Weathering and Erosion Revisions 09

Weathering and erosion generally go hand-in- expansion is responsible for the bursting of hand but the two terms have subtly different water pipes that are allowed to get too cold in scientific meanings. Weathering is the name winter. The expansion of water into ice given to processes that change the surface pushes against the sides of the cracks with material of the Earth as a result of exposure to great force, wedging the rocks apart. This the effects of water and the atmosphere. process, which is characterized by a cycle of Erosion means the removal or transportation daytime thawing and refreezing at night, is of material from one position on the surface called frost wedging (also known as ice of the Earth to a different position, again by wedging). Frost wedging causes large rock the effects of water and the atmosphere. masses, especially the rocks exposed on There are circumstances where weathering mountain tops, to be broken into smaller and erosion occur simultaneously. For pieces. Frost wedging can have the same example, storm waves along our coasts hurl effect on pavement. During the winter, water rocks at the cliffs, causing abrasion. Pieces trapped in cracks in the pavement freezes into are broken off and carried away. Weathering ice. The ice often expands enough to crack the (by breaking the solid rock) and erosion pavement and form potholes. (removal of the broken material) occur together. We will begin by discussing the two types of weathering: physical weathering and chemical weathering.

PHYSICAL WEATHERING

Physical weathering (also known as mechanical weathering) is the process by Frost wedging is most prevalent in cool, which rocks are broken down into smaller temperate climates where freezing and fragments without undergoing any change in thawing occurs many times in the year. In the chemical composition. Physical weathering is arctic, frost wedging actually occurs less mainly caused by the freezing of water, the frequently because the temperature tends to expansion of rock, and the activities of plants stay below freezing for long periods of time. and animals.

Frost Wedging. In areas with temperate and Exfoliation. Weathering continually breaks down exposed bedrock into smaller cold climates, as well as in high mountainous areas, rocks are weathered by the action of fragments, which are then carried away by wind and water. Consequently, rock formed freezing water. During the daytime, when the temperature is above the freezing point of deep underground (under great pressure) becomes exposed at the Earth's surface. The water (O°C), rainwater, melted snow, or ice trickles into cracks in the rocks. At night, release of overlying pressure causes the newly exposed bedrock to expand, forming cracks when the temperature falls below the freezing point of water, the water trapped inside the parallel to the rock's surface. Then, frost wedging causes large, curved slabs of rock to rock changes into ice. When water freezes, it increases in volume by about 9 percent. This peel away from the main body of the rock.

1 The peeling away of the outer layers from a materials, so they tend to cause the rock to rock is called exfoliation. Rounded crumble and fall apart. The breaking down of mountaintops called exfoliation domes are rocks through changes in their chemical formed in this way. composition is called chemical weathering. An example of chemical weathering is the Miners and quarry workers have been injured change of feldspar in granite to clay. When and killed by exfoliation. As a mine shaft or acted upon by water, the feldspar changes into quarry pit is dug into the bedrock, the removal powdery clay minerals. Feldspar and clay of rock causes a rapid decrease in pressure on have different chemical compositions. the surrounding bed rock. The reduced pressure can result in sudden exfoliation, Water, oxygen and carbon dioxide are the sending dangerous missiles of rock through main agents of chemical weathering. When the air. water and carbon dioxide combine chemically, they produce a weak acid that can Animals and Plants. Insects, earthworms, break down rocks. Rocks also are weathered rabbits, woodchucks, and many other animals chemically by the action of acids produced by burrow through the soil. Perhaps the most plants and animals. These acids are formed by effective of all are humans. The tunnel the living plants and by the decomposition of organisms make often expose parts of the plant and animal materials. bedrock to the weathering action of the air and water. Frost wedging or chemical action Action of Acids. You may have noticed the can then break down the bedrock. In addition, exposed surfaces of bedrock sometimes the act of burrowing through soil helps to appear to be covered by a grayish or bluish- break down rock particles into smaller and green crust. A closer look reveals that this smaller pieces. Mining and quarrying, colored crust actually consists of tiny plants highway construction projects and called lichens. Lichens "eat" rocks by excavations for building foundations and releasing acids that slowly dissolve the basements all lead to significant physical minerals in the rocks. In addition, the action weathering of the Earth's crust. of bacteria and other microscopic organisms on dead plants and animals changes the Root Wedging. You may have seen trees composition of their remains. As these that appear to be growing out of solid rock. changes take place, acids are produced. The Trees occasional grow in soil that has acids then dissolve the minerals in the rocks, collected in small cracks in the bedrock. In and the rocks crumble apart. other cases trees grow in soil in the cracks of a boulder. In either case, as the tree continues Water. When water comes into contact with to grow it exerts great pressure. The pressure certain minerals the water combines causes the rock to split apart even wider. chemically with these minerals and changes them. This kind of chemical weathering is CHEMICAL WEATHERING called hydrolysis. You learned that when water reacts with the feldspar in granite, the Exposure to air, water, and organisms can feldspar changes into clay. Clay is much change the minerals in a rock into new softer than the original feldspar. Thus, the substances that have different chemical process of hydrolysis weakens granite, compositions. These new substances are making it more susceptible to physical generally softer or weaker than the original weathering.

2 Oxygen. When an iron nail is exposed to the slowly. The world's largest underground atmosphere rusting takes place. Iron, in the caverns were formed by this type of chemical presence of moisture, combines chemically weathering. The original limestone bedrock with oxygen in the atmosphere. The was gradually dissolved by water containing combining of oxygen with another substance carbonic acid. The water then carried the is called oxidation. The oxidation of an iron dissolved limestone away, leaving the hollow nail produces a powdery reddish brown spaces called caverns. substance called rust. Rust is composed mainly of a chemical compound called iron RESISTANCE TO WEATHERING oxide (Fe2O3). The ability of a rock to resist weathering Oxidation also takes place in rocks that depends mainly on the mineral composition contain iron-bearing minerals. When these of the rock and on the number of cracks in the rocks are exposed to oxygen in the rock. Rocks that consist mostly of quartz are atmosphere, the minerals slowly change into more resistant to either physical or chemical softer, more crumbly substances. Like an iron weathering than most other kinds of rocks. nail that rusts when exposed to oxygen and The chemical weathering of quartz is very moisture, the iron-bearing minerals in these slow because quartz does not combine readily rocks also rust when exposed to the with other substances. Unlike rocks that atmosphere. contain abundant quartz, rocks that consist mainly of feldspar weather rapidly, especially The chemical weathering of rocks that contain in warm moist parts of the world. In these iron often produces reddish or brownish areas, chemical weathering rapidly changes soils. Many of the world's largest iron ore feldspar into clay. Granite consists mainly of deposits were formed by chemical quartz and feldspar. Although granite is a weathering. The great deposits of iron ore in relatively hard rock, the chemical weathering Minnesota and upper Michigan were formed of feldspar eventually causes granite to over a vast period of time by the chemical crumble. Quartz weathers also, but at a much weathering of basalt, an igneous rock that slower rate than feldspar. Eventually, contains iron. physical weathering breaks down the quartz into small grains of sand. Carbon Dioxide. When carbon dioxide and water combine chemically, they form a weak In general, sedimentary rocks weather more acid called carbonic acid (H2CO3). A common rapidly than other rock types because name for carbonic acid is soda water. Soda sedimentary rocks consist of many small water is made by dissolving carbon dioxide grains that are cemented together. Poorly gas in water. cemented sedimentary rocks, such as some When you place a drop of hydrochloric acid shales and sandstones, contain small air on a rock containing calcite, bubbles of spaces between the grains. Consequently, carbon dioxide gas appear on the rock. At the water easily penetrates the rocks. If the water same time that the hydrochloric acid is freezes within these rocks, the rocks are dissolving the calcite, carbon dioxide gas is broken apart by frost wedging. being released. If water dissolves the cement holding the Like hydrochloric acid, carbonic acid grains of a sedimentary rock together, the dissolves calcite from rocks, but much more grains will separate. Cements that consist of

3 iron compounds or calcite dissolve more organic matter. Most soil is made up of about rapidly than do cements composed of silica 50 percent, rock and mineral fragments and (quartz) compounds. In moist climates, 50 percent air, water, and organic matter. Soil therefore, sedimentary rocks cemented by can take hundreds of years to form and can calcite and iron compounds weather rapidly, range in thickness from 60 meters in some whereas those cemented by silica are more areas to just a few centimeters in others. resistant to weathering. As rock weathers into smaller and smaller Cracks in a rock also cause the rock to fragments, plants begin to grow in the weather more rapidly. The more cracks in a weathered rock. Then worms, insects, rock, the faster that weathering will split the bacteria, and fungi begin living among the rock into fragments. Once a large rock has plant roots. These organisms don't just live in been split into smaller fragments, the smaller the weathered rock they help it evolve into fragments can then be broken down more soil by adding organic matter. When the rapidly by the action of air and water. plants and animals that live in the soil eventually die, they break down in a process SOIL called decay. The dark-colored organic matter made of pieces of decaying plants and The soil consists of particles formed by animals is called humus. As worms and physical and chemical weathering. Soil insects burrow throughout the soil, they mix usually contains particles of sand, clay, the humus with the fragments of rock. As you various minerals, tiny living organisms, and can see, weathered rock is constantly humus. Humus is the decayed remains of evolving into soil. plants and animals. In addition, some types of soil have large numbers of air spaces between SOIL TYPES their particles. Soils are divided into three main classes Formation of Soil according to texture. These classes are sandy soils, clay soils, and loamy soils. You can How often have you been told, "Take off determine the texture of each soil type by those dirty shoes before you come into this squeezing and rubbing a small amount of house"? Ever since you were a young child, moist soil between your fingers. you've had many experiences with dirt. Dirt is actually soil. You can find soil in lots of Sandy soils feel gritty, and their particles do places. An empty lot may have exposed soil not bind together firmly. Sandy soils are in it. A garden or a flowerbed has soil. porous, which means that water passes through them rapidly. Consequently, sandy But what is soil and where does it come soils do not hold much water. Adding a large from? As you learned, weathering gradually amount of humus to a sandy soil permits the breaks rocks into smaller and smaller soil to hold more water. fragments. When plants and animals live in these fragments, organic matter, such as Clay soils feel smooth and greasy, and their leaves, twigs, and dead worms and insects, is particles bind together firmly. Clay soils are added. When organic matter is gradually usually moist, but they do not permit water to added to the weathered rock, soil evolves. pass through easily. Soil is a mixture of weathered rock and

4 Loamy soils feel somewhat like velvet and usually contains minerals that were washed their particles clump together. Loamy soils down from the A horizon. The process in consist of a mixture of sand, clay, and silt. A which minerals are dissolved in water and loamy soil holds water well and permits some carried down in a soil profile is called water to pass through. leaching. The process of leaching is similar to how coffee is made in an automatic drip Soil Profiles coffee maker. Like water seeping into the A horizon, hot water drips into coffee grounds You may have seen layers of soil if you've in a filter. There, like water dissolving ever been near a steep slope such as a road cut minerals in the B horizon, the water absorbs where the soil and rock are exposed. You flavor and color from the coffee grounds. might have noticed that plants grow in the top Then, like water carrying the dissolved layer of soil, which is a darker color than the minerals to the C horizon, water in a coffee other soil layers below it. These different maker flows through the filter and into the layers of soil make up what is called a soil pot. profile. Each layer in the soil profile is called a horizon. There are generally three horizons, Below the B horizon you will find the C and they are labeled A, B, and C. horizon. This is the bottom layer in a soil profile. Some of the materials in this layer were leached from the B horizon. The C horizon also contains partly weathered rock, but no humus. This rock is just beginning the long, slow process of evolving into soil. What do you suppose you will find if you dig all the way to the bottom of the C horizon? As you might have guessed, there will be solid rock.

The A horizon is the top layer of soil. It's Although the bottom two layers of the soil are also known as topsoil. If you could dig up a relatively thick, the topsoil is always a thin scoop of topsoil from the top of that steep layer. Even fertile farmlands have a relatively slope and look at it very closely, what would thin layer of topsoil. Because topsoil is a you see? The soil would be dark in color and loose material, it is easily carried away by would contain sediments, decayed leaves, the wind and water. In the 1930s, winds blew roots of plants, and even insects and worms. away billions of tons of topsoil from the The A horizon is the most fully evolved soil farmlands of the Plains states. The dust was layer in a soil profile. This means that the A carried in great clouds more than 3000 horizon has changed the most since it was kilometers (1860 miles) east to the Atlantic just weathered rock. It generally has more Ocean. The United States still loses large humus and smaller sediments in it than the amounts of topsoil each year because of the other, less evolved layers in a soil profile. destructive action of wind and water.

The next layer, below the A horizon, is the B horizon (subsoil). This layer is less evolved and lighter in color than the A horizon Erosion – Section 2 because it has little or no humus. Some plant roots reach into this layer. This horizon Have you ever ridden your bike by a river just after a heavy rain? The water was so muddy

5 that it looked like chocolate milk. Where do downward as one large mass. The material you suppose all of the mud came from? As does not travel very fast or very far, but when you might guess, some of it came from dirt it happens, a curved scar is left where the along the river's bank, but the rest of it was slumped material originally rested. Slumps carried to the river from much more distant occur because the material underlying the sources. slumped material weakened. It could no longer support the overlying material, so it Mud is a product of erosion. Erosion is the slipped down slope. process that moves or transports weathered sediments from one location to another. As you investigate the processes of erosion, you will see that eroded sediments are eventually Slump deposited. But what moves the sediments in the first place? The four major agents of erosion are gravity, wind, running water Creep. The next time you travel by car or and glaciers. bus, look along the roadway for slopes where trees, utility poles, and fence posts lean As you investigate these four agents of downhill. These indicate that another type of erosion, you will notice that they have several mass movement is happening. It's called things in common. For one thing, they all creep. Creep gets its name from the way carry sediments only when they have enough sediments slowly creep down a hill. It is energy of motion. For example, air doesn't especially common in areas where freezing have the ability to erode sediments as long as and thawing occur. Small sediments are it's standing still. But once air begins moving pushed up by the expanding water in the soil and develops into wind, it can carry dust, soil, as the ground freezes. When the soil thaws and even large rocks along with it. Another the sediments fall down slope, often less than thing that the agents of erosion have in a millimeter at a time. Several years of soil common is that they all drop their load of creeping down slope can cause objects such sediments when their energy of motion as utility poles and fence posts to lean. decreases. This dropping of sediments is called deposition. Now let's take a look at how gravity, wind, running-water and glaciers erode and deposit sediments. Creep

Gravity. Gravity causes loose materials to move down slope. When gravity alone causes materials to move down slope, it's called mass movement. Some mass Rockslides. You may have seen signs along movements are very slow; you hardly notice the road warning you, "Beware of Falling that they are happening. Other types, Rocks”. This movement is called a rockslide. however, happen very quickly. Let's examine A rockslide happens when large blocks of some different types of mass movements. rock break loose from steep slopes and tumble quickly to the bottom. As they fall, these Slump. A slump is a type of mass movement rocks crash into other rocks and they too that takes place on steep slopes. It occurs break loose. Rockslides commonly occur in when loose materials or rock layers slip mountainous areas. They happen most often after heavy rains or during earthquakes. But

6 they can happen on any rocky slope at any surface. Wherever these fine materials lie time without warning. Piles of broken rock at uncovered at the surface, the wind is an the bottom of a cliff tell you that rockslides important agent of erosion. have occurred there in the past and are likely to occur there again. The wind changes the landscape by moving

sand, silt, and clay from one place to another. Mudflows. Can you imagine being in a car Lightweight silt and clay particles can be traveling along a mountain road during a lifted and carried by light breezes. This type storm, when suddenly a wall of chocolate of erosion is deflation. However, only strong pudding slides clown a slope and covers your winds can carry and move the heavier sand car? This is similar to what might happen if grains. Strong winds lift silt and clay you were caught in a mudflow. Instead of particles high into the atmosphere and carry chocolate pudding, a thick mixture of them long distances. Heavier sand grains, sediments and water would flow down the however, are rarely lifted more than one slope in a type of mass movement called a meter off the ground and travel relatively mudflow. Mudflows usually occur in short distances. On windy days at the beach, relatively dry areas where weathering forms you may have notice while standing that most thick layers of dry sediments. When heavy sand grains strike you below knee level. Even rains fall in these areas, the water mixes with during violent desert windstorms, most of the the sediments and forms a thick, pasty flying sand grains do not rise more than one substance. Gravity causes this mass to slide or two meters above the ground. downhill. A mudflow has enough energy to move almost anything in its path, including Flying sand grains erode pebbles, boulders, houses, cars, and large rocks. When a and even large rock outcrops by wind mudflow finally reaches the bottom of a abrasion. During wind abrasion, flying sand slope, it loses its energy of motion and deposits all the sediments and debris it has grains sandblast, or chip away, any solid been carrying. These deposits are usually a surface that they are blown against. In some cone-shaped mass. windy areas of the United States, the bases of wooden telephone poles have been Now that you've thought about mudflows, completely sandblasted away by wind rock slides, creep and slump, think about how abrasion. On a longer time scale, isolated all these mass movements are similar. They're rock outcrops have been carved into strange, all more likely to happen where there are mushroom like shapes by windblown sand. steep slopes. They all depend on gravity to make them happen. And, regardless of the Loss of Topsoil. A long drought in the 1930s type of mass movement, it will occur more led to wind erosion of the fertile topsoil of the often after a heavy rain because the water Plains states. Crops withered and died, adds mass and makes the sediments slippery. leaving bare fields unprotected from the wind. Moist topsoil turned into powdery dust and Wind farmers were helpless as the wind stripped the precious topsoil from their fields. On several In dry environments, such as deserts, very few occasions, strong winds carried thick clouds plants cover the ground. For example, in parts of dust-like topsoil from the Great Plains to of the southwestern United States, deposits of the Atlantic Ocean—over 3000 km away. loose sand, silt, and clay lie exposed at the Because of the disastrous loss of topsoil during such dust storms, the region from

7 Texas to South Dakota was called the "Dust dune, some sand is blown over the top, falling Bowl." on the leeward side. Gradually, the entire dune is moved in the leeward direction. Entire DEPOSITS OF WINDBLOWN farms, towns, and forests have been buried by MATERIAL advancing sand dunes. However, in regions where grasses and shrubs grow on the dunes, Sand, silt, and clay particles are deposited the movement of sand dunes is slowed or when the wind slows down or is blocked by even stopped. an object. Two deposits often formed by wind blown sediments are sand dunes and loess.

Sand Dunes. Boulders, rock outcrops, and isolated shrubs are often found in regions that have a great deal of loose sand. Such objects Most dunes are made of quartz sand. A obstruct the wind, allowing windblown sand notable exception is the White Sands National to be deposited. For example, a boulder Monument in New Mexico, which has blocks the wind, slowing the wind velocity in beautiful white dunes made of gypsum sand. the immediate area. Consequently, sand Another exception would be islands made of grains suddenly fall to the ground and pile up coral, such as Bermuda, which have dunes around the boulder. In time, the sand grains made of calcite sand. The calcite sand is form a sand hill, or sand dune. produced by the weathering of coral reefs that

make up the islands. The side of a sand dune facing into the wind is called the windward side. The windward Loess. Unlayered deposits of windblown silt side of a sand dune has a gentler slope than are called loess (pronounced less). Loess the side facing away from the wind, or the particles are angular and may be derived from leeward side. For example, winds blowing many types of rocks. Loess weathers to form from the north will form dunes with gentle very fertile topsoil. The topsoil of several north slopes and steep south slopes. In places Mid-western states, including Kansas, Iowa, where the wind blows steadily from one Missouri, and Illinois, consists largely of direction, crescent-shaped sand dunes form. loess deposits. On some Illinois farmlands, Sand dunes range in height from one meter to fertile loess deposits are more than 90 m more than 100 m. In the Sahara Desert there thick. are sand dunes more than 200 m tall. Dunes also are found in the Great Sand Dune Loess often consists of glacial rock flour. National Monument in Colorado. When glaciers melt and the melt-water drains away, rock fragments of various sizes are left exposed on the ground. The wind picks up the lightweight particles of rock flour, leaving behind the heavier materials. The windblown rock flour eventually settles to the ground, forming deposits of loess.

The wind not only forms sand dunes but also Loess is an unusual kind of sediment. Most moves dunes from place to place. As the wind deposits of loose sediment collapse to form blows against the windward side of a sand slopes when they are cut into by streams or by

8 road builders. Loess deposits, however, form running water. Runoff water flowing downhill steep cliffs when they are cut into, because from repeated rainfalls deepens and widens a the angular loess particles stick together. gully. Eventually, the bed of the gully may Loess cliffs are often found along the banks reach the water table. If there is sufficient of the upper Mississippi and Missouri rivers. groundwater throughout the year, a permanent stream forms in the gully. Otherwise, the RUNNING WATER gully contains flowing water only when it rains a lot or when snow melts. For example, The most important agent of erosion is many gullies in the deserts of the running water. Brooks, creeks, streams, and southwestern United States are dry for much rivers are constantly at work removing of the year. They fill with water after heavy weathered fragments of bedrock and carrying rains and then dry up because water does not them away from their original location. flow continuously in such a gully. This Almost all of the water that erodes the land stream is called an intermittent stream. originates in the ocean. Water evaporates from the ocean and becomes water vapor. The Small streams often join to form larger water vapor cools and condenses into huge streams. In turn, these streams combine and masses of tiny water droplets called clouds. In form a still larger stream called a time, precipitation falls from clouds in the mainstream. All of the streams that supply a form of rain or snow. Small amounts of the mainstream are called tributaries of that precipitation that falls on Earth's surface mainstream and all of its tributaries form a soaks into the ground. Most of the water that river system. The Missouri River and Ohio falls as rain or snow quickly evaporates back River are examples of river systems; both into the atmosphere or returns to the ocean by rivers are themselves tributaries of the way of streams and rivers. Water that seeps Mississippi River. The Mississippi River and into the ground moves slowly downward and its thousands of tributaries form a huge river becomes groundwater. When groundwater system that extends from the Rocky meets an impermeable layer of rock, the Mountains to the Appalachian Mountains and groundwater flows sideways along the from Canada to the Gulf of Mexico. The area impermeable layer until it meets openings in of land drained by a river system is called its the surface. At these surface openings, lakes, watershed. River systems usually empty into ponds, and springs appear. Lakes, ponds, and a large body of water such as the ocean. The springs provide streams with a steady supply place where a river empties into a larger body of water. of water is called the mouth of the river.

RIVER SYSTEMS EROSION BY STREAMS AND RIVERS

When rain falls on sloping land, the runoff You have seen that running water carries water flowing downhill carries away loose away loose rock materials. Streams and rivers rock fragments and soil. When water erodes can also wear down solid bedrock, gradually without being in a stream channel, sheet eroding high mountains into low hills and flat erosion takes place. Soon, small channels plains. Two ways that streams and rivers called rills, might be created by this flowing erode bedrock are by abrasion and by water. If water continues to flow along the solution of minerals in the bedrock. same path a gulley may form. Gulleys are deeper channels carved into Earth's surface by

9 Abrasion. A stream erodes bedrock by the easily dissolved by streams. Running water grinding action of the sand, pebbles, and other also causes sedimentary rocks cemented rock fragments it carries. Like sandpaper together by calcite or iron compounds to against wood, these materials scrape against crumble, because these cementing agents are the streambed and banks. Particle by particle easily dissolved. The dissolved mineral the bedrock is worn away. The grinding substances in water are invisible. However, action of rock fragments scraping against the they can be detected by chemical analysis. streambed and against each other is called Geologists estimate that the rivers of the abrasion. United States carry about 250 million tons of dissolved minerals into the oceans each year. When large rock fragments first enter a stream, the fragments are usually angular with HOW A STREAM ERODES ITS BED sharp edges. These sharp edges act as cutting tools. As the sharp-edged, angular rocks How quickly a stream erodes its bed depends bounce and scrape along the riverbed, they on several factors. These factors include the chip off pieces of the bedrock. In time, the velocity of the stream, the volume of water angular rocks lose their sharp edges and carried by the stream, the size and shape of become rounded. The roundness of a stone is sediments in the stream, and the type of an indication of how long it has been carried bedrock beneath the stream. along by a stream. The lowest level to which a river can erode its Streams may carry a large volume of fine- bed is called base level. Sea level is the grained silt and clay. The silt and clay are ultimate base level, but the floor of a lake or produced when coarse-grained rock particles basin into which a river flows may become a collide local and temporary base level. Cliffs or with escarpments and differences in the resistance each of rocks create irregularities in the bed of a other river and can thus cause rapids and waterfalls. or A river tends to eliminate irregularities and to with form a smooth gradient from its source to its the base level. As it approaches base level, streambed, or when loose materials are eroded downward cutting is replaced by sideways from the banks of a stream. The motion of cutting and the river widens its bed and valley the stream prevents the silt and clay particles and develops a winding course that forms from settling to the streambed. Rivers that exaggerated loops and bends called meanders. carry abundant silt and clay have a muddy appearance. The Mississippi River and the Velocity. Rapidly flowing streams erode their Missouri River are two examples. In fact, the beds faster than slow-moving streams. A Missouri River has been nicknamed the "Big rapidly flowing stream has great cutting Muddy." It has been estimated that the rivers power because of its speed and because it can of the United States carry about 700 million carry large, coarse-grained sediments such as tons of sediments into the oceans every year. gravel, pebbles, and even small boulders. These coarse-grained materials scrape with Solution. Besides abrasion, streams also great force against the streambed. In contrast, erode rocks by the dissolving, or solution, of a slow-moving stream has little cutting power their minerals. Limestone and marble are because it can carry only fine-grained

10 sediments such as silt and clay. Such fine- streams erode bedrock made of granite, slate, grained sediments have almost no effect on and quartzite very slowly. the streambed. A stream moving about 1 kilometer (km) per hour can carry silt, clay, RUNNING WATER AND DEPOSITION and small sand grains. A stream moving at about 8 km per hour can move small boulders. You already know that the ability of a stream to carry sediments depends partly on its Volume. A deep, wide stream can transport a velocity. A stream's velocity decreases when greater volume of water than a narrow, shallow stream can. Suppose two streams move with the same velocity but carry different volumes of water. The two streams also flow over the same type of bedrock. In this case, the stream with the greater volume of water will erode its bed faster. This stream erodes its bed faster because it carries the stream (a) enters a larger body of water, more sediment. (b) overflows its banks, (c) changes direction, or (d) flows from a steep slope to a gentle Size and Shape of Sediments. Most stream slope. erosion is produced by the abrasion of coarse grained, angular sediments. Large rock As the velocity of a stream decreases, its fragments with sharp edges make much better ability to carry rock particles also decreases. cutting tools than do small, rounded particles. Thus, when a stream slows down, the coarse- Consequently, large coarse-grained sediments grained sediments settle out first, followed by such as gravel and pebbles enable a stream to the fine-grained sediments. An accumulation erode its bed most effectively. of sediments left by a stream or river is called a deposit. As a deposit of sediment grows, it Type of Bedrock. Streams readily erode may produce landscape features such as bedrock that is soft or soluble in water. For deltas, alluvial fans, levees and sandbars. example, most streams will easily erode bedrock made of limestone and shale, which Deltas. When a river enters a large body of are relatively soft rocks. Streams also readily water, (such as a lake or bay), the river erode sedimentary rocks cemented together immediately slows down. This sudden by calcite or iron compounds, which are decrease in speed causes sediments to be soluble in water. In contrast, bedrock made of deposited on the bottom of the larger body of granite, slate, or quartzite is much harder and water. In time, these deposits extend far into a more resistant to chemical weathering than lake or bay, producing a landform called a are limestone and shale. Consequently, delta. The name delta comes from the fact

11 that many of these deposits have a triangular shape that resembles the Greek letter delta. For example, the Nile River in Egypt has built a triangular delta where it empties into the Mediterranean Sea. Not all deltas have this shape. Another type of delta looks like long toes extending into a larger body of water. This type is called a bird-foot delta. In the United States, the Mississippi River has built a bird-foot delta out into the Gulf of Mexico.

Sandbars. The velocity of the river changes when it flows around a curve or bend. The Alluvial Fans. A stream that flows from steep water in the river slows down on the inside of mountains out onto a broad, flat plain the curve and speeds up on the outside of the abruptly slows down and deposits sediments curve. The loss of speed on the inside of the on the plain. The resulting fan-shaped deposit curve causes sediments to settle out of the is called an alluvial fan. Alluvial fans are water and to be deposited along the inside most common in dry regions, such as the bank of the river. This type of deposit is southwestern United States, where they are called a sandbar. A sandbar may also form often seen at the base of mountain passes. when a boulder or some other obstruction, such as a fallen tree, partly blocks a river Levees. When a river overflows its banks the channel. water that leaves the channel slows down when it enters the floodplain. As the water Water Diversion. The changing of the loses speed, it deposits sediments on the natural flow of water by people is called banks of the river. Each time the river water diversion. Dams and pipelines are overflows its banks more sediment is often successfully used to take water from one deposited on top of the previous deposit. location to another. This is usually done if Layer by layer, the river builds up long, low the demand for water is greater then the local walls of sediments along its banks. These built-up riverbanks are called levees. Towns located near a flood-prone river often build artificial levees along the riverbanks. These artificial levees usually prevent the river from overflowing and flooding the nearby towns. Many levees, both natural and artificial, are found along the banks of the Mississippi River. supply. However, this leaves less water for the people at the original location. Water diversion can significantly affect the velocity and the volume of a stream.

12 river flows over this cliff. Yellowstone Falls LIFE CYCLE OF A RIVER in Yellowstone National Park is an example of a waterfall produced in this way. As a river flows and erodes the land, the river undergoes changes, passing through different Maturity. As a young river continues to stages of development. These stages make up erode and smooth out its channel, the river the river's life cycle. There are three main loses its waterfalls and rapids. Erosion also stages in the life cycle of a river: youth, decreases the slope of its channel. As a maturity, and old age. Each stage has its own consequence, the velocity of the river characteristic features. decreases as well. The river is then said to be in its mature stage. The Missouri River is an Youth. During youth, a river flows rapidly example of a mature river. A mature river down mountains or hills. A young river flows more slowly and has less cutting power tumbles noisily over falls and races through than a young river. Consequently, when a rapids, enclosed within a narrow, rock-walled mature river encounters an obstruction, the valley. Because a young river moves swiftly, river flows around the obstruction. In this it carries large quantities of coarse-grained way, a mature river begins to form a curved, sediments. Consequently, the river cuts or S-shaped, channel. In time, the curves rapidly downward into the underlying become larger, and the river channel becomes bedrock, eroding a deep, narrow, V-shaped more winding. These looping, S-shaped channel. The Colorado River, which has curves in the river's course are called produced the Grand Canyon, is an example of meanders. During maturity, a river's once- a young river. steep valley walls are worn down by the agents of physical and chemical weathering. A waterfall is perhaps the most spectacular Instead of being V-shaped, the valley walls of feature of a young river. A waterfall occurs a mature river are somewhat rounded and wherever the elevation of a riverbed drops slope gently toward the river. suddenly. For example, as the Niagara River flows from Lake Erie to Lake Ontario, the As a mature river flows back and forth across elevation of the riverbed drops about 50 its valley, the river gradually widens the meters at one spot. The sudden drop of the valley floor. When flooding causes the river river's bed in this location produced the to overflow its channel, coarse-grained famous Niagara Falls. sediments, such as gravel and pebbles, are deposited along the banks to form levees. A waterfall also may form where a river Finer grained sediments, such as silt and clay, travels over different types of bedrock. For are carried beyond the riverbanks and example, a river flowing over a layer of basalt deposited on the valley floor. These sediments followed by shale erodes the softer shale level out the valley floor to form a relatively faster than the harder basalt. In time, the flat area called a floodplain. Swamps often uneven erosion produces a basalt cliff in the form on a floodplain because the deposits of river channel. A waterfall is formed as the fine silt and clay do not readily allow the

13 floodwaters to soak into the ground. steeper. The steeper slope increases the water velocity, enabling the river to cut through the Old Age. When the slope of a river channel underlying bedrock again. With its renewed becomes almost flat, the water flows very cutting power, the river begins to deepen its slowly. Then, a river is said to be in old age. old meanders. A deepened meander looks like The lower part of the Mississippi River is in a curved trench with steep walls. These this stage of development. Instead of causing deeply eroded curves of a river are called erosion, an old river mainly affects the land entrenched meanders. You can see these by depositing sediments. Consequently, the along the Susquehanna River in New York natural levees found along an old river are and Pennsylvania, and along the San Juan higher than the levees along a mature river. River in Utah and New Mexico. An old river also has a wider floodplain than a mature river because the old river flows back and forth over a much wider valley.

Sometimes, an old river cuts through the narrow part of one of its meanders. This causes a sharp decrease in the velocity of water flowing through the meander. As a result, sediments accumulate where the ends of the meander meet the new river channel. In time, sediments completely cut off the Glaciers Section 3 meander from the river. The cutoff meander is called an oxbow lake. When sediments fill in In some areas of the world, temperatures are an oxbow lake, a swampy area, or bayou, is low enough that it snows year-round. If the formed. snow doesn't melt, it begins piling up. When it accumulates, the weight of the snow is great enough to compress the bottom layers into ice. When the snow piles up to 50 to 60 meters high, the ice on bottom partially melts and becomes putty-like. The whole mass

begins to slide on this putty-like layer and it You should be aware that a river may not be moves downhill. This moving mass of ice and in the same stage of its life cycle along its snow is a glacier. entire course. In fact, many rivers show several different stages of development along their length, depending on the steepness of the land. For example, a single river may exhibit signs of youth in its upper reaches, near its source; characteristics of a mature river in its middle stretch; and features of old age near its mouth, where it empties into the ocean.

Sometimes an old or mature river becomes youthful again. For example, forces inside the Earth may raise the crust, causing the slope of an old, meandering river channel to become

14 Movement of Glaciers moves faster than its sides. The sides of a glacier move more slowly than the center The movement of glaciers is caused in part by because of friction between the glacier and the great pressure exerted by the surface ice the valley walls. Some glaciers move as far as on the ice located deep within the glacier. 20 meters in a day. Other glaciers move very This pressure causes the deep ice to become slowly, advancing only a few centimeters in a plastic-like. This process is called Plastic day. Because of additional weight, glaciers Deformation. In other words, the deep ice move faster after winters with heavy snowfall behaves like dough or taffy. Materials that than after winters with light snowfall. Also, look solid but that can flow slowly and the sleeper the slope, the faster a glacier change shape. moves.

You may have noticed how the asphalt along the edge of a busy highway seems to have been pushed up against the curb. That is because the weight of passing cars and trucks causes the solid asphalt to become plastic- like and flow slowly toward the curb. In a similar way, deep ice in a glacier becomes plastic-like and flows outward because of the enormous weight of the overlying snow and ice. Glaciers, along with gravity, running water, Glaciers can also move due to a process and wind, are agents of erosion. There are two called Basal Slip. Basal Slip is a process types of glaciers: continental glaciers and where the underlying ice crystals rearrange valley glaciers. Continental glaciers are huge themselves so they are parallel to each other. masses of ice and snow found near the Earth’s When the ice crystals are parallel to each polar regions. Today, they are found only in other, the massive weight of the glacier Greenland and Antarctica. But during past ice causes the ice crystals to slide across one ages, continental glaciers covered large another causing the glacier to advance. portions of the world including where the city of Baraboo is today. Glacial movement also involves the melting of ice at the base of a glacier. Deep ice in a glacier melts under great pressure, forming a thin layer of water beneath the glacier. The melt water acts as a lubricant, causing the entire glacier to slip a small distance downhill. This movement decreases the pressure at the base of the glacier, allowing the melt-water to refreeze. In time, the weight of the ice causes the deep ice to melt again, and the process is repeated.

A row of stakes driven into the surface of a glacier will reveal the center of the glacier

15 An ice age is a period of time when ice and through hard igneous rock making snow cover much of Earth's surface. The last advancement very difficult. The Green Bay ice age in Wisconsin occurred from 35,000 to and Lake Michigan lobes to the east and the 10,000 years ago. During this time glaciers Superior lobe to the west were moving faster dramatically changed the landscape of and pinching in on the sides of the Chippewa Wisconsin. Glaciers were the most important lobe. This pinching created massive friction agent of erosion during this time period. and further slowed the Chippewa lobe. Finally, the glacier began to retreat before it Wisconsin was divided into two regions: could reach the driftless area. Glaciers are glaciated and non-glaciated (driftless) areas. considered to be retreating when more ice and The glaciated area was the northern and snow are melting than are being added to the eastern 2/3 of the state. The southwestern 1/3 glacier. of the state was the driftless area. Results of Wisconsin glaciation include changes to soil quality, landscape and land features. Northern Wisconsin soil is poor and shallow, central and southeastern Wisconsin soil is very good and thick because of the rich soil deposited there by the glacier. South- western soil is also very good. Northern Wisconsin is generally flat, southeastern Wisconsin has gentle rolling hills while southwestern Wisconsin has many steep hills. The glaciers also left many land features such as drumlins, moraines, The glaciated area was covered by four main eskers, kettle glacial lobes (from east to west) the Lake lakes and Michigan lobe, Green Bay lobe, Chippewa outwash. These lobe and Superior lobe. will be discussed later. The driftless area was not covered by glacial The last ice age ice during the previous ice ages. The driftless even changed area was not covered by ice for three reasons. the route of the The highlands of hard igneous rocks in north- Wisconsin central Wisconsin slowed the Chippewa lobe. River. It was going uphill and trying to carve

16 temperatures. During times of ice ages the axis is tilted at approximately 13 degrees. The low axis tilt means less direct sunlight for the northern hemisphere resulting in cooler average temperatures allowing the snow and ice to accumulate year-round. During the last ice age the average temperature on Earth was only about 40C lower than it is today.

The final condition needed for an ice age is called the Albedo Effect. The Albedo Effect is the measure of reflectivity of light and heat from the surface of the Earth. The oceans remained dark in color compared to the landmasses that were covered in snow. The dark colored oceans continued to collect solar heat creating warm air over them When the glaciers retreated from Wisconsin because they reflected less of the sunlight they left huge quantities of water forming back into space. The snow-covered Glacial Lake Wisconsin. This glacial lake is landmasses reflected much of the solar energy believed to have drained in a matter of weeks back into space creating very cold air over the creating most of the stunning sandstone landmasses. When the two air masses of sculpture along the Wisconsin River in the differing temperatures met, the moisture- Wisconsin Dells region. laden air from over the oceans condensed forming large amounts of snow in the frigid Causes of the Ice Ages air over the landmasses. This continued year after year to provide the snow and ice for the Scientists have learned that the Earth’s orbit glaciers. This snow and ice created the changes every 100,000 years from relatively continental glaciers. circular to a more elliptical orbital pattern. The change in orbit is due to the gravitational Valley glaciers are fairly common even in pull of the sun and the moon. The Earth’s today's warmer climate. They are located in average temperatures are warmer when the mountainous areas where the average Earth’s orbit is more circular. When the orbit temperature is low enough that snow doesn't is more elliptical the Earth’s average melt over the summer season. The glaciers of temperatures are cooler thus creating Glacier National Park in Montana are valley favorable conditions for an ice age. The glaciers. How is it possible that something as elliptical orbits create cooler summers fragile as snow or ice can erode something as allowing more snow and ice to remain for hard as rock? It may not seem likely, but longer periods of time. much of Earth's landscape has been shaped by glacial ice. Let's explore how this is possible. Scientists have also discovered the Earth’s inclination or tilt of axis changes every Glacial Erosion 40,000 years. Today the Earth’s axis is tilted at approximately 23 degrees. This axis tilt As they move over land, glaciers act like allows the northern hemisphere more direct bulldozers, pushing any loose materials out of sunlight creating warmer average their path. These eroded sediments are added to

17 the mass of the glacier or piled up along its sides. Deposition by Glaciers But glaciers do more than just move loose sediments. They also weather and erode rock and When glaciers begin to melt, they no longer have soil that isn’t loose. Glacial snow and ice melt enough energy of motion to continue carrying creating runoff that flows into cracks in rocks. sediments. Therefore, these materials are Later, the water refreezes in these cracks, deposited. Glacial deposits are classified into two expands, and breaks the rock into pieces. The rock major types. One is a jumble of different-sized fragments are moved along with the glacial ice. sediments that is deposited from the glacial ice This process, called plucking results in boulders, and snow. This mixture of boulders, sand, clay, gravel, and sand being added to the bottom and and silt is called till. When a glacier stops sides of a glacier. moving, till begins dropping from its base. These sediments cover huge areas of land. Continental These materials at the base of a glacier act like the glaciers moving across the northern United States blade of a plow. They scrape the soil and bedrock dropped so much till that it completely filled the glacier moves over. They cause the glacier to valleys and covered hills during the last ice age. erode even more than the ice and snow alone Today, these areas appear quite flat. Till is also could. Striations are left behind when bedrock is deposited in front of a glacier when it stops gouged. These striations are usually long, moving forward. Because it's made of the rocks parallel scars in rocks. Very large striations are and soil that the glacier has been pushing along, it called glacial grooves. looks like a big ridge of material left behind by a bulldozer. Similar ridges are deposited along the If you live in the mountains and want to know if sides of glaciers. These mounds of material are there were ever valley glaciers in your area, how called moraines. Moraines can easily be found in could you find out? You might begin by looking the Baraboo area today. If you have ever driven for striations. But what other evidence of glacial on highway 159 between highway 12 and Devil’s erosion could you find? Glacial plucking often Lake you have driven over a moraine. You also occurs near the top of a mountain. Where a have driven over it if you have driven on highway glacier is in contact with a wall of rock a bowl- 136 past K-mart. shaped basin, called a cirque, is created in the side of the mountain. If two or more glaciers The other major type of glacial deposit is erode a mountain summit from several directions, outwash. Outwash is deposited from the glacier's a ridge or sharpened peak forms. melted ice. This melt-water carries sediments and deposits them much like a river does. For Valley glaciers flow down mountain slopes and example, one type of outwash deposit is an valleys, eroding as they go. Valleys that have alluvial fan made of sediments eroded by the been eroded by glaciers are a different shape from glacier. It forms when a stream of melt-water those that have been eroded by streams. Stream- drops sand and gravel in front of the glacier. eroded valleys are normally V-shaped; glacially Another type of outwash deposit looks like a long eroded valleys are usually U-shaped. This is winding ridge. This deposit forms beneath a because glaciers pluck and scrape soil and rock melting glacier. Melt-water forms rivers within along their sides as well as on their bottoms. the ice. These rivers carry sand and gravel and Streams tend to erode downward into underlying deposit them within their channels. When the rock more than glaciers do. glacier melts, a winding ridge of sand and gravel, called an esker, is left behind.