Concretion#Septarian concretions

File:Concretion_rock_from_Middle_Jurassic_of_Iran.jpg

Detailed studies have demonstrated that concretions form after sediments are buried but before the sediment is fully lithified during diagenesis.{{Cite journal|title=The Moeraki Boulders; anatomy of some septarian concretions|url=https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/55/3/398/97797/the-moeraki-boulders-anatomy-of-some-septarian|journal=Journal of Sedimentary Research|date=1985-05-01|issn=1527-1404|pages=398–406|volume=55|issue=3|doi=10.1306/212F86E3-2B24-11D7-8648000102C1865D|language=en|first1=P.|last1=Dale|first2=C. A.|last2=Landis|first3=J. R.|last3=Boles}}{{Cite journal|title=Isotopic evidence for origin of the Moeraki septarian concretions, New Zealand|url=https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/59/2/272/113809/isotopic-evidence-for-origin-of-the-moeraki|journal=Journal of Sedimentary Research|date=1989-03-01|issn=1527-1404|pages=272–279|volume=59|issue=2|doi=10.1306/212F8F6C-2B24-11D7-8648000102C1865D|language=en|first1=James R.|last1=Boles|first2=Geoffrey D.|last2=Thyne}}{{Cite journal|title=The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England|journal=Sedimentology|date=1991|issn=1365-3091|pages=79–106|volume=38|issue=1|doi=10.1111/j.1365-3091.1991.tb01856.x|language=en|first=I. C.|last=Scotchman|bibcode=1991Sedim..38...79S}}{{cite journal |last1=Mozley |first1=Peter S. |last2=Burns |first2=Stephen J. |title=Oxygen and Carbon Isotopic Composition of Marine Carbonate Concretions: An Overview |journal=SEPM Journal of Sedimentary Research |date=1993 |volume=63 |doi=10.1306/D4267A91-2B26-11D7-8648000102C1865D |url=https://www.researchgate.net/publication/279585759 |access-date=19 August 2021}}{{cite journal|title=Calcite-Cemented Concretions in Cretaceous Sandstone, Wyoming and Utah, U.S.A.|url=https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/73/3/462/99315/calcite-cemented-concretions-in-cretaceous|journal=Journal of Sedimentary Research|date=2003-05-01|issn=1527-1404|pages=462–483|volume=73|issue=3|doi=10.1306/111602730462|language=en|first1=Kitty L.|last1=Milliken|first2=M. Dane|last2=Picard|first3=Earle F.|last3=McBride|bibcode=2003JSedR..73..462M}}{{cite journal|title=Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale, microbially induced, chemical heterogeneity in groundwater|journal=GSA Bulletin|date=2005-11-01|issn=0016-7606|pages=1400–1412|volume=117|issue=11–12|doi=10.1130/B25618.1|language=en|first1=J. Matthew|last1=Davis|first2=Peter S.|last2=Mozley|bibcode=2005GSAB..117.1400M}} They typically form when a mineral precipitates and cements sediment around a nucleus, which is often organic, such as a leaf, tooth, piece of shell or fossil. For this reason, fossil collectors commonly break open concretions in their search for fossil animal and plant specimens.{{cite book |last1=Prothero |first1=Donald R. |last2=Schwab |first2=Fred |title=Sedimentary geology : an introduction to sedimentary rocks and stratigraphy |date=2004 |publisher=W.H. Freeman |location=New York |isbn=0716739054 |page=118 |edition=2nd}} Some of the most unusual concretion nuclei are World War II military shells, bombs, and shrapnel, which are found inside siderite concretions found in an English coastal salt marsh.{{cite journal|title=Complex cementation textures and authigenic mineral assemblages in Recent concretions from the Lincolnshire Wash (east coast, UK) driven by Fe(0) to Fe(II) oxidation|url=http://iugspace.iugaza.edu.ps/xmlui/bitstream/handle/20.500.12358/26050/Al-Aghaetal1995RecentconcretionsfromtheWash.pdf?sequence=1&isAllowed=y|journal=Journal of the Geological Society|date=1995-02-01|issn=0016-7649|pages=157–171|volume=152|issue=1|doi=10.1144/gsjgs.152.1.0157|language=en|first1=J.|last1=Esson|first2=C. D.|last2=Curtis|first3=S. D.|last3=Burley|first4=M. R.|last4=Al-AGHA |bibcode=1995JGSoc.152..157A |s2cid=129359274|archive-url=https://web.archive.org/web/20191213005520/http://iugspace.iugaza.edu.ps/xmlui/bitstream/handle/20.500.12358/26050/Al-Aghaetal1995RecentconcretionsfromtheWash.pdf?sequence=1&isAllowed=y|archive-date=2019-12-13|url-status=dead}}

Depending on the environmental conditions present at the time of their formation, concretions can be created by either concentric or pervasive growth.{{cite journal|title=Relationship between oriented calcite concretions and permeability correlation structure in an alluvial aquifer, Sierra Ladrones Formation, New Mexico|url=https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/66/1/11/98701/relationship-between-oriented-calcite-concretions|journal=Journal of Sedimentary Research|date=1996-01-01|issn=1527-1404|pages=11–16|volume=66|issue=1|doi=10.1306/D4268293-2B26-11D7-8648000102C1865D|language=en|first1=J. Matthew|last1=Davis|first2=Peter S.|last2=Mozley}}{{Cite journal|title=Mudrock-hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition|url=https://jgs.lyellcollection.org/content/157/1/239|journal=Journal of the Geological Society|date=2000-01-01|issn=0016-7649|pages=239–251|volume=157|issue=1|doi=10.1144/jgs.157.1.239|language=en|first1=Q. J.|last1=Fisher|first2=R.|last2=Raiswell|bibcode=2000JGSoc.157..239R|s2cid=128897857}} In concentric growth, the concretion grows as successive layers of mineral precipitate around a central core. This process results in roughly spherical concretions that grow with time. In the case of pervasive growth, cementation of the host sediments, by infilling of its pore space by precipitated minerals, occurs simultaneously throughout the volume of the area, which in time becomes a concretion. Concretions are often exposed at the surface by subsequent erosion that removes the weaker, uncemented material.

File:Rock concretions from Hells Hollow State Park, Pennsylvania.jpg in Pennsylvania]]

Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=9780199653065 |edition=Fourth |chapter=concretion}} to huge bodies three meters in diameter and weighing several thousand pounds.{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0131547283 |edition=4th |page=114}} The giant, red concretions occurring in Theodore Roosevelt National Park, in North Dakota, are almost {{convert|3|m|ft|abbr=on|sp=us}} in diameter.{{cite web |url=http://www.parkrec.nd.gov/byways/theodore/theodore.html |title=Theodore Roosevelt National Park North Unit Scenic Byway |access-date=2012-05-13 |url-status=dead |archive-url=https://web.archive.org/web/20120508204512/http://www.parkrec.nd.gov/byways/theodore/theodore.html |archive-date=2012-05-08 }} Spheroidal concretions, as large as {{convert|9|m|ft|abbr=on|sp=us}} in diameter, have been found eroding out of the Qasr el Sagha Formation within the Faiyum depression of Egypt.{{cite journal |last1=Abdel-Wahab |first1=A. |last2=McBride |first2=E. F. |title=Origin of Giant Calcite-Cemented Concretions, Temple Member, Qasr El Sagha Formation (Eocene), Faiyum Depression, Egypt |journal=Journal of Sedimentary Research |date=1 January 2001 |volume=71 |issue=1 |pages=70–81 |doi=10.1306/031700710070|bibcode=2001JSedR..71...70A }} Concretions occur in a wide variety of shapes, including spheres, disks, tubes, and grape-like or soap bubble-like aggregates.{{cite journal |last1=Todd |first1=J. E. |title=Concretions and their geological effects |journal=Geological Society of America Bulletin |date=1 January 1903 |volume=14 |issue=1 |pages=353–368 |doi=10.1130/GSAB-14-353|bibcode=1903GSAB...14..353T |url=https://zenodo.org/record/1671335 }}

File:Concretions embedded in sandstone in Anza Borrego Desert State Park in California.jpg in the United States]]

Concretions are commonly composed of a mineral present as a minor component of the host rock. For example, concretions in sandstones or shales are commonly formed of a carbonate mineral such as calcite; those in limestones are commonly an amorphous or microcrystalline form of silica such as chert, flint, or jasper; while those in black shale may be composed of pyrite.{{sfn|Prothero|Schwab|2004|p=118}} Other minerals that form concretions include iron oxides or hydroxides (such as goethite and hematite),{{cite journal |last1=Busigny |first1=Vincent |last2=Dauphas |first2=Nicolas |title=Tracing paleofluid circulations using iron isotopes: A study of hematite and goethite concretions from the Navajo Sandstone (Utah, USA) |journal=Earth and Planetary Science Letters |date=February 2007 |volume=254 |issue=3–4 |pages=272–287 |doi=10.1016/j.epsl.2006.11.038|bibcode=2007E&PSL.254..272B }}{{cite journal |last1=Parry |first1=W.T. |title=Composition, nucleation, and growth of iron oxide concretions |journal=Sedimentary Geology |date=January 2011 |volume=233 |issue=1–4 |pages=53–68 |doi=10.1016/j.sedgeo.2010.10.009|bibcode=2011SedG..233...53P }} dolomite, siderite,{{cite journal |last1=Curtis |first1=C.D. |last2=Coleman |first2=M.L. |last3=Love |first3=L.G. |title=Pore water evolution during sediment burial from isotopic and mineral chemistry of calcite, dolomite and siderite concretions |journal=Geochimica et Cosmochimica Acta |date=October 1986 |volume=50 |issue=10 |pages=2321–2334 |doi=10.1016/0016-7037(86)90085-2|bibcode=1986GeCoA..50.2321C }} ankerite,{{cite journal |last1=Strickler |first1=Michael E. |title=Fe Substitution for Al in Glauconite with Increasing Diagenesis in the First Wilcox Sandstone (Lower Eocene), Livingston Parish, Louisiana |journal=Clays and Clay Minerals |date=1990 |volume=38 |issue=1 |pages=69–76 |doi=10.1346/CCMN.1990.0380110|bibcode=1990CCM....38...69S |s2cid=140180525 }} marcasite,{{cite journal|last1=Van Horn |first1=F.R. |last2=Van Horn |first2=K.R. |year=1933 |title=X-ray study of pyrite or marcasite concretions in the rocks of the Cleveland, Ohio, quadrangles |journal=American Mineralogist|volume=18 |number=7 |pages=288–294 |url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/18/7/288/536696/X-ray-study-of-pyrite-or-marcasite-concretions-in |access-date=10 August 2021}} barite,{{cite journal |last1=Bréhéret |first1=Jean-G. |last2=Brumsack |first2=Hans-J. |title=Barite concretions as evidence of pauses in sedimentation in the Marnes Bleues Formation of the Vocontian Basin (SE France) |journal=Sedimentary Geology |date=February 2000 |volume=130 |issue=3–4 |pages=205–228 |doi=10.1016/S0037-0738(99)00112-8|bibcode=2000SedG..130..205B }}{{cite journal |last1=Leśniak |first1=P.M. |last2=Łącka |first2=B. |last3=Hladı́kova |first3=J. |last4=Zieliński |first4=G. |title=Origin of barite concretions in the West Carpathian flysch, Poland |journal=Chemical Geology |date=June 1999 |volume=158 |issue=1–2 |pages=155–163 |doi=10.1016/S0009-2541(99)00010-8|bibcode=1999ChGeo.158..155L }} and gypsum.{{cite journal |last1=Cruz |first1=Marí |last2=Pí |first2=A |title=Characterization of Gypsum Concretion in Loess: Some Geotechnical Considerations |journal=From Fundamentals to Applications in Geotechnics |date=2015 |issue=From Fundamentals to Applications in Geotechnics |pages=3248–3255 |doi=10.3233/978-1-61499-603-3-3248}}

Although concretions often consist of a single dominant mineral,{{sfn|Allaby|2013|loc="concretion"}} other minerals can be present depending on the environmental conditions that created them. For example, carbonate concretions, which form in response to the reduction of sulfates by bacteria, often contain minor percentages of pyrite.{{cite journal |last1=Coleman |first1=Max L. |title=Microbial processes: Controls on the shape and composition of carbonate concretions |journal=Marine Geology |date=July 1993 |volume=113 |issue=1–2 |pages=127–140 |doi=10.1016/0025-3227(93)90154-N|bibcode=1993MGeol.113..127C }} Other concretions, which formed as a result of microbial sulfate reduction, consist of a mixture of calcite, barite, and pyrite.{{cite journal |last1=Raiswell |first1=R. |last2=Bottrell |first2=S. H. |last3=Dean |first3=S. P. |last4=Marshall |first4=J. D. |last5=Carr |first5=A. |last6=Hatfield |first6=D. |title=Isotopic constraints on growth conditions of multiphase calcite-pyrite-barite concretions in Carboniferous mudstones: Diagenetic history of septarian concretions in Carboniferous mudstones |journal=Sedimentology |date=25 April 2002 |volume=49 |issue=2 |pages=237–254 |doi=10.1046/j.1365-3091.2002.00439.x|s2cid=129664903 }}

File:Vaqueros Sandstone Sanborn County Park.jpg sandstone with concretions]]

Image:07-ml-3-soil-mosaic-B019R1 br.jpg]]

Concretions are found in a variety of rocks, but are particularly common in shales, siltstones, and sandstones.{{sfn|Boggs|2006|p=114}} They often outwardly resemble fossils or rocks that look as if they do not belong to the stratum in which they were found.{{cite web |title=Concretions |date=3 April 2020 |url=https://www.priweb.org/blog-post/concretions |publisher=Paleontological Research Institution |access-date=11 August 2021}} Occasionally, concretions contain a fossil, either as its nucleus or as a component that has been incorporated during its growth but concretions are not fossils themselves.{{sfn|Prothero|Schwab|2004|p=118}} They appear in nodular patches, concentrated along bedding planes,{{sfn|Prothero|Schwab|2004|p=118}} or protruding from weathered cliffsides.{{cite journal |last1=Marshall |first1=Jim D. |last2=Pirrie |first2=Duncan |title=Carbonate concretions-explained |journal=Geology Today |date=March 2013 |volume=29 |issue=2 |pages=53–62 |doi=10.1111/gto.12002|bibcode=2013GeolT..29...53M |s2cid=129659655 }}

Small hematite concretions or Martian spherules have been observed by the Opportunity rover in the Eagle Crater on Mars.{{cite web |last=Dvorsky |first=George |title=The Enduring Mystery of the Martian 'Blueberries' Discovered by Opportunity Rover |url=https://gizmodo.com/the-enduring-mystery-of-the-martian-blueberries-discove-1832649426 |date=15 February 2019 |work=Gizmodo}}

Concretions vary considerably in their compositions, shapes, sizes and modes of origin.

{{redirect|Septaria|the genus of gastropod snail|Septaria (gastropod)}}File:MoerakiBouldersSunrise.jpg, New Zealand]]

Image:Septarian Nodule.jpg Septarian concretions (or septarian nodules) are carbonate-rich concretions containing angular cavities or cracks (septaria; {{abbr|sg.|singular}} {{linktext|septarium}}, from the Latin {{lang|la|septum}} "partition, separating element", referring to the cracks or cavities separating polygonal blocks of hardened material).{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=septarium}}{{cite web

|url=http://dictionary.reference.com/search?q=septarian|title=septarian|publisher=dictionary.reference.com|access-date=March 20, 2014}} Septarian nodules are characteristically found in carbonate-rich mudrock. They typically show an internal structure of polyhedral blocks (the matrix) separated by mineral-filled radiating cracks (the septaria) which taper towards the rim of the concretion. The radiating cracks sometimes intersect a second set of concentric cracks.{{cite book |last1=Potter |first1=Paul Edwin |last2=Maynard |first2=J. Barry |last3=Pryor |first3=Wayne A. |title=Sedimentology of shale: study guide and reference source |date=1980 |publisher=Springer-Verlag |location=New York |isbn=0387904301 |pages=23, 36}} However, the cracks can be highly variable in shape and volume, as well as the degree of shrinkage they indicate.{{cite journal |last1=Pratt |first1=Brian R. |title=Septarian concretions: internal cracking caused by synsedimentary earthquakes |journal=Sedimentology |date=27 February 2001 |volume=48 |issue=1 |pages=189, 193–194 |doi=10.1046/j.1365-3091.2001.00366.x|bibcode=2001Sedim..48..189P |s2cid=140665532 }} The matrix is typically composed of argillaceous carbonate, such as clay ironstone, while the crack filling is usually calcite. The calcite often contains significant iron (ferroan calcite) and may have inclusions of pyrite and clay minerals. The brown calcite common in septaria may also be colored by organic compounds produced by bacterial decay of organic matter in the original sediments.{{cite journal |last1=Hendry |first1=James P. |last2=Pearson |first2=Michael J. |last3=Trewin |first3=Nigel H. |last4=Fallick |first4=Anthony E. |title=Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis: Jurassic septarian concretions, NW Scotland |journal=Sedimentology |date=16 May 2006 |volume=53 |issue=3 |pages=537–565 |doi=10.1111/j.1365-3091.2006.00779.x|s2cid=130767202 |doi-access=free }}

Septarian concretions are found in many kinds of mudstone, including lacustrine siltstones such as the Beaufort Group of northwest Mozambique,{{cite journal |last1=Melezhik |first1=Victor A. |last2=Fallick |first2=Anthony E. |last3=Smith |first3=Richard A. |last4=Rosse |first4=Danta M. |title=Spherical and columnar, septarian, 18 O-depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids |journal=Sedimentology |date=December 2007 |volume=54 |issue=6 |pages=1389–1416 |doi=10.1111/j.1365-3091.2007.00886.x|bibcode=2007Sedim..54.1389M |s2cid=129030770 }} but are most commonly found in marine shales, such as the Staffin Shale Formation of Skye, the Kimmeridge Clay of England,{{cite journal |last1=Astin |first1=T. R. |last2=Scotchman |first2=I. C. |title=The diagenetic history of some septarian concretions from the Kimmeridge Clay, England |journal=Sedimentology |date=April 1988 |volume=35 |issue=2 |pages=349–368 |doi=10.1111/j.1365-3091.1988.tb00952.x|bibcode=1988Sedim..35..349A }}{{cite journal |last1=Scotchman |first1=I. C. |title=The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England |journal=Sedimentology |date=February 1991 |volume=38 |issue=1 |pages=79–106 |doi=10.1111/j.1365-3091.1991.tb01856.x|bibcode=1991Sedim..38...79S }} or the Mancos Group of North America.{{cite journal |last1=Dale |first1=Annabel |last2=John |first2=Cédric M. |last3=Mozley |first3=Peter S. |last4=Smalley |first4=P. C. |last5=Muggeridge |first5=Ann H. |title=Time-capsule concretions: Unlocking burial diagenetic processes in the Mancos Shale using carbonate clumped isotopes |journal=Earth and Planetary Science Letters |date=May 2014 |volume=394 |pages=30–37 |doi=10.1016/j.epsl.2014.03.004|bibcode=2014E&PSL.394...30D |doi-access=free }}

It is commonly thought that concretions grew incrementally from the inside outwards. Chemical and textural zoning in many concretions are consistent with this concentric model of formation. However, the evidence is ambiguous, and many or most concretions may have formed by pervasive cementation of the entire volume of the concretion at the same time.{{cite journal |last1=Mozley |first1=Peter S. |title=The internal structure of carbonate concretions in mudrocks: a critical evaluation of the conventional concentric model of concretion growth |journal=Sedimentary Geology |date=May 1996 |volume=103 |issue=1–2 |pages=85–91 |doi=10.1016/0037-0738(95)00087-9|bibcode=1996SedG..103...85M }}{{cite journal |last1=Raiswell |first1=R. |last2=Fisher |first2=Q. J. |title=Mudrock-hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition |journal=Journal of the Geological Society |date=January 2000 |volume=157 |issue=1 |pages=239–251 |doi=10.1144/jgs.157.1.239|bibcode=2000JGSoc.157..239R |s2cid=128897857 }} For example, if the porosity after early cementation varies across the concretion, then later cementation filling this porosity would produce compositional zoning even with uniform pore water composition. Whether the initial cementation was concentric or pervasive, there is considerable evidence that it occurred quickly and at shallow depth of burial.{{cite journal |last1=Thyne |first1=Geoffrey D. |last2=Boles |first2=James R. |title=Isotopic Evidence for Origin of the Moeraki Septarian Concretions, New Zealand |journal=SEPM Journal of Sedimentary Research |date=1989 |volume=59 |doi=10.1306/212F8F6C-2B24-11D7-8648000102C1865D}}{{cite journal |last1=Duck |first1=R. W. |title=Subaqueous shrinkage cracks and early sediment fabrics preserved in Pleistocene calcareous concretions |journal=Journal of the Geological Society |date=February 1995 |volume=152 |issue=1 |pages=151–156 |doi=10.1144/gsjgs.152.1.0151|bibcode=1995JGSoc.152..151D |s2cid=129928697 }}{{cite journal |last1=De Craen |first1=M. |last2=Swennen |first2=R. |last3=Keppens |first3=E. |title=Petrography and geochemistry of septarian carbonate concretions from the Boom Clay Formation (Oligocene, Belgium) |journal=Geologie en Mijnbouw |date=1998 |volume=77 |issue=1 |pages=63–76 |doi=10.1023/A:1003468328212|s2cid=126635562 }} In many cases, there is clear evidence that the initial concretion formed around some kind of organic nucleus.{{sfn|Potter|Maynard|Pryor|1980|p=23}}

The origin of the carbonate-rich septaria is still debated. One possibility is that dehydration hardens the outer shell of the concretion while causing the interior matrix to shrink until it cracks. Shrinkage of a still-wet matrix may also take place through syneresis, in which the particles of colloidal material in the interior of the concretion become gradually more tightly bound while expelling water. Another possibility is that early cementation reduces the permeability of the concretion, trapping pore fluids and creating excess pore pressure during continued burial. This could crack the interior at depths as shallow as {{convert|10|m||sp=us}}.{{cite journal |last1=Hounslow |first1=Mark W. |title=Significance of localized pore pressures to the genesis of septarian concretions |journal=Sedimentology |date=November 1997 |volume=44 |issue=6 |pages=1133–1147 |doi=10.1046/j.1365-3091.1997.d01-64.x|bibcode=1997Sedim..44.1133H |s2cid=130385560 }} A more speculative theory is that the septaria form by brittle fracturing resulting from earthquakes.{{sfn|Pratt|2001|pp=189-213}} Regardless of the mechanism of crack formation, the septaria, like the concretion itself, likely form at a relatively shallow depth of burial of less than {{convert|50|m||sp=us}}{{cite journal |last1=Astin |first1=T. R. |title=Septarian crack formation in carbonate concretions from shales and mudstones |journal=Clay Minerals |date=October 1986 |volume=21 |issue=4 |pages=617–631 |doi=10.1180/claymin.1986.021.4.12|bibcode=1986ClMin..21..617A |s2cid=128609480 }} and possibly as little as {{convert|12|m||sp=us}}. Geologically young concretions of the Errol Beds of Scotland show texture consistent with formation from flocculated sediments containing organic matter, whose decay left tiny gas bubbles (30 to 35 microns in diameter) and a soap of calcium fatty acids salts. The conversion of these fatty acids to calcium carbonate may have promoted shrinkage and fracture of the matrix.

One model for the formation of septarian concretions in the Staffin Shales suggests that the concretions started as semirigid masses of flocculated clay. The individual colloidal clay particles were bound by extracellular polymeric substances or EPS produced by colonizing bacteria. The decay of these substances, together with syneresis of the host mud, produced stresses that fractured the interiors of the concretions while still at shallow burial depth. This was possible only with the bacterial colonization and the right sedimentation rate. Additional fractures formed during subsequent episodes of shallow burial (during the Cretaceous) or uplift (during the Paleogene). Water derived from rain and snow (meteoric water) later infiltrated the beds and deposited ferroan calcite in the cracks.

Septarian concretions often record a complex history of formation that provides geologists with information on early diagenesis, the initial stages of the formation of sedimentary rock from unconsolidated sediments. Most concretions appear to have formed at depths of burial where sulfate-reducing microorganisms are active.{{cite journal |last1=Pearson |first1=M.J. |last2=Hendry |first2=J.P. |last3=Taylor |first3=C.W. |last4=Russell |first4=M.A. |title=Fatty acids in sparry calcite fracture fills and microsparite cement of septarian diagenetic concretions |journal=Geochimica et Cosmochimica Acta |date=April 2005 |volume=69 |issue=7 |pages=1773–1786 |doi=10.1016/j.gca.2004.09.024|bibcode=2005GeCoA..69.1773P }} This corresponds to burial depths of {{convert|15 to 150|m||sp=us}}, and is characterized by generation of carbon dioxide, increased alkalinity and precipitation of calcium carbonate.{{cite journal |last1=Raiswell |first1=R. |last2=Fisher |first2=Q.J. |title=Rates of carbonate cementation associated with sulphate reduction in DSDP/ODP sediments: implications for the formation of concretions |journal=Chemical Geology |date=November 2004 |volume=211 |issue=1–2 |pages=71–85 |doi=10.1016/j.chemgeo.2004.06.020 |bibcode=2004ChGeo.211...71R |url=http://eprints.whiterose.ac.uk/404/1/raiswellr6.pdf |access-date=2021-08-19 |archive-date=2022-01-30 |archive-url=https://web.archive.org/web/20220130203933/https://eprints.whiterose.ac.uk/404/1/raiswellr6.pdf |url-status=dead }} However, there is some evidence that formation continues well into the methanogenic zone beneath the sulfate reduction zone.{{cite journal |last1=Huggett |first1=J. M. |title=Diagenesis of mudrocks and concretions from the London Clay Formation in the London Basin |journal=Clay Minerals |date=October 1994 |volume=29 |issue=4 |pages=693–707 |doi=10.1180/claymin.1994.029.4.22|bibcode=1994ClMin..29..693H |s2cid=129727119 }}

A spectacular example of boulder septarian concretions, which are as much as {{convert|3|m|ft|abbr=off|sp=us}} in diameter, are the Moeraki Boulders. These concretions are found eroding out of Paleocene mudstone of the Moeraki Formation exposed along the coast near Moeraki, South Island, New Zealand. They are composed of calcite-cemented mud with septarian veins of calcite and rare late-stage quartz and ferrous dolomite.{{cite journal |last1=Boles |first1=J.R. |last2=Landis |first2=C.A. |last3=Dale |first3=P. |title=The Moeraki Boulders – Anatomy of Some Septarian Concretions |journal=SEPM Journal of Sedimentary Research |date=1985 |volume=55 |pages=398–406 |doi=10.1306/212F86E3-2B24-11D7-8648000102C1865D}}Fordyce, E., and P. Maxwell, 2003, Canterbury Basin Paleontology and Stratigraphy, Geological Society of New Zealand Annual Field Conference 2003 Field Trip 8, Miscellaneous Publication 116B, Geological Society of New Zealand, Dunedin, New Zealand. {{ISBN|0-908678-97-5}}Forsyth, P.J., and G. Coates, 1992, The Moeraki boulders. Institute of Geological & Nuclear Sciences, Information Series no. 1, (Lower Hutt, New Zealand)Thyne, G.D., and J.R. Boles, 1989, [http://jsedres.sepmonline.org/cgi/content/abstract/59/2/272?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&author1=Thyne&andorexactfulltext=and&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT Isotopic evidence for origin of the Moeraki septarian concretions, New Zealand], Journal of Sedimentary Petrology. v. 59, n. 2, p. 272–279. The much smaller septarian concretions found in the Kimmeridge Clay exposed in cliffs along the Wessex coast of England are more typical examples of septarian concretions.{{cite journal |last1=Astin |first1=T. R. |title=The diagenetic history of some septarian concretions from the Kimmeridge Clay, England |journal=Sedimentology |volume=35 |issue=2 |pages=349–368 |doi=10.1111/j.1365-3091.1988.tb00952.x |year=1988 |bibcode=1988Sedim..35..349A }}

File:Bowling Balls Beach 2 edit.jpg (Mendocino County, California, United States) weathered out of steeply tilted Cenozoic mudstone]]

Cannonball concretions are large spherical concretions, which resemble cannonballs. These are found along the Cannonball River within Morton and Sioux Counties, North Dakota, and can reach {{convert|3|m|ft|abbr=on|sp=us}} in diameter. They were created by early cementation of sand and silt by calcite. Similar cannonball concretions, which are as much as {{convert|4|to|6|m|ft|abbr=on|sp=us}} in diameter, are found associated with sandstone outcrops of the Frontier Formation in northeast Utah and central Wyoming. They formed by the early cementation of sand by calcite. Somewhat weathered and eroded giant cannonball concretions, as large as {{convert|6|m|ft|abbr=off|sp=us}} in diameter, occur in abundance at "Rock City" in Ottawa County, Kansas. Large and spherical boulders are also found along Koekohe beach near Moeraki on the east coast of the South Island of New Zealand.Dann, C., and Peat, N. (1989) Dunedin, North and South Otago. Wellington: GP Books. {{ISBN|0-477-01438-0}} The Moeraki Boulders, Ward Beach boulders and Koutu Boulders of New Zealand are examples of septarian concretions, which are also cannonball concretions. Large spherical rocks, which are found on the shore of Lake Huron near Kettle Point, Ontario, and locally known as "kettles", are typical cannonball concretions. Cannonball concretions have also been reported from Van Mijenfjorden, Spitsbergen; near Haines Junction, Yukon Territory, Canada; Jameson Land, East Greenland; near Mecevici, Ozimici, and Zavidovici in Bosnia-Herzegovina; in Alaska in the Kenai Peninsula Captain Cook State Park on north of Cook Inlet beach{{cite web |url=http://cookinletconcretions.com/Kenai%20Article.htm |title=Kenai Peninsula Online – Alaska Newspaper – |access-date=2010-05-13 |url-status=dead |archive-url=https://web.archive.org/web/20110708191432/http://cookinletconcretions.com/Kenai%20Article.htm |archive-date=2011-07-08 }} and on Kodiak Island northeast of Fossil Beach.{{cite web|url=https://books.google.com/books?id=bGlonWEE-8YC&q=Fossil+beach+kodiak+concretions&pg=RA1-PA17|title=Geological Survey Professional Paper|date=24 May 1976|publisher=U.S. Government Printing Office|via=Google Books}} This type of concretion is also found in Romania, where they are known as trovants.{{cite web |title=trovant |url=https://dexonline.ro/definitie/trovant |work=dexonline.ro |accessdate=October 3, 2024}}Emma Davies, "[https://www.sciencefocus.com/planet-earth/trovants These ‘living’ rocks can give birth to baby stones]", 8 August 2023, BBC Science Focus

Image:OrdovicianEdrio.jpg; Kope Formation (Upper Ordovician), northern Kentucky]]

Image:HiatusConcretionsIsrael060910.jpg (Upper Cretaceous), the Negev, southern Israel]]

Hiatus concretions are distinguished by their stratigraphic history of exhumation, exposure and reburial. They are found where submarine erosion has concentrated early diagenetic concretions as lag surfaces by washing away surrounding fine-grained sediments. Their significance for stratigraphy, sedimentology and paleontology was first noted by Voigt who referred to them as Hiatus-Konkretionen.{{cite journal |last1=Voigt |first1=Ehrhard |title=Über Hiatus-Konkretionen (dargestellt an Beispielen aus dem Lias) |journal=Geologische Rundschau |date=October 1968 |volume=58 |issue=1 |pages=281–296 |doi=10.1007/BF01820609|bibcode=1968GeoRu..58..281V |s2cid=128842746 }} "Hiatus" refers to the break in sedimentation that allowed this erosion and exposure. They are found throughout the fossil record but are most common during periods in which calcite sea conditions prevailed, such as the Ordovician, Jurassic and Cretaceous.{{cite journal |last1=Zatoń |first1=Michał |title=Hiatus concretions |journal=Geology Today |date=24 September 2010 |volume=26 |issue=5 |pages=186–189 |doi=10.1111/j.1365-2451.2010.00762.x|bibcode=2010GeolT..26..186Z |s2cid=247665440 }} Most are formed from the cemented infillings of burrow systems in siliciclastic or carbonate sediments.

A distinctive feature of hiatus concretions separating them from other types is that they were often encrusted by marine organisms including bryozoans, echinoderms and tube worms in the Paleozoic{{cite journal |last1=Wilson |first1=M. A. |title=Disturbance and Ecologic Succession in an Upper Ordovician Cobble-Dwelling Hardground Fauna |journal=Science |date=3 May 1985 |volume=228 |issue=4699 |pages=575–577 |doi=10.1126/science.228.4699.575|pmid=17736081 |bibcode=1985Sci...228..575W |s2cid=28818298 }} and bryozoans, oysters and tube worms in the Mesozoic and Cenozoic. Hiatus concretions are also often significantly bored by worms and bivalves.{{cite journal |last1=Wilson |first1=Mark A. |last2=Taylor |first2=Paul D. |title=Palaeocology of Hard Substrate Faunas from the Cretaceous Qahlah Formation of the Oman Mountains |journal=Palaeontology |date=February 2001 |volume=44 |issue=1 |pages=21–41 |doi=10.1111/1475-4983.00167|bibcode=2001Palgy..44...21W |s2cid=129664357 |doi-access=free }}

Elongate concretions form parallel to sedimentary strata and have been studied extensively due to the inferred influence of phreatic (saturated) zone groundwater flow direction on the orientation of the axis of elongation.{{cite journal |last1=Johnson |first1=M.R. |title=Paleogeographic Significance of Oriented Calcareous Concretions in the Triassic Katberg Formation, South Africa |journal=SEPM Journal of Sedimentary Research |date=1989 |volume=59 |pages=1008–1010 |doi=10.1306/212F90D9-2B24-11D7-8648000102C1865D}}{{cite journal |last1=McBride |first1=E. F. |last2=Picard |first2=M. D. |last3=Milliken |first3=K. L. |title=Calcite-Cemented Concretions in Cretaceous Sandstone, Wyoming and Utah, U.S.A. |journal=Journal of Sedimentary Research |date=1 May 2003 |volume=73 |issue=3 |pages=462–483 |doi=10.1306/111602730462|bibcode=2003JSedR..73..462M }}{{cite journal |last1=Mozley |first1=Peter S. |last2=Goodwin |first2=Laurel B. |title=Patterns of cementation along a Cenozoic normal fault: A record of paleoflow orientations |journal=Geology |date=1 June 1995 |volume=23 |issue=6 |pages=539–542 |doi=10.1130/0091-7613(1995)023<0539:POCAAC>2.3.CO;2|bibcode=1995Geo....23..539M }}{{cite journal |last1=Mozley |first1=Peter S. |last2=Davis |first2=J. Matthew |title=Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale, microbially induced, chemical heterogeneity in groundwater |journal=Geological Society of America Bulletin |date=2005 |volume=117 |issue=11 |pages=1400 |doi=10.1130/B25618.1|bibcode=2005GSAB..117.1400M }} In addition to providing information about the orientation of past fluid flow in the host rock, elongate concretions can provide insight into local permeability trends (i.e., permeability correlation structure; variation in groundwater velocity,{{cite journal |last1=Davis |first1=J. Matthew |title=Oriented carbonate concretions in a paleoaquifer: Insights into geologic controls on fluid flow |journal=Water Resources Research |date=June 1999 |volume=35 |issue=6 |pages=1705–1711 |doi=10.1029/1999WR900042|bibcode=1999WRR....35.1705D |s2cid=129502157 |doi-access=free }} and the types of geological features that influence flow.

Elongate concretions are well known in the Kimmeridge Clay formation of northwest Europe. In outcrops, where they have acquired the name "doggers", they are typically only a few meters across, but in the subsurface they can be seen to penetrate up to tens of meters of along-hole dimension. Unlike limestone beds, however, it is impossible to consistently correlate them between even closely spaced wells.{{citation needed|date=May 2010}}

Image:MoquiMarble1.jpg

Moqui Marbles, also called Moqui balls or "Moki marbles", are iron oxide concretions which can be found eroding in great abundance out of outcrops of the Navajo Sandstone within south-central and southeastern Utah. These concretions range in shape from spheres to discs, buttons, spiked balls, cylindrical forms, and other odd shapes. They range from pea-size to baseball-size.{{cite journal |last1=Chan |first1=M.A. |first2=W.T. |last2=Parry |year=2002 |title=Mysteries of Sandstone Colors and Concretions in Colorado Plateau Canyon Country |journal=Utah Geological Survey Public Information Series |volume=77 |pages=1–19 |url=https://ugspub.nr.utah.gov/publications/public_information/PI-77.pdf |access-date=18 August 2021}}{{cite journal |last1=Catling |first1=David C. |title=On Earth, as it is on Mars? |journal=Nature |date=June 2004 |volume=429 |issue=6993 |pages=707–708 |doi=10.1038/429707a|pmid=15201892 |s2cid=4393420 |doi-access=free }}

The concretions were created by the precipitation of iron, which was dissolved in groundwater. The iron was originally present as a thin film of iron oxide surrounding sand grains in the Navajo Sandstone. Groundwater containing methane or petroleum from underlying rock beds reacted with the iron oxide, converting it to soluble reduced iron. When the iron-bearing groundwater came into contact with more oxygen-rich groundwater, the reduced iron was converted back to insoluble iron oxide, which formed the concretions.{{cite journal |last1=Chan |first1=M.A. |first2=B.B. |last2=Beitler |first3=W.T. |last3=Parry |first4=J. |last4=Ormo |first5=G. |last5=Komatsu |year=2005 |title=Red Rock and Red Planet Diagenesis: Comparison of Earth and Mars Concretions |journal=GSA Today |volume=15 |number=8 |pages=4–10 |doi=10.1130/1052-5173(2005)015[4:RRARPD]2.0.CO;2 |url=https://www.geosociety.org/gsatoday/archive/15/8/pdf/i1052-5173-15-8-4.pdf |access-date=18 August 2021}} It is possible that reduced iron first formed siderite concretions that were subsequently oxidized. Iron-oxidizing bacteria may have played a role.{{cite journal |last1=Loope |first1=David B. |last2=Kettler |first2=Richard M. |last3=Weber |first3=Karrie A. |title=Morphologic Clues to the Origins of Iron Oxide–Cemented Spheroids, Boxworks, and Pipelike Concretions, Navajo Sandstone of South-Central Utah, U.S.A. |journal=The Journal of Geology |date=September 2011 |volume=119 |issue=5 |pages=505–520 |doi=10.1086/661110|bibcode=2011JG....119..505L |s2cid=10139364 |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1197&context=bioscifacpub }}

Kansas pop rocks are concretions of either iron sulfide, i.e. pyrite and marcasite, or in some cases jarosite, which are found in outcrops of the Smoky Hill Chalk Member of the Niobrara Formation within Gove County, Kansas. They are typically associated with thin layers of altered volcanic ash, called bentonite, that occur within the chalk comprising the Smoky Hill Chalk Member. A few of these concretions enclose, at least in part, large flattened valves of inoceramid bivalves. These concretions range in size from a few millimeters to as much as {{convert|0.7|m|ft|abbr=on|sp=us}} in length and {{convert|12|cm|ft|abbr=on|sp=us}} in thickness. Most of these concretions are oblate spheroids. Other "pop rocks" are small polycuboid pyrite concretions, which are as much as {{convert|7|cm|ft|abbr=on|sp=us}} in diameter. These concretions are called "pop rocks" because they explode if thrown in a fire. Also, when they are either cut or hammered, they produce sparks and a burning sulfur smell. Contrary to what has been published on the Internet, none of the iron sulfide concretions, which are found in the Smoky Hill Chalk Member were created by either the replacement of fossils or by metamorphic processes. In fact, metamorphic rocks are completely absent from the Smoky Hill Chalk Member.{{cite journal |last1=Hattin |first1=D.E. |year=1982 |title=Stratigraphy and depositional environment of the Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the type area, western Kansas |journal=Kansas Geological Survey Bulletin |volume=225 |pages=1–108}} Instead, all of these iron sulfide concretions were created by the precipitation of iron sulfides within anoxic marine calcareous ooze after it had accumulated and before it had lithified into chalk.

File:Marleka fairy stone from Stensö in Sweden.JPG

Iron sulfide concretions, such as the Kansas Pop rocks, consisting of either pyrite and marcasite, are nonmagnetic.{{cite journal |last1=Hobbs |first1=D |last2=Hafner |first2=J |title=Magnetism and magneto-structural effects in transition-metal sulphides |journal=Journal of Physics: Condensed Matter |date=25 October 1999 |volume=11 |issue=42 |pages=8197–8222 |doi=10.1088/0953-8984/11/42/303|bibcode=1999JPCM...11.8197H |s2cid=250900204 }} On the other hand, iron sulfide concretions, which either are composed of or contain either pyrrhotite or smythite, will be magnetic to varying degrees.{{cite journal |last1=Hoffmann |first1=Viktor |last2=Stanjek |first2=Helge |last3=Murad |first3=Enver |title=Mineralogical, magnetic and mössbauer data of symthite (Fe9S11) |journal=Studia Geophysica & Geodætica |date=December 1993 |volume=37 |issue=4 |pages=366–381 |doi=10.1007/BF01613583|bibcode=1993StGG...37..366H |s2cid=131123088 }} Prolonged heating of either a pyrite or marcasite concretion will convert portions of either mineral into pyrrhotite causing the concretion to become slightly magnetic.

Disc concretions composed of calcium carbonate are often found eroding out of exposures of interlaminated silt and clay, varved, proglacial lake deposits. For example, great numbers of strikingly symmetrical concretions have been found eroding out of outcrops of Quaternary proglacial lake sediments along and in the gravels of the Connecticut River and its tributaries in Massachusetts and Vermont. Depending the specific source of these concretions, they vary in an infinite variety of forms that include disc-shapes; crescent-shapes; watch-shapes; cylindrical or club-shapes; botryoidal masses; and animal-like forms. They can vary in length from {{convert|2|in|cm|abbr=on|sp=us}} to over {{convert|22|in|cm|abbr=on|sp=us}} and often exhibit concentric grooves on their surfaces. In the Connecticut River Valley, these concretions are often called "claystones" because the concretions are harder than the clay enclosing them. In local brickyards, they were called "clay-dogs" either because of their animal-like forms or the concretions were nuisances in molding bricks.{{cite journal |last1=Gratacap |first1=L.P. |year=1884 |title=Opinions Upon Clay Stones and Concretions |journal=The American Naturalist |volume=18 |number=9 |doi=10.1086/273756 |pages=882–892 |bibcode=1884ANat...18..882G |s2cid=84690956 |url=https://www.journals.uchicago.edu/doi/pdf/10.1086/273756 |access-date=18 August 2021}}{{cite book |last1=Sheldon |first1=J.M.A. |year=1900 |title=Concretions from the Champlain clays of the Connecticut Valley |publisher=University Press |location=Boston |page=74 |url=https://books.google.com/books?id=tUkPAAAAYAAJ&dq=Sheldon,+J.M.A.,+1900.+%27%27Concretions+from+the+Champlain+clays+of+the+Connecticut+Valley.%27%27+University+Press,+Boston.+pp.74.&pg=PA7 |access-date=18 August 2021}}{{cite journal |last1=Tarr |first1=W. A. |title=Concretions in the Champlain formation of the Connecticut River Valley |journal=Geological Society of America Bulletin |date=31 October 1935 |volume=46 |issue=10 |pages=1493–1534 |doi=10.1130/GSAB-46-1493|bibcode=1935GSAB...46.1493T }} Similar disc-shaped calcium carbonate concretions have also been found in the Harricana River valley in the Abitibi-Témiscamingue administrative region of Quebec, and in Östergötland county, Sweden. In Scandinavia, they are known as "marlekor" ("fairy stones").{{cite journal |last1=Kindle |first1=E. M. |title=Range and Distribution of Certain Types of Canadian Pleistocene Concretions |journal=Geological Society of America Bulletin |date=30 September 1923 |volume=34 |issue=3 |pages=609–648 |doi=10.1130/GSAB-34-609|bibcode=1923GSAB...34..609K }}Warkentin, B.P., 1967. Carbonate content of concretions in varved sediments. Canadian Journal of Earth Sciences, 4(2), pp.333-333.

File:Gogotte sandstone concretion (5784278101).jpg

{{ill|Gogottes|fr|Gogotte (géologie)}} are sandstone concretions found in Oligocene (~30 million years) aged sediments near Fontainebleau, France. Gogottes have fetched high prices at auction due to their sculpture-like quality.{{cite news|last=Haigney|first=Sophie|date=2021-06-18|title=Once Again, Fossils Are Hot|language=en-US|work=The New York Times|url=https://www.nytimes.com/2021/06/18/arts/fossils-private-sale-collectors.html|access-date=2021-07-14|issn=0362-4331}}

{{clear}}

• {{annotated link|Bowling Ball Beach}}
• {{annotated link|Caliche|aka=calcrete}} in arid and semi-arid soils
• {{annotated link|Champ Island}}
• {{annotated link|Diagenesis}}
• {{annotated link|Dinocochlea|Dinocochlea}} in the Natural History Museum, London
• {{annotated link|Dorodango}}
• {{annotated link|Gypcrust}}. CaSO₄ concretions in arid and semi-arid soils
• {{annotated link|Klerksdorp sphere}}
• {{annotated link|Martian spherules}}
• {{annotated link|Moeraki Boulders}}
• {{annotated link|Mushroom Rock State Park}}
• {{annotated link|Nodule (geology)}}, a replacement body, not to be confused with a concretion
• {{annotated link|Rock City, Kansas}}
• {{annotated link|Speleothem}}. CaCO₃

{{Reflist}}

{{Commons category|Concretions}}

• Dietrich, R.V., 2002, [https://web.archive.org/web/20081202111650/http://www.cst.cmich.edu/users/dietr1rv/concretions/index.htm Carbonate Concretions – A Bibliography], The Wayback Machine. and [http://condor.cmich.edu/cdm/singleitem/collection/p1610-01coll1/id/3165/rec/4228 PDF file of Carbonate Concretions – A Bibliography] {{Webarchive|url=https://web.archive.org/web/20141217052538/http://condor.cmich.edu/cdm/singleitem/collection/p1610-01coll1/id/3165/rec/4228 |date=2014-12-17 }}, CMU Online Digital Object Repository, Central Michigan University, Mount Pleasant, Michigan.
• Biek, B., 2002, [https://www.dmr.nd.gov/ndgs/ndnotes/concretions/concretions.asp Concretions and Nodules in North Dakota] North Dakota Geological Survey, Bismarck, North Dakota.
• Everhart, M., 2004, [http://www.oceansofkansas.com/FieldGuide5.html A Field Guide to Fossils of the Smoky Hill ChalkPart 5: Coprolites, Pearls, Fossilized Wood and other Remains] Part of the [http://www.oceansofkansas.com Oceans of Kansas] web site.
• Hansen, M.C., 1994, [https://web.archive.org/web/20081216220206/http://www.ohiodnr.com/Portals/10/pdf/GeoFacts/geof04.pdf Ohio Shale Concretions PDF version, 270 KB] Ohio Division of Geological Survey GeoFacts n. 4, pp. 1–2.
• Hanson, W.D., and J.M. Howard, 2005, [https://web.archive.org/web/20100527173151/http://www.geology.ar.gov/mis_pub_info/MP%2022%20Prim%20Boulders.pdf Spherical Boulders in North-Central Arkansas PDF version, 2.8 MB] Arkansas Geological Commission Miscellaneous Publication n. 22, pp. 1–23.
• Heinrich, P.V., 2007, [http://www.hgms.org/BBG/Aug07.pdf The Giant Concretions of Rock City Kansas PDF version, 836 KB] {{Webarchive|url=https://web.archive.org/web/20161020134413/http://www.hgms.org/BBG/Aug07.pdf |date=2016-10-20 }} BackBender's Gazette. vol. 38, no. 8, pp. 6–12.
• Hokianga Tourism Association, nd, [http://hokiangatourism.org.nz/activities/attractions/koutu-boulders Koutu Boulders ANY ONE FOR A GAME OF BOWLS?] and [https://web.archive.org/web/20081014032805/http://hokianga.net.nz/hokianga/koutu/koutu_boulders.htm Koutu Boulders, Hokianga Harbour, Northland, New Zealand] High-quality pictures of cannonball concretions.
• Irna, 2006, [http://irna.lautre.net/Tout-ce-que-la-nature-ne-peut-pas,45.html All that nature can never do, part IV : stone spheres] {{Webarchive|url=https://web.archive.org/web/20160303225759/http://irna.lautre.net/Tout-ce-que-la-nature-ne-peut-pas,45.html |date=2016-03-03 }}
• Irna, 2007a, [http://irna.lautre.net/Stone-balls-in-France-too.html Stone balls : in France too!] {{Webarchive|url=https://web.archive.org/web/20150713044420/http://irna.lautre.net/Stone-balls-in-France-too.html |date=2015-07-13 }}
• Irna, 2007b, [http://irna.lautre.net/Boules-de-pierre-en-Slovaquie.html Stone balls in Slovakia, Czech Republic and Poland] {{Webarchive|url=https://web.archive.org/web/20080315043453/http://irna.lautre.net/Boules-de-pierre-en-Slovaquie.html |date=2008-03-15}}
• Katz, B., 1998, [http://www.desertusa.com/mag98/oct/papr/geo_conc.html Concretions] Digital West Media, Inc.
• Kuban, Glen J., 2006–2008. [http://paleo.cc/paluxy/nevada.htm Nevada Shoe Prints?]
• McCollum, A., nd, [http://allanmccollum.net/amcimages/imperialvalleyconcretions.html Sand Concretions from Imperial Valley], a collection of articles maintained by an American artist.
• Mozley, P.S., [https://web.archive.org/web/20120304075635/http://allanmccollum.net/amcimages/mozley.html Concretions, bombs, and groundwater], on-line version of an overview paper originally published by the New Mexico Bureau of Geology and Mineral Resources.
• United States Geological Survey, nd, [https://web.archive.org/web/20141015163848/http://nd.water.usgs.gov/lewisandclark/points/concretions.html Cannonball concretion]
• University of Utah, 2004, [http://unews.utah.edu/news_releases/earth-has-039blueberries039-like-mars/ Earth Has 'Blueberries' Like Mars 'Moqui Marbles' Formed in Groundwater in Utah's National Parks] {{Webarchive|url=https://web.archive.org/web/20150802011237/http://unews.utah.edu/news_releases/earth-has-039blueberries039-like-mars/ |date=2015-08-02 }} press release about iron oxide and Martian concretions
• Tessa Koumoundouros: [https://www.sciencealert.com/these-strange-bulging-geological-manifestations-loom-over-romania These Eerie 'Living Stones' in Romania Are Fantastical, And Totally Real]. On: science^alert. 25 December 2020: About Trovants in Costești, Ulmet and other locations in Romania

Category:Sedimentary rocks

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Category:Mineralogy

Category:Pseudofossils

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