Cobble (geology)

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Beach cobbles (Nash Point, Wales) Cobbles Nash Point.jpg
Beach cobbles (Nash Point, Wales)

A cobble (sometimes a cobblestone) is a clast of rock defined on the Udden–Wentworth scale as having a particle size of 64–256 millimeters (2.5–10.1 in), larger than a pebble and smaller than a boulder. Other scales define a cobble's size in slightly different terms. A rock made predominantly of cobbles is termed a conglomerate. Cobblestone is a building material based on cobbles.

Particle size

Particle size is a notion introduced for comparing dimensions of solid particles (flecks), liquid particles (droplets), or gaseous particles (bubbles). The notion of particle size applies to colloidal particles, particles in ecology, particles present in granular material, and particles that form a granular material.

Pebble clast of rock (between 4-64 mm in diameter according to Wentworth-Udden scale)

A pebble is a clast of rock with a particle size of 2 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered larger than granules and smaller than cobbles. A rock made predominantly of pebbles is termed a conglomerate. Pebble tools are among the earliest known man-made artifacts, dating from the Palaeolithic period of human history.

Boulder natural rock fragment (larger than 200 mm in diameter according to ISO 14688)

In geology, a boulder is a rock fragment with size greater than 25.6 centimetres (10.1 in) in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In common usage, a boulder is too large for a person to move. Smaller boulders are usually just called rocks or stones. The word boulder is short for boulder stone, from Middle English bulderston or Swedish bullersten.

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Etymology

Cobbles, also called cobblestones, derive their name from the word cob, meaning a rounded lump. The term is further related to the German Kopf, meaning head. [1] Chester Wentworth referred to cobbles as cobble bowlders[ sic ] in his 1922 paper that would become the basis for the Udden–Wentworth scale. [2]

The Latin adverb sic inserted after a quoted word or passage indicates that the quoted matter has been transcribed or translated exactly as found in the source text, complete with any erroneous, archaic, or otherwise nonstandard spelling. It also applies to any surprising assertion, faulty reasoning, or other matter that might be likely interpreted as an error of transcription.

Classifications

Sandy conglomerate with cobbles in the Hazeva Formation (Miocene) of southern Israel Hazeva cobbles.jpg
Sandy conglomerate with cobbles in the Hazeva Formation (Miocene) of southern Israel

Within the widely used Krumbein phi scale of grain sizes, cobbles are defined as clasts of rock ranging from −6 to −8 φ. This classification corresponds with the Udden–Wentworth size scale which defines cobbles as clasts with diameters from 64–256 millimeters (2.5–10.1 in). On this scale, cobbles are larger than pebbles which measure 4–64 millimeters (0.16–2.52 in) in diameter and smaller than boulders, whose diameters range from 256–4,096 millimeters (10.1–161.3 in). On the Udden–Wentworth scale, an unlithified fraction of cobbles is classified as gravel while a lithified sample primarily composed of cobbles is a conglomerate. [2] The Committee on Sedimentation of the US National Research Council has recommended that in situ cobbles be identified by their process of origination, if possible (e.g. cobbles by disintegration, by exfoliation , etc.). [3]

Grain size diameter of individual grains of sediment, or of lithified particles in clastic rocks

Grain size is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, gravel, and cobbles, to boulders.

Clastic rock type of sedimentary rock

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

Lithification is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithification is a process of porosity destruction through compaction and cementation. Lithification includes all the processes which convert unconsolidated sediments into sedimentary rocks. Petrifaction, though often used as a synonym, is more specifically used to describe the replacement of organic material by silica in the formation of fossils.

In the late 1800s and early to mid-1900s, prior to the Udden–Wentworth scale's widespread adoption, size classifications tended to group all particles larger than 2 millimeters (0.079 in) together as gravel or stones. Other scales have defined the size of a cobble slightly differently than the Udden–Wentworth; the British Standards Institution denotes a cobble as any clast ranging in diameter from 60–200 millimeters (2.4–7.9 in) while the United States Department of Agriculture's definition suggests a range of 75–250 millimeters (3.0–9.8 in) and the ISO standard 14688 names cobbles as ranging from 63–200 millimeters (2.5–7.9 in) in diameter. [4]

United States Department of Agriculture U.S. federal executive department responsible for developing and executing federal government policy on farming, agriculture, forestry, and food

The United States Department of Agriculture (USDA), also known as the Agriculture Department, is the U.S. federal executive department responsible for developing and executing federal laws related to farming, forestry, and food. It aims to meet the needs of farmers and ranchers, promote agricultural trade and production, work to assure food safety, protect natural resources, foster rural communities and end hunger in the United States and internationally.

Various attempts have been made to refine the Udden–Wentworth scale, including its definition of cobbles. [4] In 1968, D. J. Doeglas proposed subdividing the cobble designation into two fractions, small cobbles (for particles with diameters from 64–125 millimeters [2.5–4.9 in]) and large cobbles (for particles with diameters from 125–250 millimeters [4.9–9.8 in]). [5] A 1999 paper by Terence C. Blair and John G. McPherson argued that the Udden–Wentworth and Krumbein scales betrayed a historical emphasis on the study of sand grains while ignoring larger gravel grains. They proposed defining fine cobbles as those with diameters from 64–128 millimeters (2.5–5.0 in) (−6 to −7 φ) and coarse cobbles as those with diameters from 128–256 millimeters (5.0–10.1 in) (−7 to −8 φ). [2] In 2012, Simon J. Blott and Kenneth Pye suggested that the cobble designation be eliminated altogether, replaced by very small boulder and small boulder designations equivalent in size to Blair and McPherson's fine and coarse cobbles, respectively. [4]

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When occurring in streams, cobbles are likely to be found in mountain valley streambeds that are moderately steep. [6] Cobbles are also transported by glaciers and deposited as with other grades of sediment as till. If the till is water-laid, finer particles like sand and pebbles may be entirely washed away, leaving a deposit of only boulders and cobbles. Glacially transported cobbles tend to share several identifying features including a tabular shape and downward diagonal striations on lateral facets. [7]

Glacier Persistent body of ice that is moving under its own weight

A glacier is a persistent body of dense ice that is constantly moving under its own weight; it forms where the accumulation of snow exceeds its ablation over many years, often centuries. Glaciers slowly deform and flow due to stresses induced by their weight, creating crevasses, seracs, and other distinguishing features. They also abrade rock and debris from their substrate to create landforms such as cirques and moraines. Glaciers form only on land and are distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.

Till Unsorted glacial sediment

Till or glacial till is unsorted glacial sediment.

Striation (geology) linear furrow in rock generated from fault movement

In structural geology, striations are linear furrows generated from fault movement. The striation's direction reveal the movement directions in the fault plane.

Cobble conglomerates may be alluvial in origin or the product of "stone avalanches", a type of debris flow resulting from unconsolidated cobbles and gravel. [8] In such stone avalanches, well-rounded cobbles may travel the farthest on account of their low rolling friction. [9] When the product of alluvial processes, the cobble conglomerate's matrix consists of gravel and coarse sand. In contrast, the matrices of flow-deposited conglomerates are primarily mud. [8]

Related Research Articles

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. 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. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, 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.

Silt is granular material of a size between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil or as sediment mixed in suspension with water and soil in a body of water such as a river. It may also exist as soil deposited at the bottom of a water body, like mudflows from landslides. Silt has a moderate specific area with a typically non-sticky, plastic feel. Silt usually has a floury feel when dry, and a slippery feel when wet. Silt can be visually observed with a hand lens, exhibiting a sparkly appearance. It also can be felt by the tongue as granular when placed on the front teeth.

Alluvial fan A fan- or cone-shaped deposit of sediment crossed and built up by streams

Alluvial fans are triangular-shaped deposits of water-transported material, often referred to as alluvium. They are an example of an unconsolidated sedimentary deposit and tend to be larger and more prominent in arid to semi-arid regions. These alluvial fans typically form in elevated or even mountainous regions where there is a rapid change in slope from a high to low gradient. The river or stream carrying the sediment flows at a relatively high velocity due to the high slope angle which is why coarse material is able to remain in the flow. When the slope decreases rapidly into a relatively plain or plateau, the stream loses the energy it needs to move its sediment. Deposition subsequently occurs and the sediment ultimately spreads out, creating an alluvial fan. Three primary zones occur within an alluvial fan which includes the proximal fan, medial fan, and the distal fan.

Lithology science of rocks

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7.

Conglomerate (geology) A coarse-grained clastic sedimentary rock with mainly rounded to subangular clasts

Conglomerate is a coarse-grained clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts, e.g., granules, pebbles, cobbles, and boulders, larger than 2 mm (0.079 in) in diameter. Conglomerates form by the consolidation and lithification of gravel. Conglomerates typically contain finer grained sediment, e.g., either sand, silt, clay or combination of them, called matrix by geologists, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay.

The Folk classification is a technical descriptive classification of sedimentary rocks devised by Robert L. Folk, an influential sedimentary petrologist and Professor Emeritus at the University of Texas.

Dunham classification classification system for carbonate sedimentary rocks

The Dunham classification system for carbonate sedimentary rocks was originally devised by Robert J. Dunham in 1962, and subsequently modified by Embry and Klovan in 1971 to include coarse-grained limestones and sediments that had been organically bound at the time of deposition. The modified Dunham Classification has subsequently become the most widely employed system for the classification of carbonate sedimentary rocks with 89% of workers currently adopting this system over the alternative Folk classification scheme

Stream substrate (sediment) is the material that rests at the bottom of a stream. There are several classification guides. One is:

Roxbury Conglomerate

The Roxbury Conglomerate, also informally known as Roxbury puddingstone, is a name for a rock formation that forms the bedrock underlying most of Roxbury, Massachusetts, now part of the city of Boston. The bedrock formation extends well beyond the limits of Roxbury, underlying part or all of Quincy, Canton, Milton, Dorchester, Dedham, Jamaica Plain, Brighton, Brookline, Newton, Needham, and Dover. It is named for exposures in Roxbury, Boston area.

Graded bedding

In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones. Normally graded beds generally represent depositional environments which decrease in transport energy as time passes, but these beds can also form during rapid depositional events. They are perhaps best represented in turbidite strata, where they indicate a sudden strong current that deposits heavy, coarse sediments first, with finer ones following as the current weakens. They can also form in terrestrial stream deposits.

Puddingstone, also known as either pudding stone or plum-pudding stone, is a popular name applied to a conglomerate that consists of distinctly rounded pebbles whose colors contrast sharply with the color of the finer-grained, often sandy, matrix or cement surrounding them. The rounded pebbles and the sharp contrast in color gives this type of conglomerate the appearance of a raisin or Christmas pudding. There are different types of puddingstone, with different composition, origin, and geographical distribution. Examples of different types of puddingstones include the Hertfordshire, Schunemunk, Roxbury, and St. Joseph Island puddingstones.

Sediment transport The movement of solid particles, typically by gravity and fluid entrainment

Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks, mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting. Sediment transport due to fluid motion occurs in rivers, oceans, lakes, seas, and other bodies of water due to currents and tides. Transport is also caused by glaciers as they flow, and on terrestrial surfaces under the influence of wind. Sediment transport due only to gravity can occur on sloping surfaces in general, including hillslopes, scarps, cliffs, and the continental shelf—continental slope boundary.

Rudite is a general name used for a sedimentary rocks that are composed of rounded or angular detrital grains, i.e. granules, pebbles, cobbles, and boulders, which are coarser than sand in size. Rudites include sedimentary rocks composed of both siliciclastic, i.e. conglomerate and breccia, and carbonate grains, i.e. calcirudite and rudstone. This term is equivalent to the Greek-derived term, psephite. Rudite was initially proposed by Grabau as "rudyte." It is derived from the Latin word, "rudus," for "crushed stone," "rubbish," "debris," and "rubble."

Psephite is either a sediment or sedimentary rock composed of fragments that are coarser than sand and which are enclosed in a matrix that varies in kind and amount. It is equivalent to a rudite. Shingle, gravel, breccia, and especially conglomerate, would all be considered psephites. It is equivalent to the Latin-derived term rudite. Psephite is more commonly used for a metamorphosed rudite.

Imbrication (sedimentology)

In sedimentology imbrication refers to a primary depositional fabric consisting of a preferred orientation of clasts such that they overlap one another in a consistent fashion, rather like a run of toppled dominoes. Imbrication is observed in conglomerates and some volcaniclastic deposits.

A matrix-supported rock is a sedimentary rock of which a defined majority is the fine-grained matrix as opposed to the clasts or allochems. For a conglomerate, a rock is considered matrix-supported when clasts constitute less than 15% of its volume. Matrix support is considered to be characteristic of debris flow deposits, in which clasts are supported within a fabric of mud as they move downstream. Wackestones and mudstones under the Dunham classification of limestones are also considered to be matrix-supported due to the predominance of micrite.

A rock veneer is a geomorphic formation in which rock fragments (clasts) of gravel or cobble size form a thin cover over a surface or hillslope. Rock veneers are typically one or two clasts thick and may partially or fully cover the ground surface. Veneers typically form in semiarid and arid regions where chemical weathering rates and the potential for mass wasting are low. Other names for a rock veneer are rock-fragment cover (RFC), stone pavement, desert pavement, stony mantle, hammada and reg.

A granule is a clast of rock with a particle size of 2 to 4 millimetres based on the Krumbein phi scale of sedimentology. Granules are generally considered to be larger than sand and smaller than pebbles. A rock made predominantly of granules is termed a granule conglomerate.

References

  1. Wentworth, C. K. (July–August 1922). "A Scale of Grade and Class Terms for Clastic Sediments". The Journal of Geology. 30 (5): 377–392. Bibcode:1922JG.....30..377W. doi:10.1086/622910. JSTOR   30063207. Closed Access logo alternative.svg
  2. 1 2 3 Blair, T. C.; McPherson, J. G. (January 1999). "Grain-size and textural classification of coarse sedimentary particles". Journal of Sedimentary Research. 69 (1): 6–19. Bibcode:1999JSedR..69....6B. doi:10.2110/jsr.69.6. Closed Access logo alternative.svg
  3. Pettijohn 1975, p. 28.
  4. 1 2 3 Blott, S. J.; Pye, K. (December 2012). "Particle size scales and classification of sediment types based on particle size distributions: Review and recommended procedures". Sedimentology. 59 (7): 2071–2096. Bibcode:2012Sedim..59.2071B. doi:10.1111/j.1365-3091.2012.01335.x. Closed Access logo alternative.svg
  5. Doeglas, D. J. (March 1968). "Grain-size indices, classification, and environment". Sedimentology. 10 (2): 83–100. Bibcode:1968Sedim..10...83D. doi:10.1111/j.1365-3091.1968.tb01101.x. Closed Access logo alternative.svg
  6. Bunte & Abt 2001, p. 1.
  7. Pettijohn 1975, pp. 173–174.
  8. 1 2 Hsü 2004, p. 81.
  9. Hsü 2004, p. 66.

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