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{{Short description|Clast of rock}} |
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[[File:Cobbles Nash Point.jpg|thumb|Beach cobbles ([[Nash Point]], Wales)|alt=A rock hammer rests atop a variety of well-rounded gray cobbles.]] |
[[File:Cobbles Nash Point.jpg|thumb|Beach cobbles ([[Nash Point]], Wales)|alt=A rock hammer rests atop a variety of well-rounded gray cobbles.]] |
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A '''cobble''' (sometimes a '''cobblestone''') is a [[clastic rocks|clast]] of rock defined on the [[Udden–Wentworth scale]] as having a [[particle size]] of {{convert|64|-|256|mm|sp=us}}, larger than a [[pebble]] and smaller than a [[boulder]]. Other scales define a cobble's size |
A '''cobble''' (sometimes a '''cobblestone''') is a [[clastic rocks|clast]] of rock defined on the [[Udden–Wentworth scale]] as having a [[particle size]] of {{convert|64|-|256|mm|sp=us}}, larger than a [[pebble]] and smaller than a [[boulder]]. Other scales define a cobble's size differently. A rock made predominantly of cobbles is termed a [[conglomerate (geology)|conglomerate]]. [[Cobblestone]] is a building material based on cobbles. |
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==Etymology== |
==Etymology== |
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Cobbles, also called [[cobblestones]], derive their name from the word [[:wiktionary:cob|cob]], meaning a rounded lump. The term is further related to the German ''{{lang|de|Kopf}}'', meaning ''head''.<ref name="wentworth">{{cite journal|last1=Wentworth|first1=C. K.|title=A Scale of Grade and Class Terms for Clastic Sediments|journal=The Journal of Geology|date=July–August 1922|volume=30|issue=5|pages=377–392|jstor=30063207|doi=10.1086/622910|bibcode=1922JG.....30..377W}} {{closed access}}</ref> Chester Wentworth referred to cobbles as ''cobble bowlders'' {{sic}} in his 1922 paper that would become the basis for the Udden–Wentworth scale.<ref name="blairmcpherson">{{cite journal|last1=Blair|first1=T. C.|last2=McPherson|first2=J. G.|title=Grain-size and textural classification of coarse sedimentary particles|journal=Journal of Sedimentary Research|date=January 1999|volume=69|issue=1|pages=6–19|doi=10.2110/jsr.69.6}} {{closed access}}</ref> |
Cobbles, also called [[cobblestones]], derive their name from the word [[:wiktionary:cob|cob]], meaning a rounded lump. The term is further related to the German ''{{lang|de|Kopf}}'', meaning ''head''.<ref name="wentworth">{{cite journal|last1=Wentworth|first1=C. K.|title=A Scale of Grade and Class Terms for Clastic Sediments|journal=The Journal of Geology|date=July–August 1922|volume=30|issue=5|pages=377–392|jstor=30063207|doi=10.1086/622910|bibcode=1922JG.....30..377W|s2cid=128682870|url=https://zenodo.org/record/1431467}} {{closed access}}</ref> Chester Wentworth referred to cobbles as ''cobble bowlders'' {{sic}} in his 1922 paper that would become the basis for the Udden–Wentworth scale.<ref name="blairmcpherson">{{cite journal|last1=Blair|first1=T. C.|last2=McPherson|first2=J. G.|title=Grain-size and textural classification of coarse sedimentary particles|journal=Journal of Sedimentary Research|date=January 1999|volume=69|issue=1|pages=6–19|doi=10.2110/jsr.69.6|bibcode=1999JSedR..69....6B}} {{closed access}}</ref> |
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==Classifications== |
==Classifications== |
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[[File:Hazeva cobbles.jpg|thumb|Sandy conglomerate with cobbles in the Hazeva Formation (Miocene) of southern Israel|alt=A brown-gray cobble conglomerate]] |
[[File:Hazeva cobbles.jpg|thumb|Sandy conglomerate with cobbles in the Hazeva Formation (Miocene) of southern Israel|alt=A brown-gray cobble conglomerate]] |
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| ⚫ | Within the widely used [[Krumbein phi scale]] of [[grain size]]s, cobbles are defined as [[clastic rock|clast]]s 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 {{convert|64|-|256|mm|sp=us}}. On this scale, cobbles are larger than pebbles which measure {{convert|4|-|64|mm|sp=us}} in diameter and smaller than boulders, whose diameters range from {{convert|256|-|4096|mm|sp=us}}. On the Udden–Wentworth scale, an [[lithification|unlithified]] fraction of cobbles is classified as [[gravel]] while a lithified sample primarily composed of cobbles is a [[Conglomerate (geology)|conglomerate]].<ref name="blairmcpherson" /> The Committee on Sedimentation of the US [[National Research Council (United States)|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 joint|exfoliation]]'', etc.).{{sfn|Pettijohn|1975|p=28}} |
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| ⚫ | Within the widely used [[Krumbein phi scale]] of [[grain size]]s, cobbles are defined as [[clastic rock|clast]]s 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 {{convert|64|-|256|mm|sp=us}}. On this scale, cobbles are larger than pebbles which measure {{convert|4|-|64|mm|sp=us}} in diameter and smaller than boulders, whose diameters range from {{convert|256|-|4096|mm|sp=us}}. On the Udden–Wentworth scale, an [[lithification|unlithified]] fraction of cobbles is classified as [[gravel]] while a lithified sample primarily composed of cobbles is a [[Conglomerate (geology)|conglomerate]].<ref name="blairmcpherson" /> The Committee on Sedimentation of the US [[National Research Council (United States)|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 joint|exfoliation]]'', etc.).{{sfn|Pettijohn|1975|p=28}} |
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| ⚫ | 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 {{convert|2|mm|sp=us}} 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 {{convert|60|-|200|mm|sp=us}} while the [[United States Department of Agriculture]]'s definition suggests a range of {{convert|75|-|250|mm|sp=us}} and the [[ISO]] standard [[ISO 14688-1:2002|14688]] names cobbles as ranging from {{convert|63|-|200|mm|sp=us}} in diameter.<ref name="blottpye">{{cite journal|last1=Blott|first1=S. J.|last2=Pye|first2=K.|title=Particle size scales and classification of sediment types based on particle size distributions: Review and recommended procedures|journal=Sedimentology|date=December 2012|volume=59|issue=7|pages=2071–2096|doi=10.1111/j.1365-3091.2012.01335.x}} {{closed access}}</ref> |
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| ⚫ | 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 {{convert|2|mm|sp=us}} 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 {{convert|60|-|200|mm|sp=us}} while the [[United States Department of Agriculture]]'s definition suggests a range of {{convert|75|-|250|mm|sp=us}} and the [[ISO]] standard [[ISO 14688-1:2002|14688]] names cobbles as ranging from {{convert|63|-|200|mm|sp=us}} in diameter.<ref name="blottpye">{{cite journal|last1=Blott|first1=S. J.|last2=Pye|first2=K.|title=Particle size scales and classification of sediment types based on particle size distributions: Review and recommended procedures|journal=Sedimentology|date=December 2012|volume=59|issue=7|pages=2071–2096|doi=10.1111/j.1365-3091.2012.01335.x|bibcode=2012Sedim..59.2071B|s2cid=130084299 }} {{closed access}}</ref> |
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| ⚫ | Various attempts have been made to refine the Udden–Wentworth scale, including its definition of cobbles.<ref name="blottpye" /> In 1968, D. J. Doeglas proposed subdividing the ''cobble'' designation into two fractions, ''small cobbles'' (for particles with diameters from {{convert|64|-|125|mm|sp=us|disp=sqbr}}) and ''large cobbles'' (for particles with diameters from {{convert|125|-|250|mm|sp=us|disp=sqbr}}).<ref name="doeglas">{{cite journal|last1=Doeglas|first1=D. J.|title=Grain-size indices, classification, and environment|journal=Sedimentology|date=March 1968|volume=10|issue=2|pages=83–100|doi=10.1111/j.1365-3091.1968.tb01101.x}} {{closed access}}</ref> 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 {{convert|64|-|128|mm|sp=us}} (−6 to −7 φ) and ''coarse cobbles'' as those with diameters from {{convert|128|-|256|mm|sp=us}} (−7 to −8 φ).<ref name="blairmcpherson" /> 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.<ref name="blottpye" /> |
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| ⚫ | Various attempts have been made to refine the Udden–Wentworth scale, including its definition of cobbles.<ref name="blottpye" /> In 1968, D. J. Doeglas proposed subdividing the ''cobble'' designation into two fractions, ''small cobbles'' (for particles with diameters from {{convert|64|-|125|mm|sp=us|disp=sqbr}}) and ''large cobbles'' (for particles with diameters from {{convert|125|-|250|mm|sp=us|disp=sqbr}}).<ref name="doeglas">{{cite journal|last1=Doeglas|first1=D. J.|title=Grain-size indices, classification, and environment|journal=Sedimentology|date=March 1968|volume=10|issue=2|pages=83–100|doi=10.1111/j.1365-3091.1968.tb01101.x|bibcode=1968Sedim..10...83D}} {{closed access}}</ref> 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 {{convert|64|-|128|mm|sp=us}} (−6 to −7 φ) and ''coarse cobbles'' as those with diameters from {{convert|128|-|256|mm|sp=us}} (−7 to −8 φ).<ref name="blairmcpherson" /> 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.<ref name="blottpye" /> |
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==Settings== |
==Settings== |
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When occurring in streams, cobbles are likely to be found in mountain valley [[streambed]]s that are moderately steep.{{sfn|Bunte & Abt|2001|p=1}} Cobbles are also transported by [[glacier]]s 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 [[Striation (geology)|striations]] on lateral facets.{{sfn|Pettijohn|1975|pp=173–174}} |
When occurring in streams, cobbles are likely to be found in mountain valley [[streambed]]s that are moderately steep.{{sfn|Bunte & Abt|2001|p=1}} Cobbles are also transported by [[glacier]]s 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. The term [[shingle beach]] refers to a [[beach]] covered with small- to medium-sized cobbles or pebbles (as opposed to fine sand). Glacially transported cobbles tend to share several identifying features including a tabular shape and downward diagonal [[Striation (geology)|striations]] on lateral facets.{{sfn|Pettijohn|1975|pp=173–174}} |
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Cobble conglomerates may be [[Alluvium|alluvial]] in origin or the product of "stone avalanches", a type of [[debris flow]] resulting from unconsolidated cobbles and gravel.{{sfn|Hsü|2004|p=81}} In such stone avalanches, well-rounded cobbles may travel the farthest on account of their low rolling friction.{{sfn|Hsü|2004|p=66}} When the product of alluvial processes, the cobble conglomerate's [[Matrix (geology)|matrix]] consists of gravel and coarse [[sand]]. In contrast, the matrices of flow-deposited conglomerates are primarily [[mud]].{{sfn|Hsü|2004|p=81}} |
Cobble conglomerates may be [[Alluvium|alluvial]] in origin or the product of "stone avalanches", a type of [[debris flow]] resulting from unconsolidated cobbles and gravel.{{sfn|Hsü|2004|p=81}} In such stone avalanches, well-rounded cobbles may travel the farthest on account of their low rolling friction.{{sfn|Hsü|2004|p=66}} When the product of alluvial processes, the cobble conglomerate's [[Matrix (geology)|matrix]] consists of gravel and coarse [[sand]]. In contrast, the matrices of flow-deposited conglomerates are primarily [[mud]].{{sfn|Hsü|2004|p=81}} |
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==References== |
==References== |
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{{ |
{{Reflist|30em}} |
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===Cited=== |
===Cited=== |
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* {{cite |
* {{cite report|last1=Bunte|first1=K.|last2=Abt|first2=S. R.|title=Sampling Surface and Subsurface Particle-Size Distributions in Wadable Gravel- and Cobble-Bed Streams for Analyses in Sediment Transport, Hydraulics, and Streambed Monitoring|url=http://www.fs.fed.us/rm/pubs/rmrs_gtr074.pdf|website=Rocky Mountain Research Station|publisher=United States Forest Service|access-date=March 18, 2016|location=Fort Collins, CO|date=2001|ref={{harvid|Bunte & Abt|2001}}|doi=10.2737/RMRS-GTR-74|hdl=2027/uc1.31210020572028|hdl-access=free}} |
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* {{cite book|last1=Hsü|first1=K. J.|title=Physics of Sedimentology: Textbook and Reference|date=2004|publisher=Springer|location=Berlin|isbn=3-540-20620- |
* {{cite book|last1=Hsü|first1=K. J.|title=Physics of Sedimentology: Textbook and Reference|date=2004|publisher=Springer|location=Berlin|isbn=978-3-540-20620-0|edition=2nd}} |
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* {{cite book|last1=Pettijohn|first1=F. J.|title=Sedimentary Rocks|date=1975|publisher=Harper and Row|location=New York City|isbn=0-06-045191-2|edition=3rd |
* {{cite book|last1=Pettijohn|first1=F. J.|title=Sedimentary Rocks|date=1975|publisher=Harper and Row|location=New York City|isbn=978-0-06-045191-2|edition=3rd}} |
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{{coastal geography}} |
{{coastal geography}} |
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[[Category:Stone]] |
[[Category:Stone (material)]] |
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[[Category:Sedimentology]] |
[[Category:Sedimentology]] |
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[[Category:Granularity of materials]] |
[[Category:Granularity of materials]] |
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Latest revision as of 14:56, 7 September 2024
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 differently. A rock made predominantly of cobbles is termed a conglomerate. Cobblestone is a building material based on cobbles.
Etymology
[edit]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]
Classifications
[edit]
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]
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]
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]
Settings
[edit]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. The term shingle beach refers to a beach covered with small- to medium-sized cobbles or pebbles (as opposed to fine sand). Glacially transported cobbles tend to share several identifying features including a tabular shape and downward diagonal striations on lateral facets.[7]
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]
References
[edit]- ^ 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. S2CID 128682870.
- ^ a b c 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.
- ^ Pettijohn 1975, p. 28.
- ^ a b c 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. S2CID 130084299.
- ^ 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.
- ^ Bunte & Abt 2001, p. 1.
- ^ Pettijohn 1975, pp. 173–174.
- ^ a b Hsü 2004, p. 81.
- ^ Hsü 2004, p. 66.
Cited
[edit]- Bunte, K.; Abt, S. R. (2001). Sampling Surface and Subsurface Particle-Size Distributions in Wadable Gravel- and Cobble-Bed Streams for Analyses in Sediment Transport, Hydraulics, and Streambed Monitoring (PDF). Rocky Mountain Research Station (Report). Fort Collins, CO: United States Forest Service. doi:10.2737/RMRS-GTR-74. hdl:2027/uc1.31210020572028. Retrieved March 18, 2016.
- Hsü, K. J. (2004). Physics of Sedimentology: Textbook and Reference (2nd ed.). Berlin: Springer. ISBN 978-3-540-20620-0.
- Pettijohn, F. J. (1975). Sedimentary Rocks (3rd ed.). New York City: Harper and Row. ISBN 978-0-06-045191-2.
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