{"id":47,"date":"2020-12-23T17:36:00","date_gmt":"2020-12-23T22:36:00","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/?post_type=chapter&#038;p=47"},"modified":"2020-12-31T14:22:16","modified_gmt":"2020-12-31T19:22:16","slug":"describing-and-naming-clastic-sedimentary-rocks","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/chapter\/describing-and-naming-clastic-sedimentary-rocks\/","title":{"raw":"Describing and Naming Clastic Sedimentary Rocks","rendered":"Describing and Naming Clastic Sedimentary Rocks"},"content":{"raw":"<h1>Describing Clastic Sedimentary Rocks<\/h1>\r\nIf you took EESC 111 you will have learned to name clastic sedimentary rocks. \u00a0In this course you will learn to <em>describe<\/em> clastic sedimentary rocks in order to better understand how and where they were formed.\r\n\r\nGeoscientists may be trying to match rocks from one location to another.\u00a0 Rocks seen at two different outcrops may have the same name (for example, sandstone), but this is far from enough information for a geologist to be able to decide if these are in fact the <em>same<\/em> sandstone.\u00a0 Having a consistent method of writing descriptions is important to be able to compare rocks from one location to another.\u00a0 A full description of a clastic sediment or sedimentary rock <em>must<\/em> include:\r\n<ul>\r\n \t<li>Colour<\/li>\r\n \t<li>Grain size category<\/li>\r\n<\/ul>\r\nIn addition, the following characteristics are included in the description when visible:\r\n<ul>\r\n \t<li>Sorting<\/li>\r\n \t<li>Rounding<\/li>\r\n \t<li>Sphericity<\/li>\r\n \t<li>Any other distinctive features<\/li>\r\n<\/ul>\r\nNote that you won't be able to determine sorting, rounding, or sphericity from a hand sample if the clasts are too small to see without magnification.\r\n\r\nA full description of a clastic sediment or sedimentary rock will take the following form, where parentheses indicate characteristics that may not be possible to determine and include:\r\n<p style=\"text-align: center\"><strong>Colour, (Sorting), (Rounding), (Sphericity), Grain Size Category, (Other Features)<\/strong><\/p>\r\nEach characteristic included in the description is explained in more detail below.\r\n<h3>Colour<\/h3>\r\nColour is an essential descriptor.\u00a0 Rocks are typically described with simple colour descriptions such as \u201cred\u201d, \u201cgrey-green\u201d, \u201ctan\u201d or \u201cblack and grey\u201d.\u00a0 Colour descriptions may also need to include some information about the arrangement such as \u201calternating bands of red and grey in 1 to 2 cm bands.\u201d\u00a0\u00a0 Typically, these colours are just named from common experience as you will be doing in this course.\u00a0 There are instances where colour becomes very important, such as when two fine-grained rocks may be only distinguishable by their exact shade of grey.\u00a0 It then becomes important that all the geoscientists are using the same colour descriptions.\u00a0 \u00a0A standardized book of colours can then be used to assign precise colours to rocks.\r\n\r\nSediment colour is controlled by many factors but can be used to infer the extent of oxidation during sediment deposition, and hence can provide a clue to the type of location where the sediment was deposited:\r\n<ul>\r\n \t<li><strong>Brown<\/strong> or <strong>red<\/strong> typically indicates that the sediment has been exposed to oxygen in the air, usually in a terrestrial environment.\u00a0 Iron in the sediments will oxidize, giving the sediments the brown to red colour.<\/li>\r\n \t<li><strong>Grey<\/strong> and <strong>grey-green<\/strong> is common for sediments deposited in shallow and\/or circulating water.<\/li>\r\n \t<li><strong>Black<\/strong> typically indicates deep water and\/or oxygen depleted water.<\/li>\r\n<\/ul>\r\nYou will learn more about this in labs 2 to 4.\r\n<h3>Average Grain Size and Grain Size Categories<\/h3>\r\nThe average grain size of a clastic rock is measured in mm and is used to assign a description of the grain size by a set of size categories: boulders, cobbles, gravel, sand, silt and clay (Table 1.2). The breakdown of grain size classifications in mm is based upon powers of 2 (e.g., boulders have a diameter of &gt; 2<sup>8<\/sup> mm), known as the Wentworth Scale. Geoscientists, engineers, soils scientists and others all use slightly different numbers for historical and practical reasons.\r\n\r\n&nbsp;\r\n\r\n[caption id=\"attachment_79\" align=\"aligncenter\" width=\"650\"]<img class=\" wp-image-79\" src=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/Table1-2.png\" alt=\"\" width=\"650\" height=\"382\" \/> <a href=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-2.pdf\" target=\"_blank\" rel=\"noopener noreferrer\">Click here<\/a> to view this table as a PDF file.[\/caption]\r\n\r\n&nbsp;\r\n\r\nThe eye, or in some cases, a hand lens and a grain-size card can be used to help determine the grain size. Sand-sized particles are easy to see with the naked eye and feel \u201cgritty\u201d to the finger tips. Silt-sized particles are at the edge of what is visible to the naked eye and will feel smooth to the fingertip but feel \u201cgritty\u201d when a small fragment is felt between the teeth. Clay-sized particles are small enough that they feel smooth to the fingertip and to the teeth.\r\n<div class=\"textbox shaded\">Although taste and feel on the teeth are mentioned in this lab manual and are sometimes used by geologists in the field, <strong>do not test any rocks in this manner<\/strong>. Hand samples may have been tested with dilute acid, and residual acid may be on the rocks.<\/div>\r\nThe size of grains generally indicates the energy conditions during deposition of the sediments, with larger grains indicating higher energy environments and smaller grains indicating lower energy or calmer environments. High energy water flows typically give way over time to lower energy water flows (e.g.: floods, storms, spring runoff), leading to a variation in grain sizes.\u00a0\u00a0 High energy water flows sometimes will erode the layers below.\r\n\r\n<hr \/>\r\n\r\n<div><span style=\"color: #800000\"><em>Note: The next three terms\u2014<strong>sorting<\/strong>, <strong>rounding<\/strong>, and <strong>sphericity<\/strong>\u2014can only be determined when the grains in the rock can be seen with the eye, or with a microscope.\u00a0 Determining these parameters may not be possible for some fine-grained sediments.<\/em><\/span><\/div>\r\n<div>\r\n\r\n<hr \/>\r\n\r\n<\/div>\r\n<h3>Sorting<\/h3>\r\nSorting refers to how much variation there is in grain sizes within a rock, and is characterized on a scale ranging from poorly sorted to well-sorted (Figure 1.1).\u00a0 For example, glacial ice can carry all particle sizes; hence, when glaciers deposit sediment it results in a poorly sorted sediment (a wide range of clast sizes). \u00a0In contrast, wind erodes and transports very specific grain sizes.\u00a0 Therefore, sediment deposited by wind tends to be extremely well-sorted. \u00a0Clastic rocks may be described as poorly sorted, moderately sorted, or well sorted.\r\n\r\n&nbsp;\r\n\r\n[caption id=\"attachment_82\" align=\"aligncenter\" width=\"909\"]<img class=\"size-full wp-image-82\" src=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-1_sorting.png\" alt=\"Left- particles with many different shapes and sizes. Right- particles that are similar in shape and size.\" width=\"909\" height=\"140\" \/> <strong>Figure 1.1 |<\/strong> Grains range from poorly sorted or unsorted at (a) to well sorted at (b).[\/caption]\r\n<h3>Rounding<\/h3>\r\nTransportation of clasts by wind or water to its final point of deposition rounds the corners off grains by abrasion. \u00a0Well-rounded grains indicate longer transportation paths. \u00a0Clastic rocks may be described as poorly, moderately or well-rounded (Figure 1.2). When clasts are microscopic, you cannot describe their rounding.\r\n\r\n&nbsp;\r\n\r\n[caption id=\"attachment_85\" align=\"alignnone\" width=\"925\"]<img class=\"size-full wp-image-85\" src=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-2_rounding1.png\" alt=\"Left-grains with sharp corners. Right- smooth grains.\" width=\"925\" height=\"126\" \/> <strong>Figure 1.2 |<\/strong> Grains range from poorly rounded at (a) to well-rounded at (b).[\/caption]\r\n\r\nThe degree of rounding can also be influenced by the mineral that makes up the grain.\u00a0 Angular grains of a soft and breakable mineral such as biotite will become rounded much sooner than grains of a hard mineral such as quartz.\r\n<h3>Sphericity<\/h3>\r\nThis refers to the closeness of a grain shape to a sphere (perfect circle).\u00a0 The sphericity of a clast can be influenced by its mineral composition and the characteristic shape of that mineral. \u00a0Clastic rocks may be described as having low, medium or high sphericity (Figure 1.3). \u00a0Note that grains may be well rounded (no sharp edges), yet they may not be spherical.\u00a0 When clasts are microscopic, you cannot describe their sphericity.\r\n\r\n&nbsp;\r\n\r\n[caption id=\"attachment_84\" align=\"aligncenter\" width=\"406\"]<img class=\"wp-image-84\" src=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-3_sphericity.png\" alt=\"Left-grains that appear flattened. Right- grains that appear closer to spherical or square.\" width=\"406\" height=\"218\" \/> <strong>Figure 1.3 |<\/strong> The grains above range from low sphericity on the left to high sphericity on the right.\u00a0 Notice that the rounding of the grains is a separate descriptor.[\/caption]\r\n<h3>Other Features<\/h3>\r\nSeveral other features of a rock can help distinguish different materials at different locations.\u00a0 These features may or may not be present and therefore are an optional part of a sediment name.\r\n<h4>Organic Matter or Fossils<\/h4>\r\nIf any organic matter is present it is described by its colour, nature (type), and any distinguishing properties.\u00a0 For example, two sandstones of different origin might be identical in the description based on the particles of sand, but the presence of \u201cblack, partially decayed wood fragments\u201d in one, and not in the other can be used to tell them apart.\u00a0 You will learn how to describe fossils in Labs 5-8.\r\n<h4>Breakage Patterns<\/h4>\r\nThe way that a rock breaks can reveal information about the rock and its history.\u00a0 There are two main ways that a sedimentary rock will break.\r\n\r\n<strong>Breakage following bedding planes.\u00a0<\/strong> A bed is a layer of sediment that is distinct from sediments above and below because of differences in grain size, mineralogy, cementing agents, fossils, sedimentary structures, and other properties.\u00a0 The boundary of these bedding planes can sometimes be a weaker plane in the rock, and the rock will tend to break parallel to the bedding.\r\n\r\n<strong>Fissility.<\/strong> Fine-grained sedimentary rocks that are composed of silt and\/or clay sized clasts may also be fissile or non-fissile.\u00a0 Fissile rocks break into thin parallel layers, even when no bedding is visible.\u00a0 This is the result of flat, plate-like clay minerals lining up during deposition from water in a manner that creates planes of weakness. \u00a0Shale is a fissile clastic sedimentary rock.\r\n<h4>Mineral Type<\/h4>\r\nSands can be composed of fragments of rock (lithic fragments), or they may be composed of grains made up of individual minerals.\u00a0 The type of mineral present in a sandstone can be useful in understanding its history and determining its name.\u00a0 Abundant feldspars and mica may indicate a sand that has not moved far from its origin, as these are physically weaker and more easily weathered minerals.\u00a0 In contrast, a sand composed of nearly pure quartz has likely been transported over a long distance, and the other weaker or more chemically unstable minerals have all weathered. You will learn about some of the most important minerals as we work through Labs 1-4.\r\n<h4>Cement<\/h4>\r\nThe mineral forming the cement holding a sedimentary rock together can be added.\u00a0 This is typically either silica (quartz) or calcite for clastic rocks. \u00a0<span style=\"color: #800000\">You will not be expected to identify the cementing minerals of clastic sedimentary rocks in this course.<\/span>\r\n<h4>Special Features or Unusual Minerals<\/h4>\r\nThe presence of some minerals in a rock can give important clues to its origins, and hence may be noted.\u00a0 For example, the presence of rusty coloured iron oxide minerals is often an important clue that a sediment formed in contact with oxygen on land as opposed to a sediment formed under water. \u00a0In contrast, the presence of pyrite may indicate that the sediment was deposited in low-oxygen conditions in water.\u00a0 You will learn more about this in Lab 2.\r\n<h4>Sedimentary Structures<\/h4>\r\nThis is a very important component of a rock description, and will be the focus of the third part of this lab. When you get to the <a href=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/chapter\/exercise-1-3-sedimentary-structures\/\" target=\"_blank\" rel=\"noopener noreferrer\">sedimentary structures exercise<\/a> you will learn about these in more detail.\r\n<h1>Naming Clastic Sedimentary Rocks<\/h1>\r\nThe names of clastic sedimentary rocks are based on the descriptions you have already learned. Table 1.3 shows how clastic rocks are named.\r\n\r\nYou must provide a full description followed by the descriptive rock type name, according to the following format:\r\n\r\n<strong>Colour, (Sorting), (Rounding), (Sphericity), Grain Size, Grain Category, (Other Features): Descriptive Rock Name<\/strong>\r\n\r\nFor example:\r\n<p style=\"padding-left: 40px\"><em>Red, well-sorted, well-rounded, high sphericity, grain size is gravel and larger, grains are rock fragments: Conglomerate<\/em><\/p>\r\n&nbsp;\r\n\r\n[caption id=\"attachment_87\" align=\"aligncenter\" width=\"700\"]<img class=\" wp-image-87\" src=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3.png\" alt=\"\" width=\"700\" height=\"457\" \/> <a href=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3.pdf\" target=\"_blank\" rel=\"noopener noreferrer\">Click here<\/a> to view this table as a PDF file.[\/caption]\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;","rendered":"<h1>Describing Clastic Sedimentary Rocks<\/h1>\n<p>If you took EESC 111 you will have learned to name clastic sedimentary rocks. \u00a0In this course you will learn to <em>describe<\/em> clastic sedimentary rocks in order to better understand how and where they were formed.<\/p>\n<p>Geoscientists may be trying to match rocks from one location to another.\u00a0 Rocks seen at two different outcrops may have the same name (for example, sandstone), but this is far from enough information for a geologist to be able to decide if these are in fact the <em>same<\/em> sandstone.\u00a0 Having a consistent method of writing descriptions is important to be able to compare rocks from one location to another.\u00a0 A full description of a clastic sediment or sedimentary rock <em>must<\/em> include:<\/p>\n<ul>\n<li>Colour<\/li>\n<li>Grain size category<\/li>\n<\/ul>\n<p>In addition, the following characteristics are included in the description when visible:<\/p>\n<ul>\n<li>Sorting<\/li>\n<li>Rounding<\/li>\n<li>Sphericity<\/li>\n<li>Any other distinctive features<\/li>\n<\/ul>\n<p>Note that you won&#8217;t be able to determine sorting, rounding, or sphericity from a hand sample if the clasts are too small to see without magnification.<\/p>\n<p>A full description of a clastic sediment or sedimentary rock will take the following form, where parentheses indicate characteristics that may not be possible to determine and include:<\/p>\n<p style=\"text-align: center\"><strong>Colour, (Sorting), (Rounding), (Sphericity), Grain Size Category, (Other Features)<\/strong><\/p>\n<p>Each characteristic included in the description is explained in more detail below.<\/p>\n<h3>Colour<\/h3>\n<p>Colour is an essential descriptor.\u00a0 Rocks are typically described with simple colour descriptions such as \u201cred\u201d, \u201cgrey-green\u201d, \u201ctan\u201d or \u201cblack and grey\u201d.\u00a0 Colour descriptions may also need to include some information about the arrangement such as \u201calternating bands of red and grey in 1 to 2 cm bands.\u201d\u00a0\u00a0 Typically, these colours are just named from common experience as you will be doing in this course.\u00a0 There are instances where colour becomes very important, such as when two fine-grained rocks may be only distinguishable by their exact shade of grey.\u00a0 It then becomes important that all the geoscientists are using the same colour descriptions.\u00a0 \u00a0A standardized book of colours can then be used to assign precise colours to rocks.<\/p>\n<p>Sediment colour is controlled by many factors but can be used to infer the extent of oxidation during sediment deposition, and hence can provide a clue to the type of location where the sediment was deposited:<\/p>\n<ul>\n<li><strong>Brown<\/strong> or <strong>red<\/strong> typically indicates that the sediment has been exposed to oxygen in the air, usually in a terrestrial environment.\u00a0 Iron in the sediments will oxidize, giving the sediments the brown to red colour.<\/li>\n<li><strong>Grey<\/strong> and <strong>grey-green<\/strong> is common for sediments deposited in shallow and\/or circulating water.<\/li>\n<li><strong>Black<\/strong> typically indicates deep water and\/or oxygen depleted water.<\/li>\n<\/ul>\n<p>You will learn more about this in labs 2 to 4.<\/p>\n<h3>Average Grain Size and Grain Size Categories<\/h3>\n<p>The average grain size of a clastic rock is measured in mm and is used to assign a description of the grain size by a set of size categories: boulders, cobbles, gravel, sand, silt and clay (Table 1.2). The breakdown of grain size classifications in mm is based upon powers of 2 (e.g., boulders have a diameter of &gt; 2<sup>8<\/sup> mm), known as the Wentworth Scale. Geoscientists, engineers, soils scientists and others all use slightly different numbers for historical and practical reasons.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_79\" aria-describedby=\"caption-attachment-79\" style=\"width: 650px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-79\" src=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/Table1-2.png\" alt=\"\" width=\"650\" height=\"382\" srcset=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/Table1-2.png 1207w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/Table1-2-300x176.png 300w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/Table1-2-1024x602.png 1024w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/Table1-2-768x451.png 768w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/Table1-2-65x38.png 65w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/Table1-2-225x132.png 225w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/Table1-2-350x206.png 350w\" sizes=\"auto, (max-width: 650px) 100vw, 650px\" \/><figcaption id=\"caption-attachment-79\" class=\"wp-caption-text\"><a href=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-2.pdf\" target=\"_blank\" rel=\"noopener noreferrer\">Click here<\/a> to view this table as a PDF file.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>The eye, or in some cases, a hand lens and a grain-size card can be used to help determine the grain size. Sand-sized particles are easy to see with the naked eye and feel \u201cgritty\u201d to the finger tips. Silt-sized particles are at the edge of what is visible to the naked eye and will feel smooth to the fingertip but feel \u201cgritty\u201d when a small fragment is felt between the teeth. Clay-sized particles are small enough that they feel smooth to the fingertip and to the teeth.<\/p>\n<div class=\"textbox shaded\">Although taste and feel on the teeth are mentioned in this lab manual and are sometimes used by geologists in the field, <strong>do not test any rocks in this manner<\/strong>. Hand samples may have been tested with dilute acid, and residual acid may be on the rocks.<\/div>\n<p>The size of grains generally indicates the energy conditions during deposition of the sediments, with larger grains indicating higher energy environments and smaller grains indicating lower energy or calmer environments. High energy water flows typically give way over time to lower energy water flows (e.g.: floods, storms, spring runoff), leading to a variation in grain sizes.\u00a0\u00a0 High energy water flows sometimes will erode the layers below.<\/p>\n<hr \/>\n<div><span style=\"color: #800000\"><em>Note: The next three terms\u2014<strong>sorting<\/strong>, <strong>rounding<\/strong>, and <strong>sphericity<\/strong>\u2014can only be determined when the grains in the rock can be seen with the eye, or with a microscope.\u00a0 Determining these parameters may not be possible for some fine-grained sediments.<\/em><\/span><\/div>\n<div>\n<hr \/>\n<\/div>\n<h3>Sorting<\/h3>\n<p>Sorting refers to how much variation there is in grain sizes within a rock, and is characterized on a scale ranging from poorly sorted to well-sorted (Figure 1.1).\u00a0 For example, glacial ice can carry all particle sizes; hence, when glaciers deposit sediment it results in a poorly sorted sediment (a wide range of clast sizes). \u00a0In contrast, wind erodes and transports very specific grain sizes.\u00a0 Therefore, sediment deposited by wind tends to be extremely well-sorted. \u00a0Clastic rocks may be described as poorly sorted, moderately sorted, or well sorted.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_82\" aria-describedby=\"caption-attachment-82\" style=\"width: 909px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-82\" src=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-1_sorting.png\" alt=\"Left- particles with many different shapes and sizes. Right- particles that are similar in shape and size.\" width=\"909\" height=\"140\" srcset=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-1_sorting.png 909w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-1_sorting-300x46.png 300w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-1_sorting-768x118.png 768w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-1_sorting-65x10.png 65w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-1_sorting-225x35.png 225w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-1_sorting-350x54.png 350w\" sizes=\"auto, (max-width: 909px) 100vw, 909px\" \/><figcaption id=\"caption-attachment-82\" class=\"wp-caption-text\"><strong>Figure 1.1 |<\/strong> Grains range from poorly sorted or unsorted at (a) to well sorted at (b).<\/figcaption><\/figure>\n<h3>Rounding<\/h3>\n<p>Transportation of clasts by wind or water to its final point of deposition rounds the corners off grains by abrasion. \u00a0Well-rounded grains indicate longer transportation paths. \u00a0Clastic rocks may be described as poorly, moderately or well-rounded (Figure 1.2). When clasts are microscopic, you cannot describe their rounding.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_85\" aria-describedby=\"caption-attachment-85\" style=\"width: 925px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-85\" src=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-2_rounding1.png\" alt=\"Left-grains with sharp corners. Right- smooth grains.\" width=\"925\" height=\"126\" srcset=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-2_rounding1.png 925w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-2_rounding1-300x41.png 300w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-2_rounding1-768x105.png 768w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-2_rounding1-65x9.png 65w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-2_rounding1-225x31.png 225w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-2_rounding1-350x48.png 350w\" sizes=\"auto, (max-width: 925px) 100vw, 925px\" \/><figcaption id=\"caption-attachment-85\" class=\"wp-caption-text\"><strong>Figure 1.2 |<\/strong> Grains range from poorly rounded at (a) to well-rounded at (b).<\/figcaption><\/figure>\n<p>The degree of rounding can also be influenced by the mineral that makes up the grain.\u00a0 Angular grains of a soft and breakable mineral such as biotite will become rounded much sooner than grains of a hard mineral such as quartz.<\/p>\n<h3>Sphericity<\/h3>\n<p>This refers to the closeness of a grain shape to a sphere (perfect circle).\u00a0 The sphericity of a clast can be influenced by its mineral composition and the characteristic shape of that mineral. \u00a0Clastic rocks may be described as having low, medium or high sphericity (Figure 1.3). \u00a0Note that grains may be well rounded (no sharp edges), yet they may not be spherical.\u00a0 When clasts are microscopic, you cannot describe their sphericity.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_84\" aria-describedby=\"caption-attachment-84\" style=\"width: 406px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-84\" src=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-3_sphericity.png\" alt=\"Left-grains that appear flattened. Right- grains that appear closer to spherical or square.\" width=\"406\" height=\"218\" srcset=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-3_sphericity.png 579w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-3_sphericity-300x161.png 300w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-3_sphericity-65x35.png 65w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-3_sphericity-225x121.png 225w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/F1-3_sphericity-350x188.png 350w\" sizes=\"auto, (max-width: 406px) 100vw, 406px\" \/><figcaption id=\"caption-attachment-84\" class=\"wp-caption-text\"><strong>Figure 1.3 |<\/strong> The grains above range from low sphericity on the left to high sphericity on the right.\u00a0 Notice that the rounding of the grains is a separate descriptor.<\/figcaption><\/figure>\n<h3>Other Features<\/h3>\n<p>Several other features of a rock can help distinguish different materials at different locations.\u00a0 These features may or may not be present and therefore are an optional part of a sediment name.<\/p>\n<h4>Organic Matter or Fossils<\/h4>\n<p>If any organic matter is present it is described by its colour, nature (type), and any distinguishing properties.\u00a0 For example, two sandstones of different origin might be identical in the description based on the particles of sand, but the presence of \u201cblack, partially decayed wood fragments\u201d in one, and not in the other can be used to tell them apart.\u00a0 You will learn how to describe fossils in Labs 5-8.<\/p>\n<h4>Breakage Patterns<\/h4>\n<p>The way that a rock breaks can reveal information about the rock and its history.\u00a0 There are two main ways that a sedimentary rock will break.<\/p>\n<p><strong>Breakage following bedding planes.\u00a0<\/strong> A bed is a layer of sediment that is distinct from sediments above and below because of differences in grain size, mineralogy, cementing agents, fossils, sedimentary structures, and other properties.\u00a0 The boundary of these bedding planes can sometimes be a weaker plane in the rock, and the rock will tend to break parallel to the bedding.<\/p>\n<p><strong>Fissility.<\/strong> Fine-grained sedimentary rocks that are composed of silt and\/or clay sized clasts may also be fissile or non-fissile.\u00a0 Fissile rocks break into thin parallel layers, even when no bedding is visible.\u00a0 This is the result of flat, plate-like clay minerals lining up during deposition from water in a manner that creates planes of weakness. \u00a0Shale is a fissile clastic sedimentary rock.<\/p>\n<h4>Mineral Type<\/h4>\n<p>Sands can be composed of fragments of rock (lithic fragments), or they may be composed of grains made up of individual minerals.\u00a0 The type of mineral present in a sandstone can be useful in understanding its history and determining its name.\u00a0 Abundant feldspars and mica may indicate a sand that has not moved far from its origin, as these are physically weaker and more easily weathered minerals.\u00a0 In contrast, a sand composed of nearly pure quartz has likely been transported over a long distance, and the other weaker or more chemically unstable minerals have all weathered. You will learn about some of the most important minerals as we work through Labs 1-4.<\/p>\n<h4>Cement<\/h4>\n<p>The mineral forming the cement holding a sedimentary rock together can be added.\u00a0 This is typically either silica (quartz) or calcite for clastic rocks. \u00a0<span style=\"color: #800000\">You will not be expected to identify the cementing minerals of clastic sedimentary rocks in this course.<\/span><\/p>\n<h4>Special Features or Unusual Minerals<\/h4>\n<p>The presence of some minerals in a rock can give important clues to its origins, and hence may be noted.\u00a0 For example, the presence of rusty coloured iron oxide minerals is often an important clue that a sediment formed in contact with oxygen on land as opposed to a sediment formed under water. \u00a0In contrast, the presence of pyrite may indicate that the sediment was deposited in low-oxygen conditions in water.\u00a0 You will learn more about this in Lab 2.<\/p>\n<h4>Sedimentary Structures<\/h4>\n<p>This is a very important component of a rock description, and will be the focus of the third part of this lab. When you get to the <a href=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/chapter\/exercise-1-3-sedimentary-structures\/\" target=\"_blank\" rel=\"noopener noreferrer\">sedimentary structures exercise<\/a> you will learn about these in more detail.<\/p>\n<h1>Naming Clastic Sedimentary Rocks<\/h1>\n<p>The names of clastic sedimentary rocks are based on the descriptions you have already learned. Table 1.3 shows how clastic rocks are named.<\/p>\n<p>You must provide a full description followed by the descriptive rock type name, according to the following format:<\/p>\n<p><strong>Colour, (Sorting), (Rounding), (Sphericity), Grain Size, Grain Category, (Other Features): Descriptive Rock Name<\/strong><\/p>\n<p>For example:<\/p>\n<p style=\"padding-left: 40px\"><em>Red, well-sorted, well-rounded, high sphericity, grain size is gravel and larger, grains are rock fragments: Conglomerate<\/em><\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_87\" aria-describedby=\"caption-attachment-87\" style=\"width: 700px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-87\" src=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3.png\" alt=\"\" width=\"700\" height=\"457\" srcset=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3.png 1619w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3-300x196.png 300w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3-1024x668.png 1024w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3-768x501.png 768w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3-1536x1002.png 1536w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3-65x42.png 65w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3-225x147.png 225w, https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3-350x228.png 350w\" sizes=\"auto, (max-width: 700px) 100vw, 700px\" \/><figcaption id=\"caption-attachment-87\" class=\"wp-caption-text\"><a href=\"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-content\/uploads\/sites\/1224\/2020\/12\/T1-3.pdf\" target=\"_blank\" rel=\"noopener noreferrer\">Click here<\/a> to view this table as a PDF file.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n","protected":false},"author":1019,"menu_order":2,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[48],"contributor":[],"license":[],"class_list":["post-47","chapter","type-chapter","status-publish","hentry","chapter-type-numberless"],"part":3,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/pressbooks\/v2\/chapters\/47","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/wp\/v2\/users\/1019"}],"version-history":[{"count":12,"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/pressbooks\/v2\/chapters\/47\/revisions"}],"predecessor-version":[{"id":154,"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/pressbooks\/v2\/chapters\/47\/revisions\/154"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/pressbooks\/v2\/parts\/3"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/pressbooks\/v2\/chapters\/47\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/wp\/v2\/media?parent=47"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/pressbooks\/v2\/chapter-type?post=47"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/wp\/v2\/contributor?post=47"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/earthhistorylab\/wp-json\/wp\/v2\/license?post=47"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}