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77-651: The Mohaka river is on the North Island of New Zealand in the east central region of Hawke’s Bay . Mohaka is a Maori word, roughly translated it means “place for dancing”. The iwi (Māori tribes) associated with the Mohaka River are Ngāti Pāhauwera , Ngāti Hineuru , Ngāti Tūwharetoa and Mana Ahuriri. The headwaters are found in the Kaweka and Kaimanawa ranges. From the range it winds southeast before twisting northeast and finally southeast again to empty into

154-400: A (usually small) angle. Sometimes multiple sets of layers with different orientations exist in the same rock, a structure called cross-bedding . Cross-bedding is characteristic of deposition by a flowing medium (wind or water). The opposite of cross-bedding is parallel lamination, where all sedimentary layering is parallel. Differences in laminations are generally caused by cyclic changes in

231-463: A bit of grade III. There are several Department of Conservation (DOC) campsites, hot pools and plenty of undisturbed native bush. After approximately 36 km, it comes out of the Kawekas and farmland becomes more common The river bends itself into an oxbow and shortly after rapids become larger as it leads into the grade III section. DOC campsites are still found along this 68 km stretch. As

308-421: A day trip or sections can be combined into multi day adventures. The Mohaka offers something for any level of kayaking . Beginners will find plenty of eddies and waves on the grade II sections while intermediate kayakers will find the grade III more challenging. The grade V is a technical run for more advanced kayakers. Local rafting companies have the most up-to-date information and can often assist in setting up

385-424: A diagenetic structure common in carbonate rocks is a stylolite . Stylolites are irregular planes where material was dissolved into the pore fluids in the rock. This can result in the precipitation of a certain chemical species producing colouring and staining of the rock, or the formation of concretions . Concretions are roughly concentric bodies with a different composition from the host rock. Their formation can be

462-463: A particular sedimentary environment. Examples of bed forms include dunes and ripple marks . Sole markings, such as tool marks and flute casts, are grooves eroded on a surface that are preserved by renewed sedimentation. These are often elongated structures and can be used to establish the direction of the flow during deposition. Ripple marks also form in flowing water. There can be symmetric or asymmetric. Asymmetric ripples form in environments where

539-465: A red colour does not necessarily mean the rock formed in a continental environment or arid climate. The presence of organic material can colour a rock black or grey. Organic material is formed from dead organisms, mostly plants. Normally, such material eventually decays by oxidation or bacterial activity. Under anoxic circumstances, however, organic material cannot decay and leaves a dark sediment, rich in organic material. This can, for example, occur at

616-489: A rock is usually expressed with the Wentworth scale, though alternative scales are sometimes used. The grain size can be expressed as a diameter or a volume, and is always an average value, since a rock is composed of clasts with different sizes. The statistical distribution of grain sizes is different for different rock types and is described in a property called the sorting of the rock. When all clasts are more or less of

693-465: A sediment after its initial deposition. This includes compaction and lithification of the sediments. Early stages of diagenesis, described as eogenesis , take place at shallow depths (a few tens of meters) and is characterized by bioturbation and mineralogical changes in the sediments, with only slight compaction. The red hematite that gives red bed sandstones their color is likely formed during eogenesis. Some biochemical processes, like

770-614: A sedimentary rock canyon. The boulders, while impressive in size, are even more intriguing because of the holes and depressions the water has carved into them. House rock worth a closer look; the center has been carved out and is accessible only by a hole in the side. Fossilized shells and organic matter have been remarkably well preserved in the stones. After the Willow Flat Bridge, the size and frequency of large boulders decreases to be replaced by steep vegetation covered terraces. Sandstone and mudstone become more prevalent as

847-431: A sedimentary rock may have been present in the original sediments or may formed by precipitation during diagenesis. In the second case, a mineral precipitate may have grown over an older generation of cement. A complex diagenetic history can be established by optical mineralogy , using a petrographic microscope . Carbonate rocks predominantly consist of carbonate minerals such as calcite, aragonite or dolomite . Both

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924-473: A shuttle. The upper section is the most popular destination for fishing in Hawke's Bay. Trout is a common catch, along with various varieties of native fish. Mainly brown trout are found above Pakaututu bridge and rainbow trout below. Fishing is not as good on the lower sections but the rapids are more exciting and challenging. Hiking is popular along the upper section. Trails lead to the hot springs and access

1001-516: A small-scale property of a rock, but determines many of its large-scale properties, such as the density , porosity or permeability . The 3D orientation of the clasts is called the fabric of the rock. The size and form of clasts can be used to determine the velocity and direction of current in the sedimentary environment that moved the clasts from their origin; fine, calcareous mud only settles in quiet water while gravel and larger clasts are moved only by rapidly moving water. The grain size of

1078-952: A term for a fissile mudrock (regardless of grain size) although some older literature uses the term "shale" as a synonym for mudrock. Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include: Chemical sedimentary rock forms when mineral constituents in solution become supersaturated and inorganically precipitate . Common chemical sedimentary rocks include oolitic limestone and rocks composed of evaporite minerals, such as halite (rock salt), sylvite , baryte and gypsum . This fourth miscellaneous category includes volcanic tuff and volcanic breccias formed by deposition and later cementation of lava fragments erupted by volcanoes, and impact breccias formed after impact events . Alternatively, sedimentary rocks can be subdivided into compositional groups based on their mineralogy: Sedimentary rocks are formed when sediment

1155-417: A texture, only the average size of the crystals and the fabric are necessary. Most sedimentary rocks contain either quartz ( siliciclastic rocks) or calcite ( carbonate rocks ). In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. However, the origin of the minerals in a sedimentary rock is often more complex than in an igneous rock. Minerals in

1232-425: A valuable indicator of the biological and ecological environment that existed after the sediment was deposited. On the other hand, the burrowing activity of organisms can destroy other (primary) structures in the sediment, making a reconstruction more difficult. Secondary structures can also form by diagenesis or the formation of a soil ( pedogenesis ) when a sediment is exposed above the water level. An example of

1309-709: A whole. During the Last Glacial Period when sea levels were over 100 metres lower than present day levels, the North and South islands were connected by a vast coastal plain which formed at the South Taranaki Bight . During this period, most of the North Island was covered in thorn scrubland and forest , while the modern-day Northland Peninsula was a subtropical rainforest . Sea levels began to rise 7,000 years ago, eventually separating

1386-503: Is deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension . This sediment is often formed when weathering and erosion break down a rock into loose material in a source area. The material is then transported from the source area to the deposition area. The type of sediment transported depends on the geology of the hinterland (the source area of the sediment). However, some sedimentary rocks, such as evaporites , are composed of material that form at

1463-421: Is a structure where beds with a smaller grain size occur on top of beds with larger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clasts in suspension settle first, then smaller clasts. Although graded bedding can form in many different environments, it is a characteristic of turbidity currents . The surface of a particular bed, called the bedform , can also be indicative of

1540-409: Is called bedding . Single beds can be a couple of centimetres to several meters thick. Finer, less pronounced layers are called laminae, and the structure a lamina forms in a rock is called lamination . Laminae are usually less than a few centimetres thick. Though bedding and lamination are often originally horizontal in nature, this is not always the case. In some environments, beds are deposited at

1617-429: Is higher when the sedimentation rate is high (so that a carcass is quickly buried), in anoxic environments (where little bacterial activity occurs) or when the organism had a particularly hard skeleton. Larger, well-preserved fossils are relatively rare. Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and

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1694-465: Is in the North Island", "my mother lives in the North Island". Maps, headings, tables, and adjectival expressions use North Island without "the". According to Māori mythology , the North and South Islands of New Zealand arose through the actions of the demigod Māui . Māui and his brothers were fishing from their canoe (the South Island) when he caught a great fish and pulled it right up from

1771-491: Is mirrored by the broad categories of rudites , arenites , and lutites , respectively, in older literature. The subdivision of these three broad categories is based on differences in clast shape (conglomerates and breccias), composition (sandstones), or grain size or texture (mudrocks). Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel. Sandstone classification schemes vary widely, but most geologists have adopted

1848-474: Is one of the two main islands of New Zealand , separated from the larger but less populous South Island by Cook Strait . With an area of 113,729 km (43,911 sq mi), it is the world's 14th-largest island , constituting 43% of New Zealand's land area. It has a population of 4,077,800 (June 2024), which is 76% of New Zealand's residents, making it the most populous island in Polynesia and

1925-401: Is reduced. Sediments are typically saturated with groundwater or seawater when originally deposited, and as pore space is reduced, much of these connate fluids are expelled. In addition to this physical compaction, chemical compaction may take place via pressure solution . Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of

2002-493: Is the most stable, followed by feldspar , micas , and finally other less stable minerals that are only present when little weathering has occurred. The amount of weathering depends mainly on the distance to the source area, the local climate and the time it took for the sediment to be transported to the point where it is deposited. In most sedimentary rocks, mica, feldspar and less stable minerals have been weathered to clay minerals like kaolinite , illite or smectite . Among

2079-448: The 28th-most-populous island in the world. Twelve main urban areas (half of them officially cities) are in the North Island. From north to south, they are Whangārei , Auckland , Hamilton , Tauranga , Rotorua , Gisborne , New Plymouth , Napier , Hastings , Whanganui , Palmerston North , and New Zealand's capital city Wellington , which is located at the south-west tip of the island. The island has been known internationally as

2156-506: The North Island for many years. The Te Reo Māori name for it, Te Ika-a-Māui , also has official recognition but it remains seldom used by most residents. On some 19th-century maps, the North Island is named New Ulster (named after Ulster province in northern Ireland) which was also a province of New Zealand that included the North Island. In 2009 the New Zealand Geographic Board found that, along with

2233-611: The Pacific Ocean near the town of Mohaka. There are many gorges on the Mohaka; some as steep as 200m (656 feet). Its main tributaries are the Waipunga, Taharua, Hautapu rivers. The full length is 172 kilometres (107 mi) and it drains a basin of 2,357 square kilometres (910 sq mi). The Mohaka Viaduct is a railway bridge over the lower Mohaka river. The Napier–Taupo road ( State Highway 5 ) has had several bridges over

2310-463: The history of life . The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology . Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in the Earth sciences , such as pedology , geomorphology , geochemistry and structural geology . Sedimentary rocks can be subdivided into four groups based on

2387-682: The organic material of a dead organism undergoes chemical reactions in which volatiles such as water and carbon dioxide are expulsed. The fossil, in the end, consists of a thin layer of pure carbon or its mineralized form, graphite . This form of fossilisation is called carbonisation . It is particularly important for plant fossils. The same process is responsible for the formation of fossil fuels like lignite or coal. Structures in sedimentary rocks can be divided into primary structures (formed during deposition) and secondary structures (formed after deposition). Unlike textures, structures are always large-scale features that can easily be studied in

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2464-590: The 2023 census, 63.1% of North Islanders identified as European ( Pākehā ), 19.8% as Māori , 10.6% as Pacific peoples , 19.3% as Asian , 1.9% as Middle Eastern/Latin American/African, and 1.1% as other ethnicities. Percentages add to more than 100% as people can identify with more than one ethnicity. Māori form the majority in three districts of the North Island: Kawerau (63.2%), Ōpōtiki (66.2%) and Wairoa (68.5%). Europeans formed

2541-501: The Dott scheme, which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of a muddy matrix between the larger grains. Six sandstone names are possible using the descriptors for grain composition (quartz-, feldspathic-, and lithic-) and the amount of matrix (wacke or arenite). For example, a quartz arenite would be composed of mostly (>90%) quartz grains and have little or no clayey matrix between

2618-418: The Mohaka to transport timber. In 2004 a water conservation order was placed on the upper section of the Mohaka because of its fantastic fishing, scenic gorges and outstanding amenity for recreation. The area is also an important habitat for the rare blue duck . More recently it is a popular spot for rafting, kayaking, tramping and fishing. Up in the Kaweka range the Mohaka is a twisting grade II river with

2695-531: The North Island is provided by fifteen District Health Boards (DHBs) . Organised around geographical areas of varying population sizes, they are not coterminous with the Local Government Regions . Sedimentary rock The sedimentary rock cover of the continents of the Earth's crust is extensive (73% of the Earth's current land surface), but sedimentary rock is estimated to be only 8% of

2772-519: The South Island, the North Island had no official name. After a public consultation, the board officially named it North Island, or the aforementioned Te Ika-a-Māui, in October 2013. In prose, the two main islands of New Zealand are called the North Island and the South Island , with the definite article. It is also normal to use the preposition in rather than on , for example "Hamilton

2849-437: The South Island, this is solely due to the North Island having higher natural increase (i.e. births minus deaths) and international migration; since the late 1980s, the internal migration flow has been from the North Island to the South Island. In the year to June 2020, the North Island gained 21,950 people from natural increase and 62,710 people from international migration, while losing 3,570 people from internal migration. At

2926-462: The South Island, with the country's largest city, Auckland, and the capital, Wellington, accounting for nearly half of it. There are 30 urban areas in the North Island with a population of 10,000 or more: The sub-national GDP of the North Island was estimated at NZ$ 282.355 billion in 2021 (78% of New Zealand's national GDP). Nine local government regions cover the North Island and its adjacent islands and territorial waters. Healthcare in

3003-470: The activity of bacteria , can affect minerals in a rock and are therefore seen as part of diagenesis. Deeper burial is accompanied by mesogenesis , during which most of the compaction and lithification takes place. Compaction takes place as the sediments come under increasing overburden (lithostatic) pressure from overlying sediments. Sediment grains move into more compact arrangements, grains of ductile minerals (such as mica ) are deformed, and pore space

3080-412: The banks get steeper and the rapids start off with the largest one on the river, Long Rapid. Scenery on this stretch includes massive boulders, waterfalls falling from the steep canyon walls and fascinating erosion patterns created by the water on the soft stones. The rapids are continuous for the full 18 km with the grade being mostly III+ with some exciting IV/V. The final stretch of the river after

3157-399: The bottom of deep seas and lakes. There is little water mixing in such environments; as a result, oxygen from surface water is not brought down, and the deposited sediment is normally a fine dark clay. Dark rocks, rich in organic material, are therefore often shales. The size , form and orientation of clasts (the original pieces of rock) in a sediment is called its texture . The texture is

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3234-504: The cement and the clasts (including fossils and ooids ) of a carbonate sedimentary rock usually consist of carbonate minerals. The mineralogy of a clastic rock is determined by the material supplied by the source area, the manner of its transport to the place of deposition and the stability of that particular mineral. The resistance of rock-forming minerals to weathering is expressed by the Goldich dissolution series . In this series, quartz

3311-421: The cement to produce secondary porosity . At sufficiently high temperature and pressure, the realm of diagenesis makes way for metamorphism , the process that forms metamorphic rock . The color of a sedimentary rock is often mostly determined by iron , an element with two major oxides: iron(II) oxide and iron(III) oxide . Iron(II) oxide (FeO) only forms under low oxygen ( anoxic ) circumstances and gives

3388-411: The circumnavigation of New Zealand. The maps described the North Island as " Ea Heinom Auwe " and " Aeheinomowe ", which recognises the "Fish of Māui" element. Another Māori name that was given to the North Island, but is now used less commonly, is Aotearoa . Use of Aotearoa to describe the North Island fell out of favour in the early 20th century, and it is now a collective Māori name for New Zealand as

3465-451: The conclusion of the Otago gold rush in the 1860s, New Zealand's European population growth has experienced a steady 'Northern drift' as population centres in the North Island have grown faster than those of New Zealand's South Island. This population trend has continued into the twenty-first century, but at a much slower rate. While the North Island's population continues to grow faster than

3542-572: The current is in one direction, such as rivers. The longer flank of such ripples is on the upstream side of the current. Symmetric wave ripples occur in environments where currents reverse directions, such as tidal flats. Mudcracks are a bed form caused by the dehydration of sediment that occasionally comes above the water surface. Such structures are commonly found at tidal flats or point bars along rivers. Secondary sedimentary structures are those which formed after deposition. Such structures form by chemical, physical and biological processes within

3619-663: The dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel (>2 mm diameter), sand (1/16 to 2 mm diameter), and mud (<1/16 mm diameter). Mud is further divided into silt (1/16 to 1/256 mm diameter) and clay (<1/256 mm diameter). The classification of clastic sedimentary rocks parallels this scheme; conglomerates and breccias are made mostly of gravel, sandstones are made mostly of sand , and mudrocks are made mostly of mud. This tripartite subdivision

3696-489: The field. Sedimentary structures can indicate something about the sedimentary environment or can serve to tell which side originally faced up where tectonics have tilted or overturned sedimentary layers. Sedimentary rocks are laid down in layers called beds or strata . A bed is defined as a layer of rock that has a uniform lithology and texture. Beds form by the deposition of layers of sediment on top of each other. The sequence of beds that characterizes sedimentary rocks

3773-404: The flow calms and the particles settle out of suspension . Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud. Mudrocks can be divided into siltstones, composed dominantly of silt-sized particles; mudstones with subequal mixture of silt- and clay-sized particles; and claystones, composed mostly of clay-sized particles. Most authors use " shale " as

3850-418: The grade V returns to grade II as it mellows and flows into the ocean. More steep walled banks of river and side creeks can be seen down this section as they cascade down, creating substantially high tiered waterfalls and smaller side gorges. The vegetation changes to become more lush, featuring silver fern or punga, eventually easing off to cleared farm land towards the ocean Water level is usually highest in

3927-485: The grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. The increased pressure and temperature stimulate further chemical reactions, such as the reactions by which organic material becomes lignite or coal. Lithification follows closely on compaction, as increased temperatures at depth hasten the precipitation of cement that binds the grains together. Pressure solution contributes to this process of cementation , as

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4004-510: The grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc. Although the Dott classification scheme is widely used by sedimentologists, common names like greywacke , arkose , and quartz sandstone are still widely used by non-specialists and in popular literature. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles. These relatively fine-grained particles are commonly transported by turbulent flow in water or air, and deposited as

4081-587: The host rock. For example, a shell consisting of calcite can dissolve while a cement of silica then fills the cavity. In the same way, precipitating minerals can fill cavities formerly occupied by blood vessels , vascular tissue or other soft tissues. This preserves the form of the organism but changes the chemical composition, a process called permineralization . The most common minerals involved in permineralization are various forms of amorphous silica ( chalcedony , flint , chert ), carbonates (especially calcite), and pyrite . At high pressure and temperature,

4158-660: The islands and linking the Cook Strait to the Tasman Sea . The North Island has an estimated population of 4,077,800 as of June 2024. The North Island had a population of 3,808,005 at the 2023 New Zealand census , an increase of 213,453 people (5.9%) since the 2018 census , and an increase of 570,957 people (17.6%) since the 2013 census . Of the total population, 733,893 people (19.3%) were aged under 15 years, 743,154 (19.5%) were 15 to 29, 1,721,427 (45.2%) were 30 to 64, and 609,534 (16.0%) were 65 or older. Ever since

4235-435: The lithologies dehydrates. Clay can be easily compressed as a result of dehydration, while sand retains the same volume and becomes relatively less dense. On the other hand, when the pore fluid pressure in a sand layer surpasses a critical point, the sand can break through overlying clay layers and flow through, forming discordant bodies of sedimentary rock called sedimentary dykes . The same process can form mud volcanoes on

4312-450: The mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. This further reduces porosity and makes the rock more compact and competent . Unroofing of buried sedimentary rock is accompanied by telogenesis , the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the sedimentary rock, such as leaching of some of

4389-496: The more technical grade III rapids begin the river flows past native bush of the Tataraakina block on the left and Waitere Station on the right. This section contains Te Hoe's rapid, which is the largest and longest in this section. There are 3 gorges to be appreciated on this run. Te Kooti’ s bridge marks the start of the grade V and the end of the grade III. After the bridge, it is an easy grade II float for about 4 km then

4466-438: The place of deposition. The nature of a sedimentary rock, therefore, not only depends on the sediment supply, but also on the sedimentary depositional environment in which it formed. As sediments accumulate in a depositional environment, older sediments are buried by younger sediments, and they undergo diagenesis. Diagenesis includes all the chemical, physical, and biological changes, exclusive of surface weathering, undergone by

4543-470: The plurality in the Auckland region (49.8%) and are the majority in the remaining 39 districts. The proportion of North Islanders born overseas at the 2018 census were 29.3%. The most common foreign countries of birth were England (15.4% of overseas-born residents), Mainland China (11.3%), India (10.1%), South Africa (5.9%), Australia (5.5%) and Samoa (5.3%). The North Island has a larger population than

4620-627: The processes responsible for their formation: clastic sedimentary rocks, biochemical (biogenic) sedimentary rocks, chemical sedimentary rocks, and a fourth category for "other" sedimentary rocks formed by impacts, volcanism , and other minor processes. Clastic sedimentary rocks are composed of rock fragments ( clasts ) that have been cemented together. The clasts are commonly individual grains of quartz , feldspar , clay minerals , or mica . However, any type of mineral may be present. Clasts may also be lithic fragments composed of more than one mineral. Clastic sedimentary rocks are subdivided according to

4697-480: The result of localized precipitation due to small differences in composition or porosity of the host rock, such as around fossils, inside burrows or around plant roots. In carbonate rocks such as limestone or chalk , chert or flint concretions are common, while terrestrial sandstones sometimes contain iron concretions. Calcite concretions in clay containing angular cavities or cracks are called septarian concretions . After deposition, physical processes can deform

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4774-437: The river approaches the ocean, as does farmland. Approximately seven fault lines cross the Mohaka river. Most are found in the upper section but the most obvious to untrained observers is found just above a rapid called Red Rock in the grade III section. It becomes apparent a fault has been crossed as the type of rock changes almost instantly. Commercial rafting is run on all sections of the Mohaka. Most sections can be run as

4851-464: The river in various places along the way. They are maintained by the Department of Conservation. Hunting for sika and red deer as well as pigs and goats is common. A permit is required. North Island The North Island ( Māori : Te Ika-a-Māui [tɛ i.kɐ ɐ mɑː.ʉ.i] , lit. 'the fish of Māui', officially North Island or Te Ika-a-Māui or historically New Ulster )

4928-454: The rock a grey or greenish colour. Iron(III) oxide (Fe 2 O 3 ) in a richer oxygen environment is often found in the form of the mineral hematite and gives the rock a reddish to brownish colour. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an important process, giving the rock a red or orange colour. Thick sequences of red sedimentary rocks formed in arid climates are called red beds . However,

5005-469: The same size, the rock is called 'well-sorted', and when there is a large spread in grain size, the rock is called 'poorly sorted'. The form of the clasts can reflect the origin of the rock. For example, coquina , a rock composed of clasts of broken shells, can only form in energetic water. The form of a clast can be described by using four parameters: Chemical sedimentary rocks have a non-clastic texture, consisting entirely of crystals. To describe such

5082-486: The sea. While he was not looking, his brothers fought over the fish and chopped it up. This great fish became the North Island, and thus a Māori name for the North Island is Te Ika-a-Māui ("The Fish of Māui"). The mountains and valleys are believed to have been formed as a result of Māui's brothers' hacking at the fish. During Captain James Cook 's voyage between 1769 and 1770 , Tahitian navigator Tupaia accompanied

5159-433: The sediment supply, caused, for example, by seasonal changes in rainfall, temperature or biochemical activity. Laminae that represent seasonal changes (similar to tree rings ) are called varves . Any sedimentary rock composed of millimeter or finer scale layers can be named with the general term laminite . When sedimentary rocks have no lamination at all, their structural character is called massive bedding. Graded bedding

5236-402: The sediment, producing a third class of secondary structures. Density contrasts between different sedimentary layers, such as between sand and clay, can result in flame structures or load casts , formed by inverted diapirism . While the clastic bed is still fluid, diapirism can cause a denser upper layer to sink into a lower layer. Sometimes, density contrasts occur or are enhanced when one of

5313-443: The sediment. They can be indicators of circumstances after deposition. Some can be used as way up criteria . Organic materials in a sediment can leave more traces than just fossils. Preserved tracks and burrows are examples of trace fossils (also called ichnofossils). Such traces are relatively rare. Most trace fossils are burrows of molluscs or arthropods . This burrowing is called bioturbation by sedimentologists. It can be

5390-469: The settlers arrived. Archeological surveys have found extensive evidence of villages and temporary camps along the lower and upper Mohaka. The Maori collected hāngī stones, taupunga, opunga, poutama, kowhaturi from the riverbed. The famous Maori, Te Kooti , reportedly frequented the area. Te Kooti's Lookout, said to be the site of a fight with the British, is visible from the river. Later, Europeans used

5467-422: The spring, dropping naturally throughout the summer but can rise quickly in heavy rains. Slips are common and trees can sometimes be found drifting or jammed between boulders. There is an automatic river gauge which posts water level information (NIWA) or contact the local rafting companies for current conditions. Much of the Mohaka runs through sedimentary rock . In the upper sections, greywacke commonly forms

5544-407: The steeper banks with smaller stones and shingle forming shallow beds and beaches. Conglomerate , sandstone and limestone begin to dominate the banks and large boulders create the rapids down through the grade III section. Moving closer to the grade V section, the boulders become larger and the banks narrow into a deep gorge. Large conglomerate blocks form the first rapid then the river eases into

5621-453: The subsurface that is useful for civil engineering , for example in the construction of roads , houses , tunnels , canals or other structures. Sedimentary rocks are also important sources of natural resources including coal , fossil fuels , drinking water and ores . The study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history , including palaeogeography , paleoclimatology and

5698-527: The three major types of rock, fossils are most commonly found in sedimentary rock. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. Often these fossils may only be visible under magnification . Dead organisms in nature are usually quickly removed by scavengers , bacteria , rotting and erosion, but under exceptional circumstances, these natural processes are unable to take place, leading to fossilisation. The chance of fossilisation

5775-457: The upper Mohaka, with the current one built in 1962. Ngati Pahauwera ’s traditional tribal territory is from the Te Hoe river junction to its mouth. The river, including its waters, bed and fisheries, is a taonga of Ngati Pahauwera. In particular they place great emphasis on the role the river plays in their tribal identity. The Mohaka was an important river for fishing and transport long before

5852-441: The volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks . Sedimentary rocks are deposited in layers as strata , forming a structure called bedding . Sedimentary rocks are often deposited in large structures called sedimentary basins . Sedimentary rocks have also been found on Mars . The study of sedimentary rocks and rock strata provides information about

5929-404: The woody tissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while they were still alive are called trace fossils , examples of which are burrows , footprints , etc. As a part of a sedimentary rock, fossils undergo the same diagenetic processes as does

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