Old Earth Ministries Online Geology Curriculum

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Geology - Chapter 6:  Sedimentary Rocks

    God's laws of nature operate on a daily basis to produce many different types of landforms.  One of the most visible of these are sedimentary rock formations.  In later chapters we will cover these rock forming processes in greater detail.  In this chapter, we will examine how eroded rocks are deposited to form new rocks, and we will also examine how rocks form from chemical processes.

Basic Principles

    Sedimentary rocks cover about three-fourths of the earth's land surface.  One of the most visible examples of sedimentary rocks is the Grand Canyon in Arizona, where you can see a mile of exposed layers of sediment.  These rock layers are called strata.  Each type of rock is indicative of the type of environment that it formed in.

     There are three sources of material that can become sedimentary rock.

1.  Fragments of other rocks and minerals, like gravel in a river bed, sand on a beach, or mud in the ocean

2.  Chemical precipitates, such as salt or calcium carbonate

3.  Organic materials, such as coral reefs or vegetation in a swamp

     Through time, these materials are compacted or cemented together (become lithified) to become rocks.  The single unifying feature of sedimentary rocks is called stratifictation...that is, they are laid down in a series of horizontal beds, one on top of another.  Each bedding plane represents a former surface of the earth, sometimes preserving features such as raindrop impressions, mud cracks, or ripple marks.  A bedding plane typically represents a change in composition of the rock, such as grain size, type, color, and so forth.

     One of the useful tools that geologists use are called geologic cross sections.  These diagrams show the rock layers in an area from a side view.  Click on the cross section on the right...it will open a new window.  This is a cross-section for the Grand Canyon.  First, notice that different rock types are represented by different patterns.  Here are the typical patterns and the rocks they represent.

  Limestone         Shale        Sandstone

  Intrusive       Metamorphic

     Typically, limestone is represented by blocks, shale by dashed lines, and sandstone by dots.  These patterns can also be mixed...for instance, in a thick layer of sandstone, there may be layers of shale, and these could be represented by a single dashed line in the middle of the sandstone dots.   Geologic cross sections are usually to scale. 

     Layers of rock are normally called formations.  A formation is simply a distinctive body of rock that serves as a convenient unit for study.  Typically, formations are homogenous units.  When a rock layer changes from sandstone to limestone, typically this is a good location to start another formation.  This is not always the case, though.  Some formations contain many small layers of limestone, sandstone, and shale, interbedded with each other.

     Another distinctive feature of sedimentary rock is fossils.  Fossils are the mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, and other organisms.

Types of Sedimentary Rocks

     There are two main types of sedimentary rocks.  The first type is known as clastic rocks.  Clastic rocks consist of mineral fragments that were previously eroded from a source rock.  The second type of sedimentary rocks are chemical and organic rocks, which consists of either chemical precipitates or organic material.  First we will examine clastic rocks.

Clastic Rocks

     Clastic rocks are subdivided according to the size of the individual grains. 

     The line between clastic rock types can be blurred.  For instance, a shale can also contain larger sand-size particles.  Such a shale could be called a sandy shale.  One way to tell the grain size of shale in the field is by tasting it.  Put a small flake of shale in your mouth and crush it.  If it feels gritty, then there is some sand-size particles present (hint...don't swallow!).

Chemical and Organic Rocks

     Also called nonclastic rocks, they typically form in an ocean, lake, or swampy environment.

     An interesting feature of carbonate rocks that you may recall from chapter three is their chemical reaction to hydrochloric acid.  To see a video of hydrochloric acid being applied to limestone, click here.

     Just like clastic rocks, you can have a mixing of mineral types within a rock.  For instance, the sea level may increase, bringing limestone-producing tidal flats over the top of what was once a beach.  As the transition occurs, you can have a "sandy limestone" form, with characteristics of both rock types.

Sedimentary Structures

     Sediment is transported to the point of deposition by current systems, usually streams, along shorelines, and in shallow seas.  Wind also is a current system, as sand and silt can also be transported this way.  This transportation yields specific features which are unique to sedimentary rocks.  

     The first feature we will examine is called stratification.  Sedimentary rocks are deposited in layers, one on top of the other.  When you look at a photo of the Grand Canyon, it is easy to see the layers of rock.  Even though a layer may be completely sandstone, it is important to remember that the other layers are still being deposited at the same time.  For example, at the shoreline, sand is being deposited which will eventually become sandstone.  Farther out to sea, where a river is pumping mud into the ocean, mud is accumulating that will later become shale.  Still further out to sea, calcium carbonate may be precipitating, and it will later become limestone.  As the sea level fluctuates, the locations of the deposition zones for these three materials moves back and forth.  This Youtube video explains the process known as transgression and regression.

     The second feature of sedimentary rocks is called cross-bedding.  As clastic sediments are transported by water or wind, they form sand waves.  The image at right shows this process for wind-blown sand.  In figure 1, the sand wave can be observed.  As the wind blows, it picks up sand particles from the rear of the wave, and deposits them over the leading edge of the wave.  This process can be repeated, making a wave on top of a wave, as in figure 4.  These waves can range in size from small ripples only a centimeter high, to giant sand dunes hundreds of meters high. 

     The structure of the bedding tells us the direction of the wind that deposited the sand.  Figure 7 in the above diagram shows the wind direction for each cross-bedded sand wave.  By the size of the cross bedding and the angle of the bedding, you can also tell if it was a beach deposit or a wind deposit in a desert environment. 

     The third feature of sedimentary rocks is called graded bedding.  This type of stratification is characterized by a decrease in grain size as you go upward through the bed.  This is commonly produced on the ocean floor, as a result of turbidity currents.  These currents transport sediment from the continental slope to the deep ocean.  These currents are usually triggered by underwater landslides, and they carry massive amounts of sediment of all grain sizes.  When they reach the point where the water current slows, the larger grains settle out onto the ocean floor first.  As the water slows, still smaller grains settle out on top of the larger grains, and finally, fine-grained silt settles out on top of this.  Click here to see a simulated turbidity current reproduced in a laboratory. 

      The fourth major feature of sedimentary rocks is surface impressions.  There are several different types that are common.  Ripple marks are small sand waves that form from the action of water and air on sediment, and they are often preserved (picture at right).  Mud cracks are another common feature that is preserved in sedimentary rocks (picture at left).  Other common markings are tracks, trails, and borings of animals.  These are called "trace fossils."  Trace fossils are indications of past life forms.  Dinosaur footprints are excellent examples of trace fossils.  A final surface impression that you may run across is raindrop impressions, as in the photo at right.

Sedimentary Rock Origins

     Where do all those sedimentary rocks come from?   The rock material that comprises sedimentary rocks starts out as parts of other rocks, before it is weathered away, and carried by some transportation system, either wind or water, to the point where it is deposited, and then it finally solidifies (lithifies) into a sedimentary rock.  This sounds simple enough, but there is more to it than that.

     Weathering is the interaction of the atmospheric elements and the rocks exposed to these elements.  Weathering can be mechanical, through processes such as frost action, or chemical, through a variety of chemical reactions.  After the individual particles are broken free of their source rock, they must be transported.  The most important thing that happens here is that the grains are separated by sorting.  Larger particles require more energy to transport, therefore they don't end up far from the source rock.  Fine-grained particles can easily be washed thousands of miles from their source.  This sorting of sizes is known as sedimentary differentiation. 

     After the grains are transported they must be deposited.  The deposition environment includes the physical, chemical, and biological conditions that are present at the location of deposition.  Each environment produces rocks which are characteristic of that environment.  Beaches produce sandstones, river deltas produce mudstone, etc.  You will learn much more about these deposition systems in later chapters.

     Finally, compaction and cementation presses the grains together, and "glues" them, making them into rocks.  Compaction occurs because the layers of rock on top of each other are compressing the bottom layers, squeezing out moisture, and allowing them to cement together.  The cement is commonly calcite, quartz, or limonite. 

     Therefore, the four stages of making a sedimentary rock are weathering, transportation, deposition, and compaction/cementation.

Tuesday - Research

     Research some of the commercial uses for sedimentary rocks.  What is limestone, sandstone, rock salt, and gypsum used for?  Name some common products that contain these rocks.  To answer these questions, utilize a search engine to locate the best webpages, or consult a textbook/encyclopedia.  You may also use the links at the bottom of this page.

Wednesday - Quiz

     Today you will complete a 10 question practice quiz.  The link to the quiz will open a new window.  You can come back here and check your answers.  Do not click the Back button on your browser during the quiz.  After the quiz, continue your research project, if necessary.

          Geology Chapter 6 Quiz

Thursday - Review

     Please review the terms in bold in the text, and ensure you have completed your research work from Tuesday.

Friday - Test

     Today you will take the end of chapter test.  Please close all other browser windows, and click on the link below.  During the test, do not click on the Back button on your browser.

          Geology Chapter 6 Test

After you have completed the test, you may proceed to the next chapter on your next school day.  Please return to the introduction page for the link to the next chapter.

Return to the Old Earth Ministries Online Geology Curriculum homepage.

Helpful Links

Gypsum (Wikipedia)

Rock Salt

Limestone

Sandstone