a bed of muddy sediment or as casts on the base of the overlying bed. When they are preserved on the top of a bed, the cracks look as they did at the time of formation. When they are preserved on the bottom of the bedrock, the cracks are filled in with younger, overlying sediment. In most bottom-of-bed examples, the cracks are the part that sticks out most. Bottom-of-bed preservation occurs when mudcracks that have already formed and are completely dried are covered with fresh, wet sediment and are buried. Through burial and pressure,

3416-434: The top of the bed and the crack tapers downward. Allen (1982) proposed a classification scheme for mudcracks based on their completeness, orientation, shape, and type of infill. Complete mudcracks form an interconnected tessellating network. The connection of cracks often occurs when individual cracks join together forming a larger continuous crack. Incomplete mudcracks are not connected to each other but still form in

3477-534: The underlying Vishnu Basement Rocks that indicate they were exhumed from depths of 25 to 10 km depths between 1,750 and 1,660 Ma and from a depth of 10 km to the surface on which the Bass Formation lies between 1,660 and 1,250 Ma. The presence of chrysotile asbestos veins in the Bass Formation was first noted in early exploration of the Grand Canyon by the Powell expedition . Shortly after 1890, mining claims were filed on these asbestos deposits. Later in

3538-459: The upper member exhibit trough cross-beds, suggesting a more northerly transport direction. The sandstone and quartzite beds of the upper member exhibit an abundance of complexly contorted, gnarly bedded, fluid expulsion structures and dramatic convolute bedding . The beds that exhibit these soft-sediment deformation structures are meters- to tens-of-meters thick. They are cited as evidence for frequent earthquake activity and fluid migration during

3599-462: The upper member is about 18 m (59 ft) thick, the upper middle member is about 80 m (260 ft) thick, and the lower middle member is about 130 m (430 ft) thick. The contact of the Shinumo Quartzite with the Hakatai Shale is the only documented unconformity that has been found within the Unkar Group . This unconformity truncates across beds and channel deposits of

3660-503: The western Grand Canyon, while stromatolites and shallow-water mud accumulated in the eastern Grand Canyon. Following the maximum incursion of marine waters, the sea slowly regressed, and it accumulated in nearshore and coastal environments as indicated by ripple marks , mudcracks , oxidized shales , and other evidence of periodic subaerial exposure found in the upper part of the Bass Formation. Evaporite -forming conditions probably occurred, also during this regressive phase. Eventually,

3721-876: Was once saturated with water. Abandoned river channels, floodplain muds, and dried ponds are localities that form mudcracks. Mudcracks can also be indicative of a predominately sunny or shady environment of formation. Rapid drying, which occurs in sunny environments, results in widely spaced, irregular mudcracks, while closer spaced, more regular mudcracks indicate that they were formed in a shady place. Similar features also occur in frozen ground, lava flows (as columnar basalt ), and igneous dykes and sills . Polygonal crack networks similar to mudcracks can form in human-made materials such as ceramic glazes , paint film, and poorly made concrete. Mudcrack patterning at smaller scales can also be observed studied using technological thin films deposited using micro and nanotechnologies . Mudcracks can be preserved as v-shaped cracks on

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