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ing, Erosion, and Sedimentary Rocks – An Introduction to Geology

The rock has a large cliff.
Light illuminates the sedimentary rocks of Notch Peak, in the House Range of western Utah.The House Range contains early Paleozoic marine rocks, highlighted by the Wheeler Formation, home to some of the best Cambrian fossils in Utah. Notch Peak contains one of the largest pure-vertical drops in North America at over 2000 feet.


By the end of this chapter, students will be able to:

  • Describe how water is an integral part of all 5.1 The Unique Properties of Water

    Water plays a role in the formation of most sedimentary rock. It is one of the main agents involved in creating the The water drops are sticking to a spider's web

    Dew on a spider’s web, drawn into droplets by cohesion and clinging to the web by adhesion.

    Polarity allows water molecules to stick to other substances. This is called

    The fact that water is attracted to itself leads to another important property, one that is extremely rare in the natural world—the liquid form is denser than the solid form. The polarity of water creates a special type of weak bonding called The negative part of the water molecules surrounds the positively-charged sodium ion.

    A sodium (Na) ion in solution.

    Even more critical for supporting life, water remains liquid over a very large range of temperatures, which is also a result of cohesion. Hydrogen 5.2 Weathering and Erosion

    Bedrock refers to the solid rock that makes up the Earth’s outer 5.2.1 Mechanical Weathering

    Mechanical Pressure Expansion

    Granite rock has a relatively thin layer that is peeling away
    The outer layer of this granite is fractured and eroding away, known as exfoliation

    Bedrock buried deep within the Earth is under high pressure and Frost Wedging

    A crack in a rock gets progressively bigger as ice freezes, prying the crack open over time.
    The process of frost wedging

    Frost wedging, also called ice wedging, uses the power of expanding ice to break apart rocks. Water works its way into various cracks, voids, and crevices. As the water freezes, it expands with great force, exploiting any weaknesses. When ice melts, the liquid water moves further into the widened spaces. Repeated cycles of freezing and melting eventually pry the rocks apart. The cycles can occur daily when fluctuations of temperature between day and night go from freezing to melting.

    Root Wedging

    The roots of the tree are breaking up the asphalt.
    The roots of this tree are demonstrating the destructive power of root wedging. Though this picture is a man-made rock (asphalt), it works on typical rock as well.

    Like Salt Expansion

    The rock has many holes from the salt erosion.
    Tafoni from Salt Point, California.

    Salt expansion, which works similarly to frost wedging, occurs in areas of high evaporation or near-5.2.2 Chemical Weathering

    The left side has one large cube, the middle has 8 medium cubes, the right side has 64 small cubes. Each group has the same overall volume.
    Each of these three groups of cubes has an equal volume. However, their surface areas are vastly different. On the left, the single cube has a length, width, and height of 4 units, giving it a surface area of 6(4×4)=96 and a volume of 4^3=64. The middle eight cubes have a length, width, and height of 2, meaning a surface area of 8(6(2×2))=8×24=192. They also have a volume of 8(2^3)=8×8=64. The 64 cubes on the right have a length, width, and height of 1, leading to a surface area of 64(6(1×1))=64×6=384. The volume remains unchanged, because 64(1^3)=64×1=64. The surface area to volume ratio (SA:V), which is related to the amount of material available for reactions, changes for each as well. On the left, it is 96/64=0.75 or 3:2. The center has a SA/V of 192/64=1.5, or 3:1. On the right, the SA:V is 384/64=6, or 6:1.

    Chemical Carbonic Acid and Hydrolysis

    The diagram on the left is before hydrolysis.
    Generic hydrolysis diagram, where the bonds in mineral in question would represent the left side of the diagram.

    Carbonic acid (H2CO3) forms when carbon dioxide, the fifth-most abundant gas in the Dissolution

    The rock is red.
    In this rock, a pyrite cube has dissolved (as seen with the negative “corner” impression in the rock), leaving behind small specks of gold.

    Dissolution is a The xenolith sits on top of a basalt rock. It has three sides like a pyramid; one of the sides is more altered to iddingsite.

    This mantle xenolith containing olivine (green) is chemically weathering by hydrolysis and oxidation into the pseudo-mineral iddingsite, which is a complex of water, clay, and iron oxides. The more altered side of the rock has been exposed to the environment longer.

    The The rocks in this area are full of holes, formed from karst dissolution.

    Eroded karst topography in Minevre, France.
    A heart-shaped formation in Timpanogos Cave
    A formation called The Great Heart of Timpanogos in Timpanogos Cave National Monument


    Goethite is in cubes, though it usually is not. Pyrite is in cubes.
    Pyrite cubes are oxidized, becoming a new mineral goethite. In this case, goethite is a pseudomorph after pyrite, meaning it has taken the form of another mineral.

    Oxidation, the chemical reaction that causes rust in 5.2.3 Erosion

    The rock is topped by a more resistant.
    A hoodoo near Moab, Utah. The more resistant cap has protected the less resistant underlying layers.

    Erosion is a mechanical process, usually driven by water, gravity, (see Chapter 10), wind, or ice (see Chapter 14) that removes sediment from the place of weathering. Liquid water is the main agent of erosion.

    The canyon has many cliffs and slopes.
    Grand Canyon from Mather Point.

    5.2.4. Soil

    The soil is sketched and labeled.
    Sketch and picture of soil.

    Soil is a combination of air, water, minerals, and organic matter that forms at the transition between The image shows the way nitrogen can move around, mostly in the soil

    Schematic of the nitrogen cycle.

    These nitrogen-fixing bacteria absorb nitrogen from the atmosphere and convert it into nitrogen compounds. These compounds are absorbed by plants and used to make DNA, amino acids, and enzymes. Animals obtain bioavailable nitrogen by eating plants, and this is the source of most of the nitrogen used by life. That nitrogen is an essential component of proteins and DNA. Soils range from poor to rich, depending on the amount of humus they contain. Soil productivity is determined by water and nutrient content. Freshly created volcanic A mountain slope has been made into artificial steps form farming.

    Agricultural terracing, as made by the Inca culture from the Andes, helps reduce erosion and promote soil formation, leading to better farming practices.

    The nature of the The image shows 5 soil layers, ranging from highly altered at the top, to unaltered at the bottom.

    A simplified soil profile, showing labeled layers.

    O Horizon: The top horizon is a thin layer of predominantly organic material, such as leaves, twigs, and other plant parts that are actively decaying into humus.

    A Horizon: The next layer, called The outside of the rock is tan and weathered, the inside is grey.

    A sample of bauxite. Note the unweathered igneous rock in the center.

    The United States governing body for agriculture, the USDA, uses a taxonomic classification to identify soil types, called soil orders. Xoxisols or laterite The black and white photo shows a giant wall of dust.

    A dust storm approaches Stratford, Texas in 1935.

    Not only is soil essential to 5.3 Sedimentary rocks

    Sedimentary rock is classified into two main categories: clastic and chemical. Clastic or detrital sedimentary rocks are made from pieces of bedrock, 5.3.1 Lithification and Diagenesis

    Lithification turns loose sediment grains, created by weathering and transported by erosion, into clastic Photo of log of petrified wood showing structures of the original wood

    Permineralization in petrified wood

    5.3.2 Detrital Sedimentary Rocks (Clastic)

    Detrital or Grain Size

    Chart with sizes ranging from clay to boulders
    Size categories of sediments, known as the Wentworth scale.

    Sorting and Rounding

    The sediment on the left is all about the same size. The sediment on the right is many sizes.
    A well-sorted sediment (left) and a poorly-sorted sediment (right).

    Sorting describes the range of grain sizes within The sediments show various stages of rounding and sphericity, from high to low.

    Degree of rounding in sediments. Sphericity refers to the spherical nature of an object, a completely different measurement unrelated to rounding.

    Composition and provenance

    The grain is round and has vesicles.
    A sand grain made of basalt, known as a microlitic volcanic lithic fragment. Box is 0.25 mm. Top picture is plane-polarized light, bottom is cross-polarized light.

    Hawiian beach composed of green olivine sand from weathering of nearby basaltic rock.

    Hawiian beach composed of green olivine sand from weathering of nearby basaltic rock.

    Geologists use Classification of Clastic Rocks

    The grey rock is broken and angular within the larger rock.
    Megabreccia in Titus Canyon, Death Valley National Park, California.

    Windblown sand grains showing rounding and frosted surfaces due to transport b wind.

    Enlarged image of frosted and rounded windblown sand grains

    Medium-grained rocks composed mainly of sand are called The rock breaks apart in very thin layers.

    The Rochester Shale, New York. Note the thin fissility in the layers.

    Fine-grained rocks include The light grey layers are very thin.

    Claystone laminations from Glacial Lake Missoula.

    Rock types found as a mixture between the main classifications, may be named using the less-common component as a descriptor. For example, a rock containing some silt but mostly rounded sand and gravel is called silty conglomerate. Sand-rich rock containing minor amounts of clay is called clayey 5.3.3. Chemical, Biochemical, and Organic

    Chemical sedimentary rocks are formed by processes that do not directly involve mechanical weathering and erosion. Inorganic chemical

    The ground is white and flat for a long distance.
    Salt-covered plain known as the Bonneville Salt Flats, Utah.

    Inorganic chemical sedimentary rocks are formed when Mineral sequence Percent Seawater remaining after evaporation Calcite 50 Gypsum/anhydrite 20 Halite 10 Various potassium and magnesium salts 5

    Table after.

    The ooids are very smooth and round
    Ooids from Joulter’s Cay, The Bahamas
    The grey limestone towers vertically stick out of the ground.
    Limestone tufa towers along the shores of Mono Lake, California.

    Calcium The white and brown natural steps show the formation of travertine.

    Travertine terraces of Mammoth Hot Springs, Yellowstone National Park, USA

    Cave deposits like stalactites and stalagmites are another form of chemical precipitation of calcite, in a form called The rock shows red and brown layering.

    Alternating bands of iron-rich and silica-rich mud, formed as oxygen combined with dissolved iron.

    The flint is dark brown/grey, and the weathered crust is light tan. The overall shape is blobby.

    A type of chert, flint, shown with a lighter weathered crust.

    Chert, another commonly found chemical Ooids forming an oolite.

    Ooids forming an oolite.


    Rock has many fossils throughout
    Fossiliferous limestone (with brachiopods and bryozoans) from the Kope Formation of Ohio. Lower image is a section of the rock that has been etched with acid to emphasize the fossils.

    Rock is broken shells

    Close-up on coquina.

    Limestone occurs in many forms, most of which originate from biological processes. Entire coral Organic

    It is very black and shiny.
    Anthracite coal, the highest grade of coal.

    Under the right conditions, intact pieces of organic material or material derived from organic sources, is preserved in the geologic record. Although not derived from sediment, this lithified organic material is associated with sedimentary Classification of Chemical Sedimentary Rocks

    The rock has many light-colored layers.
    Gyprock, a rock made of the mineral gypsum. From the Castile formation of New Mexico.

    In contrast to detrital sediment, chemical, biochemical, and organic sedimentary rocks are classified based on mineral composition. Most of these are monomineralic,

    Sedimentary rock identification chart

    5.4 Sedimentary Structures

    Sedimentary structures are visible textures or arrangements of sediments within a rock. Geologists use these structures to interpret the processes that made the rock and the environment in which it formed. They use 5.4.1. Bedding Planes

    Photo of strata in Utah lying horizontal
    Horizontal strata in southern Utah.

    The most basic sedimentary structure is bedding planes, the planes that separate the layers or strata in sedimentary and some volcanic rocks. Visible in exposed outcroppings, each Two students are looking at the layers of rock.

    Students from the University of Wooster examine beds of Ordovician limestone in central Tennessee.

    As would be expected, bed thickness can indicate 5.4.2. Graded Bedding

    Rock shows layers described.
    Image of the classic Bouma sequence. A=coarse- to fine-grained sandstone, possibly with an erosive base. B=laminated medium- to fine-grained sandstone. C=rippled fine-grained sandstone. D=laminated siltstone grading to mudstone.

    5.4.3. Flow Regime and Bedforms

    There are 7 images of increasing velocity.
    Bedforms from under increasing flow velocities.

    In fluid systems, such as moving water or wind, sand is the most easily transported and deposited sediment grain. Smaller particles like silt and clay are less movable by fluid systems because the tiny grains are chemically attracted to each other and stick to the underlying Flow Regime (part) Bedform Description Lower (lowest) Plane Beds

    There are slight groves in the rock.
    Subtle lines across this sandstone (trending from the lower left to upper right) are parting lineations.

    Plane beds created in the lower Ripples

    The sand has a steep side on the left of the ripple, and a more gentle slope on the right.
    Modern current ripple in sand from the Netherlands. The flow creates a steep side down current. In this image, the flow is from right to left.

    Ripples are known by several names: This brown rock has symmetry in its ripples.

    A bidirectional flow creates this symmetrical wave ripple. From rocks in Nomgon, Mongolia. Note the crests of the ripples have been eroded away by subsequent flows in places.

    First scientifically described by Hertha Ayrton, ripple shapes are determined by flow type and can be straight-crested, sinuous, or complex. Asymmetrical ripples form in a unidirectional flow. Symmetrical The ripples are on top, slightly offset, from each other.

    Climbing ripple deposit from India.


    The mountain has a large variety of angles of beds, resulting from dunes moving in all directions.
    Lithified cross-bedded dunes from the high country of Zion National Park, Utah. The complexity of bedding planes results from the three-dimensional network of ancient dune flows.

    Dunes are very large and prominent versions of The red dune sand is rippled on one side (the steep side) and smooth on the other.

    Modern sand dune in Morocco.

    Dunes are the most common sedimentary structure found within channelized flows of air or water. The biggest difference between The flow is to the left on the bottom, and the right on the top.

    Herringbone cross-bedding from the Mazomanie Formation, upper Cambrian of Minnesota.
    The up and down waves are famous from hummocky-cross stratification.
    Hummocky-cross stratification, seen as wavy lines throughout the middle of this rock face. Best example is just above the pencil in the center.

    Another Antidunes

    The large waves are in place.
    Antidunes forming in Urdaibai, Spain.

    Antidunes are so named because they share similar characteristics with dunes, but are formed by a different, opposing process. While 5.4.4. Bioturbation

    There are several ovals and lines representing places where organisms crawled through the sediment.
    Bioturbated dolomitic siltstone from Kentucky.

    5.4.5. Mudcracks

    The cracks are in several directions, forming squares, triangles, and other polygonal shapes.
    Lithified mudcracks from Maryland.

    5.4.6. Sole Marks

    The bulge is sticking out of a rock layer above the head of the observer.
    This flute cast shows a flow direction toward the upper right of the image, as seen by the bulge sticking down out of the layer above. The flute cast would have been scoured into a rock layer below that has been removed by erosion, leaving the sandy layer above to fill in the flute cast.

    The rock is filled with narrow, parallel ridges.

    Groove casts at the base of a turbidite deposit in Italy.

    Formed similarly to The drill core is cylindrical.

    A drill core showing a load cast showing light-colored sand sticking down into dark mud.

    Load 5.4.7. Raindrop Impressions

    This grey rock has round circles left by raindrops
    Mississippian raindrop impressions over wave ripples from Nova Scotia.

    Like their name implies, 5.4.8. Imbrication

    The rocks in this conglomerate are tilted, leaning toward the right.
    Cobbles in this conglomerate are positioned in a way that they are stacked on each other, which occurred as flow went from left to right.

    5.4.9. Geopetal Structures

    Line is horizontal in picture as well.
    This bivalve (clam) fossil was partially filled with tan sediment, partially empty. Later fluids filled in the fossil with white calcite minerals. The line between the sediment and the later calcite is paleo-horizontal.

    Geopetal structures, also called up-direction indicators, are used to identify which way was up when the sedimentary rock layers were originally formed. This is especially important in places where the rock layers have been deformed, tilted, or overturned. Well preserved mudcracks, This footprint of a dinosaur is three toes.

    Eubrontes trace fossil from Utah, showing the geopetal direction is into the image.
    • Vugs: Small voids in the rock that usually become filled during diagenesis. If the void is partially filled or filled in stages, it serves as a permanent record of a level bubble, frozen in time.
    • 5.5 Depositional Environments
      Many different environments are representative environments from high elevation to deep under water.
      A representation of common depositional environments.

      The ultimate goal of many stratigraphy studies is to understand the original depositional environment. Knowing where and how a particular sedimentary rock was formed can help geologists paint a picture of past environments—such as a mountain glacier, gentle Location Sediment Common Rock Types Typical Fossils Sedimentary Structures 5.5.1. Marine

      Marine depositional environments are completely and constantly submerged in seawater. Their depositional characteristics are largely dependent on the depth of water with two notable exceptions, submarine fans and turbidites.


      The thickness is low in the abyssal plain.
      Marine sediment thickness. Note the lack of sediment away from the continents.

      Abyssal sedimentary rocks form on the abyssal plain. The plain encompasses relatively flat The rock is powdery and white.

      Diatomaceous earth

      The third sediment type is pelagic clay. Very fine-grained clay particles, typically brown or red, descend through the water column very slowly. Pelagic clay deposition occurs in areas of remote open ocean, where there is little plankton accumulation.

      The canyon allows stacking of these deposits on the ocean floor.
      Turbidites inter-deposited within submarine fans.

      Two notable exceptions to the fine-grained nature of abyssal Continental Slope

      The deposit is a large, dipping pile of sediment
      Contourite drift deposit imaged with seismic waves.

      Continental slope deposits are not common in the rock record. The most notable type of continental slope deposits are contourites. Contourites form on the slope between the continental shelf and deep Lower shoreface

      The diagram shows that wavebase is 1/2 the wavelength of waves of water.
      Diagram describing wavebase.

      The lower shoreface lies below the normal depth of wave agitation, so the Upper shoreface

      The image shows the many complexities of the shoreline described in the text.
      Diagram of zones of the shoreline.

      The upper shoreface contains sediments within the zone of normal wave action, but still submerged below the beach environment. These 5.5.2. Transitional coastline environments

      Onlap is sediments moving toward the land. Offlap is moving away.
      The rising sea levels of transgressions create onlapping sediments, regressions create offlapping.

      Transitional environments, more often called Littoral

      The tan rock has dark streaks of minerals.
      Lithified heavy mineral sand (dark layers) from a beach deposit in India.

      The littoral zone, better known as the beach, consists of highly weathered, homogeneous, well-sorted sand grains made mostly of Tidal Flats

      The tidal flat it a network of channels.
      General diagram of a tidal flat and associated features.


      The fold is a long ridge.
      Waterpocket fold, Capitol Reef National Park, Utah

      Reefs, which most people would immediately associate with tropical coral reefs found in the oceans, are not only made by living things. Natural buildups of sand or rock can also create The reef has many intricacies.

      A modern coral reef.

      Most reefs, now and in the geologic past, originate from the biological processes of living organisms. The growth habits of coral The reef is offshore of the island proper.

      The light blue reef is fringing the island of Vanatinai. As the island erodes away, only the reef will remain, forming a reef-bound seamount.

      Sediment found in coral reefs is typically fine-grained, mostly carbonate, and tends to deposit between the intact coral skeletons. Water with high levels of silt or clay particles can inhibit the The map shows locations.

      Seamounts and guyots in the North Pacific.


      The lagoon is just inside the coastline.
      Kara-Bogaz Gol lagoon, Turkmenistan.


      The river, as it flows north and enters the sea, spreads out.
      The Nile delta, in Egypt.
      Birdfoot river-dominated delta of the Mississippi River
      Birdfoot river-dominated delta of the Mississippi River

      Deltas form where rivers enter lakes or oceans and are of three basic shapes: Tidal delta of the Ganges River.

      Tidal delta of the Ganges River.

      Deltas are organized by the dominant process that controls their shape: tide-dominated, wave-dominated, or river-dominated. Wave-dominated deltas generally have smooth coastlines and beach-ridges on the land that represent previous shorelines. The Nile River 5.5.3. Terrestrial

      Terrestrial Fluvial

      The river wiggles back and forth.
      The Cauto River in Cuba. Note the sinuosity in the river, which is meandering.

      Fluvial (river) systems are formed by water flowing in channels over the land. They generally come in two main varieties: The river has many inter-braided channels.

      The braided Waimakariri river in New Zealand.

      Braided fluvial systems generally contain coarser sediment grains, and form a complicated series of intertwined channels that flow around gravel and sand bars (see Chapter 11, Water).


      This broad valley in the desert has alluvial deposition.
      An alluvial fan spreads out into a broad alluvial plain. From Red Rock Canyon State Park, California.

      A distinctive characteristic of Lacustrine

      The mountain has a large hole in the center that is filled with the lake.
      Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama.

      Lake systems and deposits, called lacustrine, form via processes somewhat similar to Paludal

      Paludal systems include bogs, marshes, swamps, or other wetlands, and usually contain lots of organic matter. Paludal systems typically develop in coastal environments, but are common occur in humid, low-lying, low-Aeolian

      The chart has the way dunes are made and four dune types.
      Formation and types of dunes.

      Aeolian, sometimes spelled Loess Plateau in China. The loess is so highly compacted that buildings and homes have been carved in it.

      Loess Plateau in China. The loess is so highly compacted that buildings and homes have been carved in it.

      Compacted layers of wind-blown Glacial

      Large boulders and smaller sand are seen together.
      Wide range of sediments near Athabaska Glacier, Jasper National Park, Alberta, Canada.

      Glacial sedimentation is very diverse, and generally consists of the most poorly-sorted sediment deposits found in nature. The main clast type is called 5.5.4. Facies

      In addition to

      5.5 Did I Get It?

      Use this quiz to check your comprehension of this section. Click directly on the answer button, not on the answer bar.

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      3. On the dynamic Earth, shifting environments over time cause facies distributions that allow interpretation of ancient shifting shorelines. In the Middle Cambrian strata of the Grand Canyon and surrounding regions, 1. East to west

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4. A stratum with a mix of sandstone and conglomerate sedimentary rocks with 1. Lagoonal

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6. 1.


Sedimentary rocks are grouped into two main categories:

Chapter 5 Review

Use this quiz to check your comprehension of this chapter. Click directly on the answer button, not on the answer bar.

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1. Which of the following is NOT part of the process of diagenesis and lithification of sediment into 1. Compaction

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2. What story does a sedimentary rock tell you?

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4. Which is the correct order of grain sizes from smallest to largest for clastic (1. sand, silt, clay, pebble, cobble, boulder