{"id":508,"date":"2021-09-16T19:29:41","date_gmt":"2021-09-16T19:29:41","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/accphysicalgeography\/chapter\/12-2-folding-physical-geology-2nd-edition\/"},"modified":"2022-02-06T23:14:30","modified_gmt":"2022-02-06T23:14:30","slug":"12-2-folding-physical-geology-2nd-edition","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/chapter\/12-2-folding-physical-geology-2nd-edition\/","title":{"raw":"12.2 Folding \u2014 Physical Geology \u2013 2nd Edition","rendered":"12.2 Folding \u2014 Physical Geology \u2013 2nd Edition"},"content":{"raw":"<div>\r\n<div>\r\n<h1 class=\"entry-title\">12.2 Folding<\/h1>\r\nWhen a body of rock, especially sedimentary rock, is squeezed from the sides by tectonic forces, it is likely to fracture and\/or become faulted if it is cold and brittle, or become folded if it is warm enough to behave in a plastic manner.\r\n\r\nThe nomenclature and geometry of folds are summarized on Figure 12.2.1.\u00a0 An upward fold is called an\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1694\">anticline<\/button><\/strong>\u00a0(or, more accurately, an antiform if we don\u2019t know if the beds have been overturned or not), while a downward fold is called a\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1695\">syncline<\/button><\/strong>, (or a synform if we don\u2019t if the beds have been overturned).\u00a0 In many areas it\u2019s common to find a series of antiforms and synforms (as in Figure 12.5), although some sequences of rocks are folded into a single antiform or synform.\u00a0A plane drawn through the crest of a fold in a series of beds is called the\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1696\">axial plane<\/button><\/strong>\u00a0of the fold.\u00a0The sloping beds on either side of an axial plane are\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1698\">limbs<\/button><\/strong>.\u00a0An antiform or synform is described as\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1697\">symmetrical<\/button><\/strong>\u00a0if the angles between each of limb and the axial plane are generally similar, and\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1699\">asymmetrical<\/button><\/strong>\u00a0if they are not.\u00a0If the axial plane is sufficiently tilted that the beds on one side have been tilted past vertical, the fold is known as an\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1700\">overturned<\/button><\/strong>\u00a0antiform or synform.\r\n\r\n<\/div>\r\n<div><img class=\"wp-image-503\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2021\/09\/antiform-synform.png\" alt=\"\" width=\"900\" height=\"238\" \/>\r\n<div id=\"caption-attachment-529\" class=\"wp-caption-text\">Figure 12.2.1 Examples of different types of folds and fold nomenclature.\u00a0Axial planes are only shown for the antiforms, but synforms also have axial planes.<\/div>\r\n<\/div>\r\n<div class=\"wp-caption alignright\" style=\"width: 400px\"><a>\r\n<img class=\"wp-image-504\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/isoclinal-recumbent-fold.png\" alt=\"&quot;&quot;\" width=\"400\" height=\"195\" \/>\r\n<\/a>\r\n<div class=\"wp-caption-text\">Figure 12.2.2 An isoclinal recumbent fold.<\/div>\r\n<\/div>\r\nA very tight fold, in which the limbs are parallel or nearly parallel to one another is called an <strong><span class=\"glossary-term\">isoclinal fold<\/span><\/strong> (Figure 12.2.2).\u00a0Isoclinal folds that have been overturned to the extent that their limbs are nearly horizontal are called <strong><span class=\"glossary-term\">recumbent folds<\/span><\/strong>.\r\n\r\nFolds can be of any size, and it\u2019s very common to have smaller folds within larger folds (Figure 12.2.3).\u00a0 Large folds can have wavelengths of tens of kilometers, and very small ones might be visible only under a microscope.\r\n<div class=\"wp-caption aligncenter\" style=\"width: 800px\"><img src=\"https:\/\/coloradogeologicalsurvey.org\/wp-content\/uploads\/G-20111220-085912-600x371.jpg\" alt=\"Folding in Owiyukuts Complex, 2.7 billion-year-old metamorphic rocks in Moffat County, Colorado. Photo credit: Colorado Geological Survey.\" \/>\r\n<div class=\"wp-caption-text\">Figure 12.2.3 Folded metamorphic rocks in Moffat County, CO.<\/div>\r\n<\/div>\r\nAntiforms are not necessarily, or even typically, expressed as ridges in the terrain, nor synforms as valleys.\u00a0Folded rocks get eroded just like all other rocks and the topography that results is typically controlled mostly by the resistance of different layers to erosion (Figure 12.2.4).\r\n<div class=\"wp-caption aligncenter\" style=\"width: 900px\"><img class=\"wp-image-506\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/fold-topography.png\" alt=\"\" width=\"900\" height=\"215\" \/>\r\n<div class=\"wp-caption-text\">Figure 12.2.4 Example of the topography in an area of folded rocks that has been eroded.\u00a0In this case the blue and green rocks are most resistant to erosion, and are represented by hills.<\/div>\r\n<\/div>\r\n<div class=\"textbox textbox--exercises\">\r\n<div class=\"textbox__header\">\r\n\r\nFigure 12.2.5 shows folding in the same area of the Rocky Mountains as Figure 12.0.1.\u00a0 Describe the types of folds using the appropriate terms from above (symmetrical, asymmetrical, isoclinal, overturned, recumbent etc.).\u00a0 You might find it useful to first sketch in the axial planes.\r\n<div id=\"attachment_507\" class=\"wp-caption aligncenter\" style=\"width: 800px\"><img class=\"wp-image-507\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Folding-style.jpg\" alt=\"\" width=\"800\" height=\"421\" \/>\r\n<div id=\"caption-attachment-530\" class=\"wp-caption-text\">Figure 12.2.5<\/div>\r\n<\/div>\r\nSee Appendix 3 for <a href=\"back-matter-005-appendix-3-answers-to-exercises.html#exercisea12.1\">Exercise 12.1 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\n<h3>Media Attributions<\/h3>\r\n<ul>\r\n \t<li>Figures 12.2.1, 12.2.2, 12.2.4, 12.2.5: \u00a9 Steven Earle. CC BY.<\/li>\r\n \t<li>12.2.3: Colorado Geological Survey<\/li>\r\n<\/ul>\r\n<\/div>\r\n<!-- pb_fixme -->","rendered":"<div>\n<div>\n<h1 class=\"entry-title\">12.2 Folding<\/h1>\n<p>When a body of rock, especially sedimentary rock, is squeezed from the sides by tectonic forces, it is likely to fracture and\/or become faulted if it is cold and brittle, or become folded if it is warm enough to behave in a plastic manner.<\/p>\n<p>The nomenclature and geometry of folds are summarized on Figure 12.2.1.\u00a0 An upward fold is called an\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1694\">anticline<\/button><\/strong>\u00a0(or, more accurately, an antiform if we don\u2019t know if the beds have been overturned or not), while a downward fold is called a\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1695\">syncline<\/button><\/strong>, (or a synform if we don\u2019t if the beds have been overturned).\u00a0 In many areas it\u2019s common to find a series of antiforms and synforms (as in Figure 12.5), although some sequences of rocks are folded into a single antiform or synform.\u00a0A plane drawn through the crest of a fold in a series of beds is called the\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1696\">axial plane<\/button><\/strong>\u00a0of the fold.\u00a0The sloping beds on either side of an axial plane are\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1698\">limbs<\/button><\/strong>.\u00a0An antiform or synform is described as\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1697\">symmetrical<\/button><\/strong>\u00a0if the angles between each of limb and the axial plane are generally similar, and\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1699\">asymmetrical<\/button><\/strong>\u00a0if they are not.\u00a0If the axial plane is sufficiently tilted that the beds on one side have been tilted past vertical, the fold is known as an\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"531-1700\">overturned<\/button><\/strong>\u00a0antiform or synform.<\/p>\n<\/div>\n<div><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-503\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2021\/09\/antiform-synform.png\" alt=\"\" width=\"900\" height=\"238\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2021\/09\/antiform-synform.png 1024w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2021\/09\/antiform-synform-300x79.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2021\/09\/antiform-synform-768x203.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2021\/09\/antiform-synform-65x17.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2021\/09\/antiform-synform-225x60.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2021\/09\/antiform-synform-350x93.png 350w\" sizes=\"auto, (max-width: 900px) 100vw, 900px\" \/><\/p>\n<div id=\"caption-attachment-529\" class=\"wp-caption-text\">Figure 12.2.1 Examples of different types of folds and fold nomenclature.\u00a0Axial planes are only shown for the antiforms, but synforms also have axial planes.<\/div>\n<\/div>\n<div class=\"wp-caption alignright\" style=\"width: 400px\"><a><br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-504\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/isoclinal-recumbent-fold.png\" alt=\"&quot;&quot;\" width=\"400\" height=\"195\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/isoclinal-recumbent-fold.png 1003w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/isoclinal-recumbent-fold-300x146.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/isoclinal-recumbent-fold-768x374.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/isoclinal-recumbent-fold-65x32.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/isoclinal-recumbent-fold-225x109.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/isoclinal-recumbent-fold-350x170.png 350w\" sizes=\"auto, (max-width: 400px) 100vw, 400px\" \/><br \/>\n<\/a><\/p>\n<div class=\"wp-caption-text\">Figure 12.2.2 An isoclinal recumbent fold.<\/div>\n<\/div>\n<p>A very tight fold, in which the limbs are parallel or nearly parallel to one another is called an <strong><span class=\"glossary-term\">isoclinal fold<\/span><\/strong> (Figure 12.2.2).\u00a0Isoclinal folds that have been overturned to the extent that their limbs are nearly horizontal are called <strong><span class=\"glossary-term\">recumbent folds<\/span><\/strong>.<\/p>\n<p>Folds can be of any size, and it\u2019s very common to have smaller folds within larger folds (Figure 12.2.3).\u00a0 Large folds can have wavelengths of tens of kilometers, and very small ones might be visible only under a microscope.<\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 800px\"><img decoding=\"async\" src=\"https:\/\/coloradogeologicalsurvey.org\/wp-content\/uploads\/G-20111220-085912-600x371.jpg\" alt=\"Folding in Owiyukuts Complex, 2.7 billion-year-old metamorphic rocks in Moffat County, Colorado. Photo credit: Colorado Geological Survey.\" \/><\/p>\n<div class=\"wp-caption-text\">Figure 12.2.3 Folded metamorphic rocks in Moffat County, CO.<\/div>\n<\/div>\n<p>Antiforms are not necessarily, or even typically, expressed as ridges in the terrain, nor synforms as valleys.\u00a0Folded rocks get eroded just like all other rocks and the topography that results is typically controlled mostly by the resistance of different layers to erosion (Figure 12.2.4).<\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 900px\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-506\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/fold-topography.png\" alt=\"\" width=\"900\" height=\"215\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/fold-topography.png 1024w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/fold-topography-300x72.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/fold-topography-768x184.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/fold-topography-65x16.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/fold-topography-225x54.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/fold-topography-350x84.png 350w\" sizes=\"auto, (max-width: 900px) 100vw, 900px\" \/><\/p>\n<div class=\"wp-caption-text\">Figure 12.2.4 Example of the topography in an area of folded rocks that has been eroded.\u00a0In this case the blue and green rocks are most resistant to erosion, and are represented by hills.<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<div class=\"textbox__header\">\n<p>Figure 12.2.5 shows folding in the same area of the Rocky Mountains as Figure 12.0.1.\u00a0 Describe the types of folds using the appropriate terms from above (symmetrical, asymmetrical, isoclinal, overturned, recumbent etc.).\u00a0 You might find it useful to first sketch in the axial planes.<\/p>\n<div id=\"attachment_507\" class=\"wp-caption aligncenter\" style=\"width: 800px\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-507\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Folding-style.jpg\" alt=\"\" width=\"800\" height=\"421\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Folding-style.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Folding-style-300x158.jpg 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Folding-style-768x404.jpg 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Folding-style-65x34.jpg 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Folding-style-225x118.jpg 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Folding-style-350x184.jpg 350w\" sizes=\"auto, (max-width: 800px) 100vw, 800px\" \/><\/p>\n<div id=\"caption-attachment-530\" class=\"wp-caption-text\">Figure 12.2.5<\/div>\n<\/div>\n<p>See Appendix 3 for <a href=\"back-matter-005-appendix-3-answers-to-exercises.html#exercisea12.1\">Exercise 12.1 answers<\/a>.<\/p>\n<\/div>\n<\/div>\n<h3>Media Attributions<\/h3>\n<ul>\n<li>Figures 12.2.1, 12.2.2, 12.2.4, 12.2.5: \u00a9 Steven Earle. CC BY.<\/li>\n<li>12.2.3: Colorado Geological Survey<\/li>\n<\/ul>\n<\/div>\n<p><!-- pb_fixme --><\/p>\n","protected":false},"author":32,"menu_order":91,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-508","chapter","type-chapter","status-publish","hentry"],"part":17,"_links":{"self":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/508","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/users\/32"}],"version-history":[{"count":3,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/508\/revisions"}],"predecessor-version":[{"id":1229,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/508\/revisions\/1229"}],"part":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/parts\/17"}],"metadata":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/508\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/media?parent=508"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapter-type?post=508"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/contributor?post=508"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/license?post=508"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}